1. Astro Physics

Category Archives: Astro Physics

The Hertzsprung-Russell Diagram of Star Clusters

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The nearby stars are of all ages, which gives them
a broad variety of luminosities and colors.  To see stars of the
same age, to see the effects of mass and composition alone on
a star's color and luminosity, one must examine star clusters.
All of the stars in a star cluster are born at about same time.
The open clusters scattered in the Galactic disk provide us with
collections of young stars.  The ancient globular clusters that
swarm around the Galactic center provide us with collections of
old stars.  By creating Herzsprung-Russell diagrams for both types
of star cluster—plots of the colors and luminosities of
stars—astrophysicists gain insight into how stars, especially
stars more massive than the Sun, change over billions of years.

Table of the 10 Brightest stars within 10 Parsecs of the Sun

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Little more than 350 stars are known to be
within 10 parsecs of the Sun.  Most of these are too dim
to see with the unaided eye. Several, however, are among
the brightest stars in the night sky.  The 10 brightest are
listed in a table on this page, along with their distances,
apparent visual magnitudes, absolute visual magnitudes,
color indices, and stellar types.

The Hertzsprung-Russell Diagram of the Nearest Stars

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The HR diagram of the stars within 10 parsecs is
presented on this page.  The diagram reveals that we are
surrounded largely by two types of star: dark main-sequence
stars and degenerate dwarfs.  Stars like the Sun are the
exception rather than the rule, and the more luminous A stars
and red giants are rather rare.  The brilliant and massive
supergiant O and B stars, of which Rigel in the constellation
Orion is an example, are completely absent from the local
stellar neighborhood, despite their prominence in the night
sky.  Most stars in the Galactic disk are much less luminous
than the Sun, and most of the stellar mass of the Galactic
disk is in these stars.

Magnitude and Color in Astronomy

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The magnitude system used by astronomers ranks
stars by brightness, with the brightest stars having the
lowest values of magnitude.  A star's magnitude is generally
measured after the starlight has passed through a colored
filter, which gives a measure of a star's color. Literally
dozens of filter systems are used by astronomers.  The
most common system for measuring color over the infrared,
visible, and ultraviolet wavelengths is the Johnson-Cousins
UBVRI system.

First AGILE catalog of high-confidence gamma-ray sources

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Authors: C. Pittori, F. Verrecchia, A. W. Chen, A. Bulgarelli, A. Pellizzoni, A. Giuliani, S. Vercellone, F. Longo, M. Tavani, P. Giommi, G. Barbiellini, M. Trifoglio, F. Gianotti, A. Argan, A. Antonelli, F. Boffelli, P. Caraveo, P. W. Cattaneo, V. Cocco, S. Colafrancesco, T. Contessi, E. Costa, S. Cutini, F. D'Ammando, E. Del Monte, G. De Paris, G. Di Cocco, G. Di Persio, I. Donnarumma, Y. Evangelista, G. Fanari, M. Feroci, A. Ferrari, M. Fiorini, F. Fornari, F. Fuschino, T. Froysland, M. Frutti, M. Galli, D. Gasparrini, C. Labanti, I. Lapshov, F. Lazzarotto, F. Liello, P. Lipari, E. Mattaini, M. Marisaldi, M. Mastropietro, A. Mauri, F. Mauri, S. Mereghetti, E. Morelli, E. Moretti, A. Morselli, L. Pacciani, F. Perotti, G. Piano, P. Picozza, M. Pilia, C. Pontoni, G. Porrovecchio, B. Preger, M. Prest, R. Primavera, G. Pucella, M. Rapisarda, A. Rappoldi, E. Rossi, A. Rubini, S. Sabatini, P. Santolamazza, E. Scalise, P. Soffitta, S. Stellato, E. Striani, F. Tamburelli, A. Traci, A. Trois, E. Vallazza, V. Vittorini, A. Zambra, D. Zanello and L. Salotti
A&A 506, 1563 (2009) Received 4 February 2009 / Accepted 3 August 2009
Keywords: gamma rays: observations, catalogs

Stellar Types

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In the nineteenth century, astronomers recognized
that stars could be classified by their spectra into a handful
of types.  Over time, this system was refined to characterize
a star in terms of prototypical stars with similar spectra.
This is the meaning of the jargon that the Sun is a G2 V
star: the G2 refers to the pattern of lines in the Sun's
spectrum, which is directly dependent on temperature, and
the V refers to the widths of these lines, which are
dependent on luminosity.  The advantage of this system
is that astronomers can determine what stars are like the
Sun in temperature and luminosity simply by looking at
the patterns of lines in the stars' spectra.


  1. Astro Physics