1. Astro Physics

Category Archives: Astro Physics

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.

Brown Dwarfs

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A class of object, long predicted by astrophysicists,
sits in the mass range between the giant gaseous planets
and the M dwarf stars.  These objects are called brown
dwarfs.  They are massive enough to burn deuterium, but they
are too light to burn hydrogen.  The first brown dwarf was
observed orbiting an M dwarf star in 1988, and since that
time, hundreds of additional brown dwarfs have been found.
They are cool, so they are primarily emitters of infrared
radiation.  In the early stages of their lives, they are
powered by deuterium fusion and gravitational potential
energy, but when they consume their deuterium, and when
the electron degeneracy pressure stops their shrinkage,
they grow cold and dark.

Extrasolar planets and brown dwarfs around A–F type stars – VII. \theta Cygni radial velocity variations: planets or stellar phenomenon?

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Authors: M. Desort, A.-M. Lagrange, F. Galland, S. Udry, G. Montagnier, H. Beust, I. Boisse, X. Bonfils, F. Bouchy, X. Delfosse, A. Eggenberger, D. Ehrenreich, T. Forveille, G. Hébrard, B. Loeillet, C. Lovis, M. Mayor, N. Meunier, C. Moutou, F. Pepe, C. Perrier, F. Pont, D. Queloz, N. C. Santos, D. Ségransan and A. Vidal-Madjar
A&A 506, 1469 (2009) Received 26 January 2009 / Accepted 27 July 2009

Keywords: techniques: radial velocities, stars: early-type, stars: planetary systems, stars: individual: \theta Cygni


  1. Astro Physics