KJZZ | ASU Astronomy Researcher Breaks Down Tonight's Eclipse KJZZ ... a lunar eclipse and a comet will be visible, too. To help break down the night-sky fun, I'm joined by Patrick Young. He's an associate professor in ASU's School of Earth and Space Exploration and a researcher in astronomy, astrophysics and ... |
Visit link:
ASU Astronomy Researcher Breaks Down Tonight's Eclipse - KJZZ
As part of an ongoing series inviting guests to experience the nightlife of the State Botanical Garden of Georgia, participants will have a chance to explore the garden during an upcoming Full Moon Hike on Feb. 11.
From 7-8:30 p.m., attendees armed with walking shoes and wanderlust, will take a guided tour through trails and gardens, exploring topics like constellations, astronomy, nocturnal creatures and more.
Tours begin at the Visitor Center & Conservatory Front Fountain before weaving two miles under a full moon with informative activities and facts along the way.
This monthly tour, which has been running for around four years, is the idea of Andie Bisceglia, 29, a University of Georgia second year graduate student.
Previously a middle school ecology teacher before going to UGA, she moved here with her husband and began working in the crop and soil sciences and education department at UGA.
Bisceglia has been working closely the the Gardens since being hired by their children's program in 2011 when she began the Full Moon Hike program.
In the summertime theres frog and insect calls you can identify because thats when theyre active, Bisceglia said. In the winter when theres less sounds going on I might focus more on astronomy or constellations. It depends on whats happening out in nature at the time.
Currently as a full-time graduate student she is not employed by the Gardens but still organizes and guides the monthly Full Moon Hikes, modeled after a nature at night program she used to teach children in Maine and Connecticut.
Bisceglia strongly encourages the surrounding community to enjoy the cost-efficient activities and programs offered by the Gardens, especially the Night Hike, which allows for a glimpse into nightlife.
Its really easy in this day and age even for someone like myself who love nature to go weeks at a time without really getting out of your neighborhood or comfort zone, Bisceglia said. Especially at nighttime when most people arent comfortable going out on their own, this is a safe environment thats lots of fun for the people who go on the hikes to learn.
For this months tour, Bisceglia hasnt yet decided on a topic, but strongly suggested that she would focus on the lunar eclipse set to happen early Saturday morning. She mentioned doing a demonstration on the science behind eclipses, keeping with a timely theme.
Although attendance fluctuates during the colder months and spikes in the summer with children out of school, Bisceglia noted that her tours run the gamut, including attendees aged 285. She also said the hikes are almost too popular, as nightly attendance, which should be capped at 25, often climbS into the 60s.
Pre-registration for the Full Moon Hike on Feb. 11 is strongly encouraged at $5 per person and $15 for families.
View original post here:
In Earths upper atmosphere, blue jets, red sprites, pixies, halos, trolls and elves streak toward space, rarely caught in the act by human eyes.
This mixed-bag of quasi-mythological terms are all names for transient luminous events, or, quite simply, forms of lightning that dance atop thunderstorm clouds. Airplane pilots have reported seeing them, but their elusive nature makes them hard to study. But ESA astronaut Andreas Morgensen, while aboard the International Space Station in September 2015, filmed hundreds of blue jets flashing over a thunderstorm that was pounding the Bay of Bengal, confirming a mysterious atmospheric phenomenon.
According to Morgensen and researchers at the Denmark National Space Institute, these observations are the first of their kind, and offer a rare glimpse of poorly understood atmospheric phenomena. His work, which was published in the journal Geophysical Research Letters, also proves that the ISS is a perfect lab for studying elves and pixies, and a planned follow-up mission should reveal even more.
A Great View
Storm clouds are like electrical cakes with alternating layers of negatively and positively charged ions. Air currents tear through the sky cake, squashing the layers and reversing their stacking order. When the base layers charge differs from the earths chargezap. The seems to be true for layers higher in the atmosphere, but instead of striking earth, transient luminous events discharge into space. Of course, you need to be above the clouds to see them; thus, the reason they are difficult to study.
Read more:
Elusive blue lightning filmed dancing above a thunderstorm - Astronomy Magazine
Plans to explore the nearest star system rely on light sails reflective panels that are propelled by light. These craft travel so fast that they will have little time to explore their destination, but altering the way the sails are used could help.
An Earth-sized planet orbits Proxima Centauri, the Sun's nearest neighbour, which is 1.3 parsecs (4.2 light years) from Earth. Astronomers hope to send a fleet of miniature probes to explore it and the neighbouring twinned stars of Alpha Centauri. Under current proposals, these laser-propelled craft would take 20 years to reach the stars and zip past them in just a few hours (see Nature 542, 2022; 2017). But Ren Heller of the Max Planck Institute for Solar System Research in Gttingen, Germany, and Michael Hippke of Neukirchen-Vluyn, Germany, say starlight could be used to slow down a sail-carrying probe, allowing more data about the planet to be collected.
In their proposal, the sail would shift direction as it passed Alpha Centauri so that starlight and the stars' gravitational pull could slow it down. The probe would then swing into orbit around Proxima Centauri, allowing multiple fly-bys of Proxima's planet. Using this set-up, a probe would take roughly a century to get from Earth to Alpha Centauri, and another half-century to reach Proxima.
Astrophys. J. 835, L32 (2017)
More:
Some black holes are small. Some black holes are giant. But oddly enough, in the cosmic fight between innocent passing stars and voracious black holes, scientists have never found a mid-sized black hole. Until now.
The star cluster 47 Tucanae, located about 13,000 to 16,000 light years from Earth, is a dense ball of stars. Hundreds of thousands of stars compacted into a 120 light-year span give off gamma rays and X-rays and more energetic events, but to date, no black holes had been found there. The center seemed ripe for opportunities to find one, but a lack of tidal disruption events and a jumble of stars hard to sift through obscured finding any lurking black holes there.
The Harvard-Smithsonian Center for Astrophysics turned to two tactics to find the black hole instead. First, they observed the motion of the stars in aggregate, and compared the rotation rate to what would happen if a black hole were present. Secondly, they observed the position of pulsars in the globular cluster.
Black holes are the densest objects in the universe. But neutron stars (which include pulsars) are a close second, as both can result from similar events in which a giant star goes supernova and its dense stellar core collapses (though a few other mechanisms can create black holes.)
If the pulsars were the biggest objects in the globular cluster, theyd be nearer to the core and act as a chief gravitational attractor. But instead, pulsars are scattered across the cluster rather than congregating in the center of the cluster.
This all suggests that a black hole of 2,200 solar masses is lying at the center of 47 Tucanae. Until now, though, astronomers have typically only found black holes of below 100 solar masses or above 10,000, the latter of which are the behemoths that power galaxies. These intermediate-mass black holes are believed to be seeds of supermassive black holes. As black holes feast, they gain mass.
The intermediate-mass black holes may form from several stars in a dense cluster collapsing, with the resulting black holes merging and creating a bigger black hole. They could also be black holes that have accumulated mass over time and indeed, 47 Tucanae is 12 billion years old, giving plenty of time to slurp up matter. There is also a scenario under which, shortly after the Big Bang, certain areas of the expanding universe were so dense they formed black holes shortly after the event.
Finding more mid-range black holes can be hard. Black holes, especially larger ones, typically clean their general area of debris. But if an unfortunate star happens to cross paths with one, the resulting event could be detected by astronomers, allowing them to see an intermediate-mass black hole in action.
Link:
Astronomers find a new class of black holes - Astronomy Magazine
Four centuries ago, Galileo began a revolution by pointing his telescope at the sky. Now a multidisciplinary team of astrophysicists and ecologists has reversed the perspective, pointing cameras towards the Earth to help the conservation of endangered species. In this case, the revolution consists of combining the use of unmanned aircraft (drones), equipped with infrared cameras, with detection techniques used to analyze astronomical images.
An important task in conservation research is to monitor the distribution and density of animal populations, which has usually been undertaken by surveys on the ground (either on foot or by car), from the air with manned aircraft or from space using satellites. In recent years, the use of drones equipped with cameras has allowed a reduction in the costs of these studies, as well as reaching areas with difficult access.
So far, most drones studies have used cameras in the visible range (the light detected by the human eye), which has two limitations. In the first place, these cameras are useful only during day time, so that they cannot be used to monitor the large number of species that are active at night or to identify poaching.
Secondly, in the visible all objects have very similar brightness, which makes it extremely difficult to make an automatic separation between the objects studied and everything that surrounds them. Infrared cameras, on the other hand, can be used both by day and by night. In addition, the difference between the body temperature of animals and the environment makes their emission in the thermal infrared range easy to separate.
However, the analysis tools in the infrared range are less developed than in the visible and, in fact, many studies use tedious manual techniques for the detection and identification of species. The study that is published this week in the International Journal of Remote Sensing, has used free software of astronomical source detection and applied it to the detection of humans and different species of animals in infrared images obtained with drones.
The study is led by researchers at the Liverpool John Moores University (LJMU), with the participation of Johan Knapen, a researcher at the Institute of Astrophysics of the Canary Islands (IAC).
Dr Steven Longmore, from the LJMU Astrophysics Research Institute and first author of the article, explains why this is possible: "Astrophysicists have been using thermal cameras for many decades. Crucially, it turns out the techniques we've developed to find and characterise the faintest objects in the Universe are exactly those needed to find and identify objects in thermal images taken with drones."
Each species has a different heat profile that acts as a 'thermal fingerprint'. "Our goal - Longmore says- is to build the definitive fingerprint libraries and automated pipeline that all future efforts in this field will rely upon."
Johan Knapen is excited about this new application: "Not only is this a fantastic collaboration between two different fields of science: astronomy and ecology, but it also introduces the use of drones into the set of technological tools that we use to obtain thermal imaging, including space- and ground-based telescopes."
The experience in the use of drones has been provided by Serge Wich, professor at the LJMU's School of Natural Sciences and Psychology and the founder of conservationdrones.org.
This pioneer in using drones for conservation work commented: "As an 'eye in the sky', conservation drones are helping the fight against illegal deforesting, poaching and habitat destruction, all leading to many species being endangered, including rhinos, orangutans, and elephants. Now, by applying the astrophysics analysis techniques used to find and identify objects in the far-distant Universe, we can try to do this more efficiently."
Biodiversity loss and consequent ecosystem collapse is one of the ten foremost dangers facing humanity. "Ultimately - says Wich- we hope this research will help tackle these problems by allowing anyone in the world to upload their aerial data and in real time get back geolocations of anything, whether survivors of natural disasters, or poachers approaching endangered species, or even the size, weight and health of livestock."
This new drone technology is part of the growing technological innovation within the LJMU. The Astrophysics Research Institute is also developing the world's largest fully robotic telescope, a scaled up version of the Liverpool Telescope, located on the Roque de Los Muchachos Observatory, in the Canary Island of La Palma.
Research paper
Read more:
An application of astronomy to save endangered species - Space Daily
KRQE News 13 | Fundraiser to help Albuquerque astronomy club replace stolen ... KRQE News 13 The show went on for the Albuquerque Astronomical Society, even without $30000 of its equipment. |
Continued here:
Fundraiser to help Albuquerque astronomy club replace stolen ... - KRQE News 13
FOX31 Denver | Astronomy trifecta this weekend FOX31 Denver DENVER Mark you calendar for both Friday February 10 and Saturday February 11. The trifecta includes the Full Snow Moon, a Lunar Eclipse, and Comet 45P all visible in Colorado. The big question is the weather. Will it be clear enough to see the trifecta? Lunar eclipse, Snow Moon and New Year Comet set to appear in the skies on the same day in February |
See the original post here:
Since their discovery by Jocelyn Bell and Antony Hewish in 1967, pulsars have intrigued astronomers as unique and exotic objects. A pulsar is a type of neutron star that emits focused beams of radiation from its poles as it spins. But now, astronomers have discovered a pulsar thats not a neutron star at all, but a white dwarf. Its the first white dwarf pulsar ever discovered, after more than 50 years of searching the skies for such an object.
The discovery, published in Nature Astronomy, was made by Professors Tom Marsh and Boris Gnsicke at the University of Warwicks Astrophysics Group, and Dr. David Buckley of the South African Astronomical Observatory. They found that the binary system AR Scorpii (AR Sco), which sits 380 light-years away in the constellation of Scorpius, contains a white dwarf acting as a pulsar.
Shortly after the first pulsars discovery, astronomers sought to understand the type of star responsible for such a signal. The pulsar that Bell and Hewish discovered had a period of slightly over one second (1.3373011 seconds); based on stellar models, this fell just within the rotational limits of a white dwarf. However, much faster pulsars were soon discovered, with periods of milliseconds. Only neutron stars were capable of rotating so quickly, so astronomers settled on these stellar remnants as the common mechanism behind pulsars.
The AR Sco system is comprised of a white dwarf and a red dwarf star, which orbit each other every 3.6 hours at a distance of about 1.4 million kilometers, or three times the distance between Earth and the Moon. As the white dwarf rotates around its axis every two minutes, it blasts its companion with a beam of radiation that excites electrons in the red dwarfs atmosphere, accelerating these particles to nearly the speed of light. This causes brightness changes that can be observed from Earth at exactly the period of the white dwarfs rotation.
AR Sco is like a gigantic dynamo: a magnet, size of the Earth, with a field that is ~10,000 stronger than any field we can produce in a laboratory, and it is rotating every two minutes. This generates an enormous electric current in the companion star, which then produces the variations in the light we detect, ProfessorBoris Gnsicke said in the press release announcing their discovery.
Whats in a name?
Neutron stars, which comprise the entire set of pulsars that had been discovered to date, are the dense remnants of a massive stars core left over after it ends its life in a supernova explosion. It takes a star several times the mass of the Sun to leave such a remnant behind. White dwarfs have a much less violent past they are the smaller, slightly less dense cores of stars like the Sun, left over once the stars outer layers have been blown away like a bubble as a planetary nebula. Most white dwarfs are roughly the size of Earth, while most neutron stars are only the size of, say, New York City. Red dwarf stars, thought to be the most common and longest-lived stars, are cool, low-mass Main Sequence stars (in the hydrogen-burning stages of their life) that only contain about 7.5 to 50 percent of the mass of our Sun.
The white dwarf pulsar in AR Sco might be Earth-sized, but its 200,000 times as massive as our planet. It also has an electromagnetic field 100 million times the strength of Earths, which is responsible for the beams of radiation it emits as a pulsar. The new data show that AR Sco's light is highly polarized, showing that the magnetic field controls the emission of the entire system, and a dead ringer for similar behavior seen from the more traditional neutron star pulsars, said Professor Tom Marsh.
This discovery, and others like it, point to the possibility that white dwarfs arent simply the inert remnants of Sun-like stars destined to simply fade away, but continue to play an active role long after the stars hydrogen-burning phase is complete.
Read more here:
Astronomers discover a white dwarf that acts like a pulsar - Astronomy Magazine
A supermassive black hole has been tearing apart and eating a star for so long it set a new record.
According to researchers, this tidal disruption event was 10 times longer than any other stars death, which either means the black hole was destroying an incredibly large star or it thoroughly torn apart a smaller star.
The team of researchers began observing the TDE that destroyed the too close star in July 2005, using NASAs Chandra X-ray Observatory and Swift satellite and ESAs WMM-Newton.
We have witnessed a stars spectacular and prolonged demise, Dacheng Lin from the University of New Hampshire in Durham, New Hampshire, who led the study, said in a press release. Dozens of tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one.
This black hole, known as XJ1500+0154, is at the center of a host galaxy about 1.8 billion light-years from Earth. It reached peak brightness in June 2008, and has been on researchers radars ever since.
For most of the time weve been looking at this object, it has been growing rapidly, James Guillochon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and co-author of the study said. This tells us something unusual like a star twice as heavy as our Sun is being fed into the black hole.
Finding this drawn out death of a star by black hole shows not only that supermassive black holes can grow, but it also gives researchers more information about advanced black holes and how they came to be.
According to the researchers, the star that the black hole is feeding on will diminish in the next several years, and will therefore cause the brightness of XJ1500+0154 to fade as well.
Continued here:
A supermassive black hole spent more than a decade consuming a star - Astronomy Magazine
A giant fireball lit up the skies over portions of the midwest last night, with more than 200 witnesses reporting seeing the real fireworks of last night.
The National Weather Service detected the meteor around 1:29 a.m. It flew over Lake Michigan between Sheboygan and Manitowoc, Wisconsin. The meteor was spotted as far south as Kentucky and as far east as New York.
Due to a visible explosion, its likely a bolide, a larger meteor that enters with enough force to explode as it enters the Earths atmosphere. Most bolides are larger than one meter. A 20-meter bolide caused an explosion in Russia in 2013 that injured thousands and damaged more than 7,000 buildings. That event, the Chelyabinsk meteor, wasnt even the biggest in Russias history. The 1908 Tunguska event levelled trees in Siberia when it exploded with the force of 10-30 megatons of TNT.
NASA-JPL keeps a running list of bolide reports here, though this latest entry has not, as of yet, been added.
Source: Washington Post
Originally posted here:
A fireball blazed over the midwest last night - Astronomy Magazine
This artists illustration depicts what astronomers call a tidal disruption event, or TDE. This is when an object, such as a star, wanders too close to a black hole and is destroyed by tidal forces generated from the black holes intense gravitational forces. During a TDE, some of the stellar debris is flung outward at high speeds, while the rest (shown as the red material in the illustration) becomes hotter as it falls toward the black hole, generating a distinct X-ray flare. A wind blowing away from this infalling material is shown in blue. X-ray: NASA/CXC/UNH/D.Lin et al, Optical: CFHT, Illustration: NASA/CXC/M.Weiss
A giant black hole ripped apart a star and then gorged on its remains for about a decade, according to astronomers. This is more than 10 times longer than any observed episode of a stars death by black hole.
Researchers made this discovery using data from NASAs Chandra X-ray Observatory and Swift satellite as well as ESAs XMM-Newton.
The trio of orbiting X-ray telescopes found evidence for a tidal disruption event (TDE), wherein the tidal forces due to the intense gravity from a black hole can destroy an object such as a star that wanders too close. During a TDE, some of the stellar debris is flung outward at high speeds, while the rest falls toward the black hole. As it travels inwards to be ingested by the black hole, the material becomes heats up to millions of degrees and generates a distinct X-ray flare.
We have witnessed a stars spectacular and prolonged demise, said Dacheng Lin from the University of New Hampshire in Durham, New Hampshire, who led the study. Dozens of tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one.
The extraordinary long bright phase of this event spanning over 10 years means that among observed TDEs this was either the most massive star ever to be completely torn apart during one of these events, or the first where a smaller star was completely torn apart.
The X-ray source containing this force-fed black hole, known by its abbreviated name of XJ1500+0154, is located in a small galaxy about 1.8 billion light-years from Earth.
The source was not detected in a Chandra observation on April 2, 2005, but was detected in an XMM-Newton observation on July 23, 2005, and reached peak brightness in a Chandra observation on June 5, 2008. These observations show that the source became at least 100 times brighter in X-rays. Since then, Chandra, Swift, and XMM-Newton have observed it multiple times.
The sharp X-ray vision of Chandra data shows that XJ1500+0154 is located at the center of its host galaxy, the expected location for a supermassive black hole.
The X-ray data also indicate that radiation from material surrounding this black hole has consistently surpassed the so-called Eddington limit, defined by a balance between the outward pressure of radiation from the hot gas and the inward pull of the gravity of the black hole.
For most of the time weve been looking at this object, it has been growing rapidly, said co-author James Guillochon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. This tells us something unusual like a star twice as heavy as our Sun is being fed into the black hole.
The conclusion that supermassive black holes can grow, from TDEs and perhaps other means, at rates above those corresponding to the Eddington limit has important implications. Such rapid growth may help explain how supermassive black holes were able to reach masses about a billion times higher than the Sun when the universe was only about a billion years old.
This event shows that black holes really can grow at extraordinarily high rates, said co-author Stefanie Komossa of QianNan Normal University for Nationalities in Duyun City, China. This may help understand how precocious black holes came to be.
Based on the modeling by the researchers the black holes feeding supply should be significantly reduced in the next decade. This would result in XJ1500+0154 fading in X-ray brightness over the next several years.
See more here:
Black hole meal sets record for duration and size - Astronomy Now Online
Willy Stevens: Aboriginal astronomy guide.
Supplied
Willy Stevens grew up knowing little about his ancestral Australian Aboriginal culture. Born a Muruwari man in New South Wales, he was adopted into a Kamilaroi family.
But when at the age of 17 he met his biological mother, she regaled him with traditional stories, many featuring the stars.
Today, the 28-year-olds passions lie in sharing those stories of Aboriginal astronomy, passed down through millennia, with students and the wider community.
While working as a tour guide in the Sydney suburb of The Rocks, he was recruited into a new Dreamtime Astronomy program at the nearby Sydney Observatory.
As the observatorys first Indigenous Australian presenter, he quickly became the face of the program, appearing on numerous radio and TV shows, including a French documentary called Between Heaven and Earth.
Given this, it might seem curious that Stevens doesnt actually hold much interest in astronomy as an academic discipline.
For me, its about teaching culture, he explains. I love to share my knowledge with my community.
But as a tour guide and university guest lecturer, he continues to develop new ways of sharing culture, especially traditions relating to the sun, moon, and stars.
His passion is palpable as he explains to undergraduate students the importance of the sky-world in Aboriginal cultures, and how this informs kinship, ceremony, and survival practices.
He notes, for instance, how the star cluster called the Pleiades in Western discourse is known to the Muruwari as Gambu Gambu a group of girls.
Some of the girls were shy and tried to hide, making them difficult to see. The ability to identify the fainter stars was used to test a persons eyesight. Their appearance in the early morning sky signalled the start of winter and the arrival of morning frost.
An optical image of the Pleiades star cluster, around 440 light-years from Earth otherwise known as Gambu Gambu.
RUSSELL CROMAN / SCIENCE PHOTO LIBRARY / Getty Images
For Stevens, its the effect his work has on Aboriginal people that most inspires him. Because of the devastating effects of ongoing colonisation, many Indigenous Australians have limited knowledge of their heritage or culture. When they attend his talks, he sees it an opportunity to share knowledge.
I feel very humbled when Aboriginal people thank me after coming on my tour and discovering things about their culture they had never known, he says. Learning these stories gave them a sense of pride and made them want to learn more.
His passion also inspires indigenous people from other cultures: I was once given a gift by a Native American lady and her granddaughter who came to one of my tours. Its a seashell necklace worn by men.
But the most important thing for him is teaching his community about history, morals and traditions. When visiting family in Narrabri and Lightning Ridge, he instructs his younger brothers about making age-old tools and learning from elders.
Stevens recently developed a potential higher education course on Aboriginal astronomy, geared toward Indigenous Australian students. He hopes participants will develop skills to work as guides and rangers across Australia, sharing more ancient stories of the stars.
Read the original here:
The passions of Aboriginal astronomy guide, Willy Stevens - Cosmos
6 February 2017 Stephen Clark
The Calabash Nebula, pictured here which has the technical name OH 231.8+04.2 is a spectacular example of the death of a low-mass star like the Sun.
This image taken by the NASA/ESA Hubble Space Telescope shows the star going through a rapid transformation from a red giant to a planetary nebula, during which it blows its outer layers of gas and dust out into the surrounding space. The recently ejected material is spat out in opposite directions with immense speed the gas shown in yellow is moving close to a million kilometres an hour.
Astronomers rarely capture a star in this phase of its evolution because it occurs within the blink of an eye in astronomical terms. Over the next thousand years the nebula is expected to evolve into a fully fledged planetary nebula.
The nebula is also known as the Rotten Egg Nebula because it contains a lot of sulphur, an element that, when combined with other elements, smells like a rotten egg but luckily, it resides over 5000 light-years away in the constellation of Puppis (The Poop deck).
Read this article:
There are two types of plates. The majority are photographic: clear glass sheets scattered with dark specks of stars. Today, these plates draw the most interest from scientists. But about one in five is a spectrographic plate, with each star depicted as a grey smear (representing the rainbow of visible light) perhaps a quarter inch long. In the early days of the collection, these spectra were the cutting edge of astronomy.
The first major developments to come out of the Harvard plate collection were systems that classified stars based on the tiny white lines slicing the spectrums grey rainbow. Its almost incomprehensible that this was all done visually by examining the plates under magnifying glasses, said Josh Grindlay, an astronomer at Harvard. Thank heavens they did because it was not something that just immediately popped out, what this spectral classification system was.
The first system was designed by Williamina Fleming, whom Pickering originally hired as a housemaid only to discover her astronomical potential. Flemings system, published in 1890, sorted more than 10,000 spectra into an alphabetical sequence of 15 letters. A second, independent systemcomplicated and criticized for its complexitywas created by Antonia Maury and used 22 letters, with further subclasses denoting how wide or narrow certain spectral lines were. (Years later, astronomers realized some of these characteristics identify binary stars and supergiants.).
These two systems were expanded and reconciled by Annie Jump Cannon, whom Grindlay calls a wonder woman. Over her career, Cannon classified 340,000 spectra on the Harvard plates, sometimes managing a hundred in one day. She was an extraordinary woman who did one thing extremely well all her life, said Hearnshaw. In her free time, she also spotted 300 variable stars and five novae Cannon built on Flemings and Maurys work to create the O, B, A, F, G, K, M sequence that still underlies stellar classification.
Nevertheless, other astronomers likely could have tackled the classification problem. This would have been done, but not in the concerted way that it was done here, Grindlay said. Hearnshaw agreed, pointing particularly to observatories in California and Germany. Harvard was a pioneer, but they had other people chasing on their tails, and of course thats good for science.
The spectrographic plates had another secret hidden among their streaks: the recipe for stars, deciphered by Cecilia Payne-Gaposchkin. Scientists elsewhere had tied specific elements and their charged varieties to the wavelengths of light they absorb, which match the white lines chopping through a spectrum; Payne-Gaposchkin applied this work to translate the Harvard spectra into elemental ingredients.
What she found was that the stars all seemed enormously uniform in composition, Gingerich said. The spectra looked very different, but that was because of the temperature difference of the stars. Hot hydrogen and very hot hydrogen have very different spectra, but both are hydrogen. That was a very important finding, he added. Payne-Gaposchkins work also proved that the vast majority of the universe is hydrogen and helium.
Even while the spectra were still unveiling their secrets, the photographic plates were beginning to shine, thanks to what is widely recognized as the single most important discovery to come out of the Harvard plates.
Astronomers wanted to sort out how bright different stars were but theres usually no way to determine whether a star looks bright because its nearby or because its genuinely a bright star. Harvard astronomer Henrietta Leavitt looked at the Large and Small Magellanic Clouds and saw clusters of stars.
She could see there was this big clump of what looked like a gazillion stars, said Grindlay. It was clearly one thing. Stars clumped together must all be about the same distance from Earthwhich in turn meant the ones that appeared brighter really were brighter. By studying a specific type of variable star, called Cepheids, in the Magellanic Clouds, Leavitt realized that brighter Cepheids took longer to dim and brighten, establishing a relationship between intrinsic luminosity and period.
That relationship meant astronomers could reverse the process: Measuring how quickly a Cepheid brightens and dims would reveal how many watts were in the lightbulb, said Grindlay, and from there, out pops the distance. That conversion required other techniques to pin down how far away specific Cepheids were, Gingerich added. But Edwin Hubble was still able to use Leavitts work to prove the Milky Way and Andromeda were two separate galaxies.
The collections full-sky scope was crucial for Leavitt, since the clouds are only visible from the often-ignored southern hemisphere. Other galaxies could have yielded the same realization, but they were too far away for the small telescopes of the time to truly see into them. It would have been very difficult and I would say essentially impossible without having the Magellanic Clouds, Grindlay said.
Follow this link:
How Harvard's vast collection of glass plates still shapes astronomy - Astronomy Magazine
Among lifes many mysteries, the answer to the question "how much wood would a woodchuck chuck" has to rank pretty low. Higher on my list: Why are woodchucks also called groundhogs? After all, wood and ground are hardly synonymous, and a chuck has nothing to do with a hog.
But the biggest question about woodchucks and groundhogs has to be why these furry rodents became associated with weather forecasting. Today is Groundhog Day, and tradition holds that if the groundhog sticks his head out of his burrow and sees his shadow, well have 6 more weeks of winter. But if the weather is cloudy, it means spring is right around the corner. The tradition was brought to America long ago by German immigrants, although the hedgehog seemed to be the animal of choice in the old country.
What does any of this have to do with astronomy? It turns out the origin of Groundhog Day is connected to the Suns movement across the sky. Groundhog Day is one of the four so-called cross-quarter days, which mark the midpoints between the solstices and equinoxes. Groundhog Day comes approximately midway between the winter solstice and the vernal equinox. Other cross-quarter days fall near May Day and Halloween.
If you pull out a calendar and count the number of days between solstices and equinoxes, youll find the cross-quarter days dont fall precisely on the days we celebrate them, but rather a few days later. And the origins of this discrepancy also come from astronomy. The holidays were fixed in our calendars relatively recently, long after the traditions sprang up. And the precession of the equinoxes caused by the Suns and Moons gravitational pull on Earths equatorial bulge has pushed the holidays earlier than the cross-quarter days. Its something to think about when your local newscasters breathlessly report groundhog behavior this Thursday.
This story is adapted from a 2007 blog post by the author. Wisconsin has had a relatively mild winter, unlike the post back then.
More here:
How Groundhogs Day and astronomy intersect - Astronomy Magazine
In 2015, NASAs Dawn spacecraft discovered a lone 4-km (2.5-mile) high mountain on the dwarf planet Ceres. Identified as a cryovolcano, which erupts ice and other volatiles instead of lava like a traditional volcano, Ahuna Mons was magnificent but alone on Ceres surface. Now, however, scientists say that Ceres may have been home to many more such cryovolcanoes in the past, which could have slowly disappeared and left Ahuna Mons as the only remaining feature of recent geologic activity.
Michael Sori of the Lunar and Planetary Laboratory at the University of Arizona in Tucson is the lead author of a new paper accepted for publication in the American Geophysical Unions journal, Geophysical Research Letters. In recent a press release, Sori explains why Ahuna Mons is such a mystery, saying, Imagine if there was just one volcano on all of Earth. That would be puzzling. But Soris team has now proposed a possible solution to this exact puzzle, a process called viscous relaxation. If this process is truly at work on Ceres, then, We think we have a very good case that there have been lots of cryovolcanoes on Ceres but they have deformed, Sori says.
Viscous relaxation refers to the flow of solids over time. On Earth, the best example of this process is the flow of glaciers, which are solid ice but move and flow slowly, given enough time. While viscous relaxation doesnt apply to Earths volcanoes, which are made of rock, it could apply to cryovolcanoes, which are made of ice, just like glaciers. Soris team speculates that older cryovolcanoes have undergone such viscous relaxation over the past few millions or billions of years, possibly aided by Ceres close orbit to the sun. This would leave only the younger Ahuna Mons clearly visible on Ceres surface. Based on computer models of this process, and assuming that Ahuna Mons is composed of at least 40 percent water, Soris team estimates that this feature would flatten out at a rate of 10-50 meters (30-160 feet) every million years. But because Ahuna Mons is only 200 million years old at most, it just hasnt had time to deform, Sori explains.
More here:
The case of Ceres' disappearing volcanoes - Astronomy Magazine
It is an exciting time to be studying astronomy. The astronomy news regularly reports on the existence and characteristics of exoplanets, the search for water on Mars, gravitational lensing of distant young galaxies, wildly energetic explosions of dying stars, attempts to detect particles of dark matter, space missions to asteroids or Pluto or the edge of the solar system, and on and on and on, from the tiniest particles and earliest moments of time to the large-scale structure of the universe as a whole.
Whitman has an Astronomy major and standard combined majors in Physics-Astronomy and Astronomy-Geology. There is also an Astronomy Minor. Students interested in graduate work in astronomy are encouraged to major in Physics-Astronomy or in Physics with an Astronomy minor, since most graduate schools look for some equivalent of an undergraduate degree in physics. Some students have also designed individually planned majors such as Astronomy-Mathematicsor Astro(geo)biology. Astronomy graduates in recent years have gone on to graduate study at places such as New Mexico State University, the University of Wyoming, Colorado State, Dartmouth, and the University of Wisconsin - Madison. For more details about major and minor requirements, look at thecollege catalog.
Students can choose between two types of introductory courses, depending on their level of comfort with math and physical sciences. All of the introductory courses have a lab component; on clear nights, lab work includes using the department's 8- and 10-inch telescopes on the roof of the science building. Those students who have taken the more mathematically rigorous introductory courses may then take upper-division courses inStellar Astrophysics, Galactic Astronomy, Cosmology,andObservational Astronomy,all of which are offered regularly in alternate years; Planetary Astrophysics or special topics (e.g., on globular clusters) areoffered somewhat less often. Students with an interest in history and philosophy may be interested in taking Finding Our Place in the Universe, a course in the history of cosmology. Several students each year also conduct research projects with faculty or undertake directed readings on topics of personal interest. The "Links and resources" tab to the left includes links to information about off-campus summer research opportunities and graduate study; for more information about department equipment, check the "Facilities" tab.
Here is the original post:
Were fitting in one more episode this week, before Fraser heads off to Costa Rica for a week. Our next episode will record on Dec. 23, 2016. We usually record Astronomy Cast every Friday at 1:30 pm Pacific / 4:30 pm Eastern / 8:30 PM UTC (20:30 GMT). You can watch us live on AstronomyCast, []
Join us as we try to finish the interrupted episode Coming Home from Mars! Landing on the surface of Mars is very difficult. In fact, its probably the toughest planet to land on in the whole Solar System. Today well talk about what its going to take to get to and return from Mars! We []
Podcast: Play in new window | Download (0.0KB)
Subscribe: iTunes | Android |
Hey folks, just letting you know that due to Fraser being at conference this week, we wont be recording an Astronomy Cast any time. Unfortunately, our last show was interrupted by a power outage, and well try to get back together to finish that one on Monday, 12/5. We will keep you posted as info []
Note This episode was interrupted due to a power outage on Frasers end. Theyll reschedule to finish episode on 12/5 (most likely Fraser is traveling this week!), and well put the pieces together when we have them and release as audio podcast! Landing on the surface of Mars is very difficult. In fact, []
When Elon Musk announced plans to send humans to Mars, he conveniently left out one important aspect. How are we supposed to survive on a place this hostile to life? Seriously, Mars sucks, and its going to take some impressive techniques and technologies to make it on the Red Planet. We usually record Astronomy Cast []
Podcast: Play in new window | Download (26.7MB)
Subscribe: iTunes | Android |
We begin a miniseries on Mars. How many episodes will we do? Who knows? But we start today with a discussion of the two Mars moons, Phobos and Deimos. We usually record Astronomy Cast every Friday at 1:30 pm Pacific / 4:30 pm Eastern / 8:30 PM UTC (20:30 GMT). You can watch us live []
Podcast: Play in new window | Download (28.1MB)
Subscribe: iTunes | Android |
Did you hear that Dark Energy doesnt exist any more? Neither does Dark Matter? It turns out that NASA recalculated the Zodiac and now youre an Ophiuchan! Science is hard enough, but communicating that science out to the public when there are publications hungry for traffic is even harder! Heres how to parse the click-bait []
Podcast: Play in new window | Download (0.0KB)
Subscribe: iTunes | Android |
I hate to tell you this, but that meat computer in your skull is constantly betraying you. Dont worry, weve all got the same, but fortunately, scientists have learned how this happens, and can help us make sure our science, and lives dont suffer because of it. (more)
Podcast: Play in new window | Download (0.0KB)
Subscribe: iTunes | Android |
Have you ever noticed spacecraft missions have some pretty cool names? How does anyone decide what to call these things? (more)
Podcast: Play in new window | Download (0.0KB)
Subscribe: iTunes | Android |
It turns out that nature figured out how to use electricity long before humans did. Lightning storms are common across the Earth, and even the Solar System. What causes this electricity in the sky, and how can science use it? We usually record Astronomy Cast every Friday at 1:30 pm Pacific / 4:30 pm Eastern []
Podcast: Play in new window | Download (0.0KB)
Subscribe: iTunes | Android |
Continue reading here:
Astronomy, science that encompasses the study of all extraterrestrial objects and phenomena. Until the invention of the telescope and the discovery of the laws of motion and gravity in the 17th century, astronomy was primarily concerned with noting and predicting the positions of the Sun, Moon, and planets, originally for calendrical and astrological purposes and later for navigational uses and scientific interest. The catalog of objects now studied is much broader and includes, in order of increasing distance, the solar system, the stars that make up the Milky Way Galaxy, and other, more distant galaxies. With the advent of scientific space probes, Earth also has come to be studied as one of the planets, though its more detailed investigation remains the domain of the geologic sciences.
Since the late 19th century astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects and the physical processes that control their formation, evolution, and emission of radiation. In addition, the gases and dust particles around and between the stars have become the subjects of much research. Study of the nuclear reactions that provide the energy radiated by stars has shown how the diversity of atoms found in nature can be derived from a universe that, following the first few minutes of its existence, consisted only of hydrogen, helium, and a trace of lithium. Concerned with phenomena on the largest scale is cosmology, the study of the evolution of the universe. Astrophysics has transformed cosmology from a purely speculative activity to a modern science capable of predictions that can be tested.
Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science. Almost all measurements must be performed at great distances from the objects of interest, with no control over such quantities as their temperature, pressure, or chemical composition. There are a few exceptions to this limitationnamely, meteorites, rock and soil samples brought back from the Moon, samples of comet dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere. These can be examined with laboratory techniques to provide information that cannot be obtained in any other way. In the future, space missions may return surface materials from Mars, asteroids, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.
A central undertaking in astronomy is the determination of distances. Without a knowledge of its distance, the size of an observed object in space would remain nothing more than an angular diameter, and the brightness of a star could not be converted into its true radiated power, or luminosity. Astronomical distance measurement began with a knowledge of Earths diameter, which provided a base for triangulation. Within the inner solar system, some distances can now be better determined through the timing of radar reflections or, in the case of the Moon, through laser ranging. For the outer planets, triangulation is still used. Beyond the solar system, distances to the closest stars are determined through triangulation, with the diameter of Earths orbit serving as the baseline and shifts in stellar parallax being the measured quantities. Stellar distances are commonly expressed by astronomers in parsecs (pc), kiloparsecs, or megaparsecs. (1pc=3.0861018 cm, or about 3.26 light-years [1.92 1013 miles].) Distances can be measured out to around a kiloparsec by trigonometric parallax (see star: Determining stellar distances). The accuracy of measurements made from Earths surface is limited by atmospheric effects, but measurements made from the Hipparcos satellite in the 1990s have extended the scale to stars as far as 650 parsecs, with an accuracy of about a thousandth of an arc second. Less-direct measurements must be used for more-distant stars and for galaxies.
Two general methods for determining galactic distances are described here. In the first, a clearly identifiable type of star is used as a reference standard because its luminosity has been well determined. This requires observation of such stars that are close enough to Earth that their distances and luminosities have been reliably measured. Such a star is termed a standard candle. Examples are Cepheid variables, whose brightness varies periodically in well-documented ways, and certain types of supernova explosions that have enormous brilliance and can thus be seen out to very great distances. Once the luminosities of such nearer standard candles have been calibrated, the distance to a farther standard candle can be calculated from its calibrated luminosity and its actual measured intensity. (The measured intensity [I] is related to the luminosity [L] and distance [d] by the formula I=L/4d2). A standard candle can be identified by means of its spectrum or the pattern of regular variations in brightness. (Corrections may have to be made for the absorption of starlight by interstellar gas and dust over great distances.) This method forms the basis of measurements of distances to the closest galaxies.
Test Your Knowledge
Astronomy and Space Quiz
The second method for galactic distance measurements makes use of the observation that the distances to galaxies generally correlate with the speeds with which those galaxies are receding from Earth (as determined from the Doppler shift in the wavelengths of their emitted light). This correlation is expressed in the Hubble law: velocity=Hdistance, in which H denotes Hubbles constant, which must be determined from observations of the rate at which the galaxies are receding. There is widespread agreement that H lies between 70 and 76 kilometres per second per megaparsec (km/sec/Mpc), with leading research groups offering estimates that have an average value of about 71 km/sec/Mpc. H has been used to determine distances to remote galaxies in which standard candles have not been found. (For additional discussion of the recession of galaxies, the Hubble law, and galactic distance determination, see physical science: Astronomy.)
The solar system took shape 4.57 billion years ago, when it condensed within a large cloud of gas and dust. Gravitational attraction holds the planets in their elliptical orbits around the Sun. In addition to Earth, five major planets (Mercury, Venus, Mars, Jupiter, and Saturn) have been known from ancient times. Since then only two more have been discovered: Uranus by accident in 1781 and Neptune in 1846 after a deliberate search following a theoretical prediction based on observed irregularities in the orbit of Uranus. Pluto, discovered in 1930 after a search for a planet predicted to lie beyond Neptune, was considered a major planet until 2006, when it was redesignated a dwarf planet by the International Astronomical Union.
The average Earth-Sun distance, which originally defined the astronomical unit (AU), provides a convenient measure for distances within the solar system. The astronomical unit is now defined dynamically (using Keplers third law; see Keplers laws of planetary motion) and has the value 1.495978706911013 cm (about 93 million miles), with an uncertainty of about 2,000 cm. The mean radius of Earths orbit is 1+(3.1108) AU. Mercury, at 0.4 AU, is the closest planet to the Sun, while Neptune, at 30.1 AU, is the farthest. Plutos orbit, with a mean radius of 39.5, is sufficiently eccentric that at times it is closer to the Sun than is Neptune. The planes of the planetary orbits are all within a few degrees of the ecliptic, the plane that contains Earths orbit around the Sun. As viewed from far above Earths North Pole, all planets move in the same (counterclockwise) direction in their orbits.
All of the planets apart from the two closest to the Sun (Mercury and Venus) have natural satellites (moons) that are very diverse in appearance, size, and structure, as revealed in close-up observations from long-range space probes. Pluto has at least three moons, including one fully half the size of Pluto itself. Four planetsJupiter, Saturn, Uranus, and Neptunehave rings, disklike systems of small rocks and particles that orbit their parent planets.
Britannica Lists & Quizzes
Philosophy & Religion Quiz
Animals List
Science Quiz
Sports & Recreation List
Most of the mass of the solar system is concentrated in the Sun, with its 1.991033 grams. Together, all of the planets amount to 2.71030 grams (i.e., about one-thousandth of the Suns mass), with Jupiter alone accounting for 71 percent of this amount. The solar system also contains a few known objects of intermediate size classified as dwarf planets and a very large number of much smaller objects collectively called small bodies. The small bodies, roughly in order of decreasing size, are the asteroids, or minor planets; comets, including Kuiper belt and Oort cloud objects; meteoroids (see meteor and meteoroid); and interplanetary dust particles. Because of their starlike appearance when discovered, the largest of these bodies were termed asteroids, and that name is widely used, but, now that the rocky nature of these bodies is understood, their more descriptive name is minor planets.
The four inner, terrestrial planetsMercury, Venus, Earth, and Marsalong with the Moon have average densities in the range of 3.95.5 grams per cubic cm, setting them apart from the four outer, giant planetsJupiter, Saturn, Uranus, and Neptunewhose densities are all close to 1 gram per cubic cm, the density of water. The compositions of these two groups of planets must therefore be significantly different. This dissimilarity is thought to be attributable to conditions that prevailed during the early development of the solar system (see below Theories of origin). Planetary temperatures now range from around 170 C (330 F, 440 K) on Mercurys surface through the typical 15 C (60 F, 290 K) on Earth to 135 C (210 F, 140 K) on Jupiter near its cloud tops and down to 210 C (350 F, 60 K) near Neptunes cloud tops. These are average temperatures; large variations exist between dayside and nightside for planets closest to the Sun, except for Venus with its thick atmosphere.
The surfaces of the terrestrial planets and many satellites show extensive cratering, produced by high-speed impacts (see meteorite crater). On Earth, with its large quantities of water and an active atmosphere, many of these cosmic footprints have eroded, but remnants of very large craters can be seen in aerial and spacecraft photographs of the terrestrial surface. On Mercury, Mars, and the Moon, the absence of water and any significant atmosphere has left the craters unchanged for billions of years, apart from disturbances produced by infrequent later impacts. Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets. Seismic activity on the Moon has been monitored by means of seismometers left on its surface by Apollo astronauts and by Lunokhod robotic rovers. Cratering on the largest scale seems to have ceased about three billion years ago, although on the Moon there is clear evidence for a continued cosmic drizzle of small particles, with the larger objects churning (gardening) the lunar surface and the smallest producing microscopic impact pits in crystals in the lunar rocks.
During the U.S. Apollo missions a total weight of 381.7 kg (841.5 pounds) of lunar material was collected; an additional 300 grams (0.66 pounds) was brought back by unmanned Soviet Luna vehicles. About 15 percent of the Apollo samples have been distributed for analysis, with the remainder stored at the NASA Johnson Space Center, Houston, Texas. The opportunity to employ a wide range of laboratory techniques on these lunar samples has revolutionized planetary science. The results of the analyses have enabled investigators to determine the composition and age of the lunar surface. Seismic observations have made it possible to probe the lunar interior. In addition, retroreflectors left on the Moons surface by Apollo astronauts have allowed high-power laser beams to be sent from Earth to the Moon and back, permitting scientists to monitor the Earth-Moon distance to an accuracy of a few centimetres. This experiment, which has provided data used in calculations of the dynamics of the Earth-Moon system, has shown that the separation of the two bodies is increasing by 4.4 cm (1.7 inches) each year. (For additional information on lunar studies, see Moon.)
Mercury is too hot to retain an atmosphere, but Venuss brilliant white appearance is the result of its being completely enveloped in thick clouds of carbon dioxide, impenetrable at visible wavelengths. Below the upper clouds, Venus has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planets surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long-wavelength infrared radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 C (870 F, 740 K). Radar, which can penetrate the thick Venusian clouds, has been used to map the planets surface. In contrast, the atmosphere of Mars is very thin and is composed mostly of carbon dioxide (95 percent), with very little water vapour; the planets surface pressure is only about 0.006 that of Earth. The outer planets have atmospheres composed largely of light gases, mainly hydrogen and helium.
Each planet rotates on its axis, and nearly all of them rotate in the same directioncounterclockwise as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotation axis very nearly in the plane of the ecliptic.
Some of the planets have magnetic fields. Earths field extends outward until it is disturbed by the solar windan outward flow of protons and electrons from the Sunwhich carries a magnetic field along with it. Through processes not yet fully understood, particles from the solar wind and galactic cosmic rays (high-speed particles from outside the solar system) populate two doughnut-shaped regions called the Van Allen radiation belts. The inner belt extends from about 1,000 to 5,000 km (600 to 3,000 miles) above Earths surface, and the outer from roughly 15,000 to 25,000 km (9,300 to 15,500 miles). In these belts, trapped particles spiral along paths that take them around Earth while bouncing back and forth between the Northern and Southern hemispheres, with their orbits controlled by Earths magnetic field. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into Earths atmosphere, where they collide with atoms and molecules to produce auroras.
Jupiter has a magnetic field far stronger than Earths and many more trapped electrons, whose synchrotron radiation (electromagnetic radiation emitted by high-speed charged particles that are forced to move in curved paths, as under the influence of a magnetic field) is detectable from Earth. Bursts of increased radio emission are correlated with the position of Io, the innermost of the four Galilean moons of Jupiter. Saturn has a magnetic field that is much weaker than Jupiters, but it too has a region of trapped particles. Mercury has a weak magnetic field that is only about 1 percent as strong as Earths and shows no evidence of trapped particles. Uranus and Neptune have fields that are less than one-tenth the strength of Saturns and appear much more complex than that of Earth. No field has been detected around Venus or Mars.
More than 200,000 asteroids with well-established orbits are known, and several hundred additional objects are discovered each year. Hundreds of thousands more have been seen, but their orbits have not been as well-determined. It is estimated that several million asteroids exist, but most are small, and their combined mass is estimated to be less than a thousandth that of Earth. Most of the asteroids have orbits close to the ecliptic and move in the asteroid belt, between 2.3 and 3.3 AU from the Sun. Because some asteroids travel in orbits that can bring them close to Earth, there is a possibility of a collision that could have devastating results (see Earth impact hazard).
Comets are considered to come from a vast reservoir, the Oort cloud, orbiting the Sun at distances of 20,00050,000 AU or more and containing trillions of icy objectslatent comet nucleiwith the potential to become active comets. Many comets have been observed over the centuries. Most make only a single pass through the inner solar system, but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times. Halleys Comet is the best-known of these periodic comets, with its next return into the inner solar system predicted for 2061 ce. Many short-period comets are thought to come from the Kuiper belt, a region lying mainly between 30 AU and 50 AU from the Sunbeyond Neptunes orbit but including part of Plutosand housing perhaps hundreds of millions of comet nuclei. Comet masses have not been well determined, but most are probably less than 1018 grams, one billionth the mass of Earth.
Since the 1990s more than a thousand comet nuclei in the Kuiper belt have been observed with large telescopes; a few are about half the size of Pluto, and at least one, Eris, is estimated to be slightly larger. Plutos orbital and physical characteristics had long caused it to be regarded as an anomaly among the planets, and, after the discovery of numerous other Pluto-like objects beyond Neptune, Pluto was seen to be no longer unique in its neighbourhood but rather a giant member of the local population. Consequently, in 2006 astronomers at the general assembly of the International Astronomical Union elected to create the new category of dwarf planets for objects with such qualifications. Pluto, Eris, and Ceres, the latter being the largest member of the asteroid belt, were given this distinction. Two other Kuiper belt objects, Makemake and Haumea, were also designated as dwarf planets.
Smaller than the observed asteroids and comets are the meteoroids (see meteor and meteoroid), lumps of stony or metallic material believed to be mostly fragments of asteroids and comets. Meteoroids vary from small rocks to boulders weighing a ton or more. A relative few have orbits that bring them into Earths atmosphere and down to the surface as meteorites. Most if not all meteorites that have been collected on Earth are probably from asteroids.
Meteorites are classified into three broad groups: stony (chondrites and achondrites; about 94 percent), iron (5 percent), and stony-iron (1 percent). Most meteoroids that enter the atmosphere heat up sufficiently to glow and appear as meteors (see meteor and meteoroid), and the great majority of these vaporize completely or break up before they reach the surface. Many, perhaps most, meteors occur in showers (see meteor shower) and follow orbits that seem to be identical with those of certain comets, thus pointing to a cometary origin. For example, each May, when Earth crosses the orbit of Halleys Comet, the Eta Aquarid meteor shower occurs. Micrometeorites (interplanetary dust particles), the smallest meteoroidal particles, can be detected from Earth-orbiting satellites or collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection. Since the late 1960s numerous meteorites have been found in the Antarctic on the surface of stranded ice flows (see Antarctic meteorites). Detailed analyses have shown that some of these meteorites have come from the Moon and others from Mars. Yet others contain microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars.
The age of the solar system, taken to be close to 4.6 billion years, has been derived from measurements of radioactivity in meteorites, lunar samples, and Earths crust. Abundances of isotopes of uranium, thorium, and rubidium and their decay products, lead and strontium, are the measured quantities.
Assessment of the chemical composition of the solar system is based on data from Earth, the Moon, and meteorites as well as on the spectral analysis of light from the Sun and planets. In broad outline, the solar system abundances of the chemical elements decrease with increasing atomic weight. Hydrogen atoms are by far the most abundant, constituting 91 percent; helium is next, with 8.9 percent; and all other types of atoms together amount to only 0.1 percent.
The origin of Earth, the Moon, and the solar system as a whole is a problem that has not yet been settled in detail. The Sun probably formed by condensation of the central region of a large cloud of gas and dust, with the planets and other bodies of the solar system forming soon after, their composition strongly influenced by the temperature and pressure gradients in the evolving solar nebula. Less-volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets. The abundant, volatile lighter elements could condense only at much greater distances to form the giant gas planets. After the early 1990s astronomers confirmed that stars other than the Sun have one or more planetlike objects revolving around them. Studies of the properties of these solar systems have both supported and challenged astronomers theoretical models of how Earths solar system formed. (See also solar system: Origin of the solar system.)
The origin of the planetary satellites is not entirely settled. As to the origin of the Moon, the opinion of astronomers had long oscillated between theories that saw its origin and condensation simultaneous with formation of Earth and those that posited a separate origin for the Moon and its later capture by Earths gravitational field. Similarities and differences in abundances of the chemical elements and their isotopes on Earth and Moon had challenged each group of theories. Finally, in the 1980s a model emerged that has gained the support of most lunar scientiststhat of a large impact on Earth with the expulsion of material that subsequently formed the Moon. (See Moon: Origin and evolution.) For the outer planets with their multiple satellites, many very small and quite unlike one another, the picture is less clear. Some of these moons have relatively smooth icy surfaces, whereas others are heavily cratered; at least one, Jupiters Io, is volcanic. Some of the moons may have formed along with their parent planets, and others may have formed elsewhere and been captured.
The measurable quantities in stellar astrophysics include the externally observable features of the stars: distance, temperature, radiation spectrum and luminosity, composition (of the outer layers), diameter, mass, and variability in any of these. Theoretical astrophysicists use these observations to model the structure of stars and to devise theories for their formation and evolution. Positional information can be used for dynamical analysis, which yields estimates of stellar masses.
In a system dating back at least to the Greek astronomer-mathematician Hipparchus in the 2nd century bce, apparent stellar brightness (m) is measured in magnitudes. Magnitudes are now defined such that a first-magnitude star is 100 times brighter than a star of sixth magnitude. The human eye cannot see stars fainter than about sixth magnitude, but modern instruments used with large telescopes can record stars as faint as about 30th magnitude. By convention, the absolute magnitude (M) is defined as the magnitude that a star would appear to have if it were located at a standard distance of 10 parsecs. These quantities are related through the expression mM=5log10r5, in which r is the stars distance in parsecs.
The magnitude scale is anchored on a group of standard stars. An absolute measure of radiant power is luminosity, usually expressed in ergs per second (ergs/sec). (Sometimes the luminosity is stated in terms of the solar luminosity, 3.861033 ergs/sec.) Luminosity can be calculated when m and r are known. Correction might be necessary for the interstellar absorption of starlight.
There are several methods for measuring a stars diameter. From the brightness and distance the luminosity (L) can be calculated, and from observations of the brightness at different wavelengths the temperature (T) can be calculated. Because the radiation from many stars can be well approximated by a Planck blackbody spectrum (see Plancks radiation law), these measured quantities can be related through the expression L=4R2T4, thus providing a means of calculating R, the stars radius. In this expression, is the Stefan-Boltzmann constant, 5.67105ergs/cm2K4sec, in which K is the temperature in kelvins. (The radius R refers to the stars photosphere, the region where the star becomes effectively opaque to outside observation.) Stellar angular diameters can be measured through interference effects. Alternatively, the intensity of the starlight can be monitored during occultation by the Moon, which produces diffraction fringes whose pattern depends on the angular diameter of the star. Stellar angular diameters of several milliarcseconds can be measured, but so far only for relatively bright and close stars.
Many stars occur in binary systems (see binary star), with the two partners in orbits around their mutual centre of mass. Such a system provides the best measurement of stellar masses. The period (P) of a binary system is related to the masses of the two stars (m1 and m2) and the orbital semimajor axis (mean radius; a) via Keplers third law: P2=42a3/G(m1+m2). (G is the universal gravitational constant.) From diameters and masses, average values of the stellar density can be calculated and thence the central pressure. With the assumption of an equation of state, the central temperature can then be calculated. For example, in the Sun the central density is 158 grams per cubic cm; the pressure is calculated to be more than one billion times the pressure of Earths atmosphere at sea level and the temperature around 15 million K (27 million F). At this temperature, all atoms are ionized, and so the solar interior consists of a plasma, an ionized gas with hydrogen nuclei (i.e., protons), helium nuclei, and electrons as major constituents. A small fraction of the hydrogen nuclei possess sufficiently high speeds that, on colliding, their electrostatic repulsion is overcome, resulting in the formation, by means of a set of fusion reactions, of helium nuclei and a release of energy (see proton-proton cycle). Some of this energy is carried away by neutrinos, but most of it is carried by photons to the surface of the Sun to maintain its luminosity.
Other stars, both more and less massive than the Sun, have broadly similar structures, but the size, central pressure and temperature, and fusion rate are functions of the stars mass and composition. The stars and their internal fusion (and resulting luminosity) are held stable against collapse through a delicate balance between the inward pressure produced by gravitational attraction and the outward pressure supplied by the photons produced in the fusion reactions.
Stars that are in this condition of hydrostatic equilibrium are termed main-sequence stars, and they occupy a well-defined band on the Hertzsprung-Russell (H-R) diagram, in which luminosity is plotted against colour index or temperature. Spectral classification, based initially on the colour index, includes the major spectral types O, B, A, F, G, K and M, each subdivided into 10 parts (see star: Stellar spectra). Temperature is deduced from broadband spectral measurements in several standard wavelength intervals. Measurement of apparent magnitudes in two spectral regions, the B and V bands (centred on 4350 and 5550 angstroms, respectively), permits calculation of the colour index, CI=mBmV, from which the temperature can be calculated.
For a given temperature, there are stars that are much more luminous than main-sequence stars. Given the dependence of luminosity on the square of the radius and the fourth power of the temperature (R2T4 of the luminosity expression above), greater luminosity implies larger radius, and such stars are termed giant stars or supergiant stars. Conversely, stars with luminosities much less than those of main-sequence stars of the same temperature must be smaller and are termed white dwarf stars. Surface temperatures of white dwarfs typically range from 10,000 to 12,000 K (18,000 to 21,000 F), and they appear visually as white or blue-white.
The strength of spectral lines of the more abundant elements in a stars atmosphere allows additional subdivisions within a class. Thus, the Sun, a main-sequence star, is classified as G2 V, in which the V denotes main sequence. Betelgeuse, a red giant with a surface temperature about half that of the Sun but with a luminosity of about 10,000 solar units, is classified as M2 Iab. In this classification, the spectral type is M2, and the Iab indicates a giant, well above the main sequence on the H-R diagram.
The range of physically allowable masses for stars is very narrow. If the stars mass is too small, the central temperature will be too low to sustain fusion reactions. The theoretical minimum stellar mass is about 0.08 solar mass. An upper theoretical limit of approximately 100 solar masses has been suggested, but this value is not firmly defined. Stars as massive as this will have luminosities about one million times greater than that of the Sun.
A general model of star formation and evolution has been developed, and the major features seem to be established. A large cloud of gas and dust can contract under its own gravitational attraction if its temperature is sufficiently low. As gravitational energy is released, the contracting central material heats up until a point is reached at which the outward radiation pressure balances the inward gravitational pressure, and contraction ceases. Fusion reactions take over as the stars primary source of energy, and the star is then on the main sequence. The time to pass through these formative stages and onto the main sequence is less than 100 million years for a star with as much mass as the Sun. It takes longer for less massive stars and a much shorter time for those much more massive.
Once a star has reached its main-sequence stage, it evolves relatively slowly, fusing hydrogen nuclei in its core to form helium nuclei. Continued fusion not only releases the energy that is radiated but also results in nucleosynthesis, the production of heavier nuclei.
Stellar evolution has of necessity been followed through computer modeling because the timescales for most stages are generally too extended for measurable changes to be observed, even over a period of many years. One exception is the supernova, the violently explosive finale of certain stars. Different types of supernovas can be distinguished by their spectral lines and by changes in luminosity during and after the outburst. In Type Ia, a white dwarf star attracts matter from its nearby companion; when the white dwarfs mass exceeds about 1.4 solar masses, the star implodes and is completely destroyed. Type II supernovas are not as luminous as Type Ia and are the final evolutionary stage of stars more massive than about eight solar masses.
The nature of the final products of stellar evolution depend on stellar mass. Some stars pass through an unstable stage in which their dimensions, temperature, and luminosity change cyclically over periods of hours or days. These so-called Cepheid variables serve as standard candles for distance measurements (see above Determining astronomical distances). Some stars blow off their outer layers to produce planetary nebulas. The expanding material can be seen glowing in a thin shell as it disperses into the interstellar medium, while the remnant core, initially with a surface temperature as high as 100,000 K (180,000 F), cools to become a white dwarf. The maximum stellar mass that can exist as a white dwarf is about 1.4 solar masses and is known as the Chandrasekhar limit. More-massive stars may end up as either neutron stars or black holes.
The average density of a white dwarf is calculated to exceed one million grams per cubic cm. Further compression is limited by a quantum condition called degeneracy (see degenerate gas), in which only certain energies are allowed for the electrons in the stars interior. Under sufficiently great pressure, the electrons are forced to combine with protons to form neutrons. The resulting neutron star will have a density in the range of 10141015 grams per cubic cm, comparable to the density within atomic nuclei. The behaviour of large masses having nuclear densities is not yet sufficiently understood to be able to set a limit on the maximum size of a neutron star, but it is thought to be in the region of three solar masses.
Still more-massive remnants of stellar evolution would have smaller dimensions and would be even denser that neutron stars. Such remnants are conceived to be black holes, objects so compact that no radiation can escape from within a characteristic distance called the Schwarzschild radius (see gravitational radius). This critical dimension is defined by Rs=2GM/c2. (Rs is the Schwarzschild radius, G is the gravitational constant, M is the objects mass, and c is the speed of light.) For an object of three solar masses, the Schwarzschild radius would be about three kilometres. Radiation emitted from beyond the Schwarzschild radius can still escape and be detected.
Although no light can be detected coming from within a black hole, the presence of a black hole may be manifested through the effects of its gravitational field, as, for example, in a binary star system. If a black hole is paired with a normal visible star, it may pull matter from its companion toward itself. This matter is accelerated as it approaches the black hole and becomes so intensely heated that it radiates large amounts of X-rays from the periphery of the black hole before reaching the Schwarzschild radius. A few candidates for stellar black holes have been founde.g., the X-ray source Cygnus X-1. Each of them has an estimated mass clearly exceeding that allowable for a neutron star, a factor crucial in the identification of possible black holes. (Supermassive black holes that do not originate as individual stars are thought to exist at the centres of active galaxies; see below Study of other galaxies and related phenomena.)
Whereas the existence of stellar black holes has been strongly indicated, the existence of neutron stars was confirmed in 1968 when they were identified with the then newly discovered pulsars, objects characterized by the emission of radiation at short and extremely regular intervals, generally between 1 and 1,000 pulses per second and stable to better than a part per billion. Pulsars are considered to be rotating neutron stars, remnants of some supernovas.
Stars are not distributed randomly throughout space. Many stars are in systems consisting of two or three members separated by less than 1,000 AU. On a larger scale, star clusters may contain many thousands of stars. Galaxies are much larger systems of stars and usually include clouds of gas and dust.
The solar system is located within the Milky Way Galaxy, close to its equatorial plane and about 7.9 kiloparsecs from the galactic centre. The galactic diameter is about 30 kiloparsecs, as indicated by luminous matter. There is evidence, however, for nonluminous matterso-called dark matterextending out nearly twice this distance. The entire system is rotating such that, at the position of the Sun, the orbital speed is about 220 km per second (almost 500,000 miles per hour) and a complete circuit takes roughly 240 million years. Application of Keplers third law leads to an estimate for the galactic mass of about 100 billion solar masses. The rotational velocity can be measured from the Doppler shifts (see Doppler effect) observed in the 21-cm emission line of neutral hydrogen and the lines of millimetre wavelengths from various molecules, especially carbon monoxide. At great distances from the galactic centre, the rotational velocity does not drop off as expected but rather increases slightly. This behaviour appears to require a much larger galactic mass than can be accounted for by the known (luminous) matter. Additional evidence for the presence of dark matter comes from a variety of other observations. The nature and extent of the dark matter (or missing mass) constitutes one of todays major astronomical puzzles.
There are about 100 billion stars in the Milky Way Galaxy. Star concentrations within the galaxy fall into three types: open clusters, globular clusters, and associations (see star cluster). Open clusters lie primarily in the disk of the galaxy; most contain between 50 and 1,000 stars within a region no more than 10 parsecs in diameter. Stellar associations tend to have somewhat fewer stars; moreover, the constituent stars are not as closely grouped as those in the clusters and are for the most part hotter. Globular clusters, which are widely scattered around the galaxy, may extend up to about 100 parsecs in diameter and may have as many as a million stars. The importance to astronomers of globular clusters lies in their use as indicators of the age of the galaxy. Because massive stars evolve more rapidly than do smaller stars, the age of a cluster can be estimated from its H-R diagram. In a young cluster the main sequence will be well-populated, but in an old cluster the heavier stars will have evolved away from the main sequence. The extent of the depopulation of the main sequence provides an index of age. In this way, the oldest globular clusters have been found to be about 14 billion 1 billion years old, which should therefore be the minimum age for the galaxy.
The interstellar medium, composed primarily of gas and dust, occupies the regions between the stars. On average, it contains less than one atom in each cubic centimetre, with about 1 percent of its mass in the form of minute dust grains. The gas, mostly hydrogen, has been mapped by means of its 21-cm emission line. The gas also contains numerous molecules. Some of these have been detected by the visible-wavelength absorption lines that they impose on the spectra of more-distant stars, while others have been identified by their own emission lines at millimetre wavelengths. Many of the interstellar molecules are found in giant molecular clouds, wherein complex organic molecules have been discovered.
In the vicinity of a very hot O- or B-type star, the intensity of ultraviolet radiation is sufficiently high to ionize the surrounding hydrogen out to a distance as great as 100 parsecs to produce an H II region, known as a Strmgren sphere. Such regions are strong and characteristic emitters of radiation at radio wavelengths, and their dimensions are well calibrated in terms of the luminosity of the central star. Using radio interferometers, astronomers are able to measure the angular diameters of H II regions even in some external galaxies and can thereby deduce the great distances to those remote systems. This method can be used for distances up to about 30 megaparsecs. (For additional information on H II regions, see nebula: Diffuse nebulae (H II regions).)
Interstellar dust grains (see nebula: Interstellar dust) scatter and absorb starlight, with the effect being roughly inversely proportional to wavelength from the infrared to the near ultraviolet. As a result, stellar spectra tend to be reddened. Absorption amounts typically to about one magnitude per kiloparsec but varies considerably in different directions. Some dusty regions contain silicate materials, identified by a broad absorption feature around a wavelength of 10 m. Other prominent spectral features in the infrared range have been sometimes, but not conclusively, attributed to graphite grains and polycyclic aromatic hydrocarbons.
Starlight often shows a small degree of polarization (a few percent), with the effect increasing with stellar distance. This is attributed to the scattering of the starlight from dust grains that have been partially aligned in a weak interstellar magnetic field. The strength of this field is estimated to be a few microgauss, very close to the strength inferred from observations of nonthermal cosmic radio noise. This radio background has been identified as synchrotron radiation, emitted by cosmic-ray electrons traveling at nearly the speed of light and moving along curved paths in the interstellar magnetic field. The spectrum of the cosmic radio noise is close to what is calculated on the basis of measurements of the cosmic rays near Earth.
Cosmic rays constitute another component of the interstellar medium. Cosmic rays that are detected in the vicinity of Earth comprise high-speed nuclei and electrons. Individual particle energies, expressed in electron volts (eV; 1 eV=1.61012 erg), range with decreasing numbers from about 106 eV to more than 1020 eV. Among the nuclei, hydrogen nuclei are the most plentiful at 86 percent, helium nuclei next at 13 percent, and all other nuclei together at about 1 percent. Electrons are about 2 percent as abundant as the nuclear component. (The relative numbers of different nuclei vary somewhat with kinetic energy, while the electron proportion is strongly energy-dependent.)
A minority of cosmic rays detected in Earths vicinity are produced in the Sun, especially at times of increased solar activity (as indicated by sunspots and solar flares). The origin of galactic cosmic rays has not yet been conclusively identified, but they are thought to be produced in stellar processes such as supernova explosions, perhaps with additional acceleration occurring in the interstellar regions. (For additional information on interstellar matter, see Milky Way Galaxy: The general interstellar medium.)
The central region of the Milky Way Galaxy is so heavily obscured by dust that direct observation has become possible only with the development of astronomy at nonvisual wavelengthsnamely, radio, infrared, and, more recently, X-ray and gamma-ray wavelengths. Together, these observations have revealed a nuclear region of intense activity, with a large number of separate sources of emission and a great deal of dust. Detection of gamma-ray emission at a line energy of 511,000 eV, which corresponds to the annihilation of electrons and positrons (the antimatter counterpart of electrons), along with radio mapping of a region no more than 20 AU across, points to a very compact and energetic source, designated Sagittarius A*, at the centre of the galaxy (see Sagittarius A). Sagittarius A* is a supermassive black hole with a mass equivalent to 4,310,000 Suns.
Galaxies are normally classified into three principal types according to their appearance: spiral, elliptical, and irregular. Galactic diameters are typically in the tens of kiloparsecs and the distances between galaxies typically in megaparsecs.
Spiral galaxiesof which the Milky Way system is a characteristic exampletend to be flattened, roughly circular systems with their constituent stars strongly concentrated along spiral arms. These arms are thought to be produced by traveling density waves, which compress and expand the galactic material. Between the spiral arms exists a diffuse interstellar medium of gas and dust, mostly at very low temperatures (below 100 K [280 F, 170 C]). Spiral galaxies are typically a few kiloparsecs in thickness; they have a central bulge and taper gradually toward the outer edges.
Ellipticals show none of the spiral features but are more densely packed stellar systems. They range in shape from nearly spherical to very flattened and contain little interstellar matter. Irregular galaxies number only a few percent of all stellar systems and exhibit none of the regular features associated with spirals or ellipticals.
Properties vary considerably among the different types of galaxies. Spirals typically have masses in the range of a billion to a trillion solar masses, with ellipticals having values from 10 times smaller to 10 times larger and the irregulars generally 10100 times smaller. Visual galactic luminosities show similar spreads among the three types, but the irregulars tend to be less luminous. In contrast, at radio wavelengths the maximum luminosity for spirals is usually 100,000 times less than for ellipticals or irregulars.
Quasars are objects whose spectra display very large redshifts, thus implying (in accordance with the Hubble law) that they lie at the greatest distances (see above Determining astronomical distances). They were discovered in 1963 but remained enigmatic for many years. They appear as starlike (i.e., very compact) sources of radio waveshence their initial designation as quasi-stellar radio sources, a term later shortened to quasars. They are now considered to be the exceedingly luminous cores of distant galaxies. These energetic cores, which emit copious quantities of X-rays and gamma rays, are termed active galactic nuclei and include the object Cygnus A and the nuclei of a class of galaxies called Seyfert galaxies. They may be powered by the infall of matter into supermassive black holes.
The Milky Way Galaxy is one of the Local Group of galaxies, which contains more than three dozen members and extends over a volume about one megaparsec in diameter. Two of the closest members are the Magellanic Clouds, irregular galaxies about 50 kiloparsecs away. At about 740 kiloparsecs the Andromeda Galaxy is one of the most distant in the Local Group. Some members of the group are moving toward the Milky Way system, while others are traveling away from it. At greater distances all galaxies are moving away from the Milky Way Galaxy. Their speeds (as determined from the redshifted wavelengths in their spectra) are generally proportional to their distances. The Hubble law relates these two quantities (see above Determining astronomical distances). In the absence of any other method, the Hubble law continues to be used for distance determinations to the farthest objectsthat is, galaxies and quasars for which redshifts can be measured.
Cosmology is the scientific study of the universe as a unified whole, from its earliest moments through its evolution to its ultimate fate. The currently accepted cosmological model is the big bang. In this picture, the expansion of the universe started in an intense explosion 13.8 billion years ago. In this primordial fireball, the temperature exceeded one trillion K, and most of the energy was in the form of radiation. As the expansion proceeded (accompanied by cooling), the role of the radiation diminished, and other physical processes dominated in turn. Thus, after about three minutes, the temperature had dropped to the one-billion-K range, making it possible for nuclear reactions of protons to take place and produce nuclei of deuterium and helium. (At the higher temperatures that prevailed earlier, these nuclei would have been promptly disrupted by high-energy photons.) With further expansion, the time between nuclear collisions had increased and the proportion of deuterium and helium nuclei had stabilized. After a few hundred thousand years, the temperature must have dropped sufficiently for electrons to remain attached to nuclei to constitute atoms. Galaxies are thought to have begun forming after a few million years, but this stage is very poorly understood. Star formation probably started much later, after at least a billion years, and the process continues today.
Observational support for this general model comes from several independent directions. The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. Confirmation of this relic energy came in 1965 with one of the most striking cosmic discoveries of the 20th centurythe observation, at short radio wavelengths, of a widespread cosmic radiation corresponding to a temperature of almost 3 K (about 454 F or 270 C). The shape of the observed spectrum is an excellent fit to the theoretical Planck blackbody spectrum. (The present best value for this temperature is 2.735 K, but it is still called three-degree radiation or the cosmic microwave background.) The spectrum of this cosmic radio noise peaks at approximately one-millimetre wavelength, which is in the far infrared, a difficult region to observe from Earth; however, the spectrum has been well mapped by the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe satellites. Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium. Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang.
Early surveys of the cosmic background radiation indicated that it is extremely uniform in all directions (isotropic). Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy posed problems for models of galaxy formation. Galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted. Such density variations were difficult to reconcile with the isotropy required by observations of the 3 K radiation. This problem was solved when the COBE satellite was able to detect the minute fluctuations in the cosmic background from which the galaxies formed.
The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics. Subatomic particles are usually studied in laboratories with giant accelerators, but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available. Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist.
The age of the universe can be calculated in several ways. Assuming the validity of the big bang model, one attempts to answer the question: How long has the universe been expanding in order to have reached its present size? The numbers relevant to calculating an answer are Hubbles constant (i.e., the current expansion rate), the density of matter in the universe, and the cosmological constant, which allows for change in the expansion rate. In 2003 a calculation based on a fresh determination of Hubbles constant yielded an age of 13.7billion 200 million years, although the precise value depends on certain assumed details of the model used. Independent estimates of stellar ages have yielded values less than this, as would be expected, but other estimates, based on supernova distance measurements, have arrived at values of about 15 billion years, still consistent, within the errors. In the big bang model the age is proportional to the reciprocal of Hubbles constant, hence the importance of determining H as reliably as possible. For example, a value for H of 100 km/sec/Mpc would lead to an age less than that of many stars, a physically unacceptable result.
A small minority of astronomers have developed alternative cosmological theories that are seriously pursued. The overwhelming professional opinion, however, continues to support the big bang model.
Finally, there is the question of the future behaviour of the universe: Is it open? That is to say, will the expansion continue indefinitely? Or is it closed, such that the expansion will slow down and eventually reverse, resulting in contraction? (The final collapse of such a contracting universe is sometimes termed the big crunch.) The density of the universe seems to be at the critical density; that is, the universe is neither open nor closed but flat. So-called dark energy, a kind of repulsive force that is now believed to be a major component of the universe, appears to be the decisive factor in predictions of the long-term fate of the cosmos. If this energy is a cosmological constant (as proposed in 1917 by Albert Einstein to correct certain problems in his model of the universe), then the result would be a big chill. In this scenario, the universe would continue to expand, but its density would decrease. While old stars would burn out, new stars would no longer form. The universe would become cold and dark. The dark (nonluminous) matter component of the universe, whose composition remains unknown, is not considered sufficient to close the universe and cause it to collapse; it now appears to contribute only a fourth of the density needed for closure.
An additional factor in deciding the fate of the universe might be the mass of neutrinos. For decades the neutrino had been postulated to have zero mass, although there was no compelling theoretical reason for this to be so. From the observation of neutrinos generated in the Sun and other celestial sources such as supernovas, in cosmic-ray interactions with Earths atmosphere, and in particle accelerators, investigators have concluded that neutrinos have some mass, though only an extremely small fraction of the mass of an electron. Although there are vast numbers of neutrinos in the universe, the sum of such small neutrino masses appears insufficient to close the universe.
Original post: