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[sci.astro] Stars (Astronomy Frequently Asked Questions) (7/9)
Section - G.05 Where can I get stellar data (especially distances)?

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Top Document: [sci.astro] Stars (Astronomy Frequently Asked Questions) (7/9)
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See reader questions & answers on this topic! - Help others by sharing your knowledge
	John Ladasky Jr. <ladasky@my-deja.com>

Two key sites for stellar data are the Astronomical Data Center,
<URL:http://adc.gsfc.nasa.gov/adc.html>, and the CDS Service for
Astronomical Catalogues,
<URL:http://cdsweb.u-strasbg.fr/cats/Cats.htx>, both of which maintain
large inventories of astronomical catalogs, including star catalogs.
Another important site is SIMBAD,
<URL:http://simbad.u-strasbg.fr/sim-fid.pl>, as one can use it to find
alternate names for a star.  (For instance, what is another name for
the variable star V* V645 Cen?)

Distances in astronomy are always problematic, and it is important to
keep in mind that all astronomical data have uncertainties.  It is
vital to understand what the uncertainties are.  Moreover, if one is
interested in constructing 3-D star maps, one should recognize that
astronomical data are not stored in XYZ coordinates.  Science-fiction
writers and people who want to make 3-D maps of local space like them,
but astronomers don't use them.  Astronomers need polar coordinates
(right ascension and declination) centered on Earth, so that they know
where to point their telescopes.

Three useful sites for distance data are 

 * One large (3803 stars) compilation of nearby stars is the
   "Preliminary Version of the Third Catalogue of Nearby Stars," which
   aims to catalog all known stars within 25 pc (~ 75 light years) of
   the Sun.  The "ReadMe" file for the catalog is at
   <URL:ftp://adc.gsfc.nasa.gov/pub/adc/archives/catalogs/5/5070A/ReadMe>.

 * The Internet Stellar Database
   <URL:http://www.stellar-database.com/> attempts to synthesize
   information about the nearest stars from various catalogs.

 * Recent research on refining astronomical data for the nearby stars
   can be found at the Research Consortium on Nearby Stars (RECONS),
   <URL:http://tarkus.pha.jhu.edu/%7Ethenry/RECONS.html>.

(Note that these sites tend to focus on *nearby* stars---that's a
result of the difficulty of obtaining accurate distances for distant
stars.)

If an object is close enough to Earth to have a significant parallax
(an apparent yearly wobble in the sky that results from the change in
observing position of the Earth), then its distance can be determined
by triangulation.  With two angles and a distance, you can compute
Cartesian coordinates if you want them.  If you'd like to use the
astronomical data, say, to calculate distances between stars, a useful
reference is <URL:http://www.projectrho.com/starmap.html>.  (Note that
many astronomical catalogs do not include parallax measurements.)

The best parallax data collected thus far comes from the European
astrometry satellite, Hipparcos,
<URL:http://astro.estec.esa.nl/Hipparcos/>, and it represents a
gigantic improvement both in systematic accuracy and in precision over
previous catalogs, but it is limited to fairly bright stars (magnitude
limit around 11).

Both the CDS and the Hipparcos Web site offer online tools for
searching the Hipparcos catalog as well as the full catalog itself.
Two important aspects of the Hipparcos catalog are how distances are
described and the names given to stars.  First, distances are
described by the parallax in milliarcseconds.  The distance d in
parsecs is given by d = 1000/p for a parallax p in milliarcseconds.
To obtain a distance in light years, multiply by 3.26.  Thus, a star
with a parallax of 100 milliarcseconds is at a distance of 10 pc (~ 30
light years).

Second, all of the Hipparcos catalog "names" will be unfamiliar to
you, as they are just numbers.  One can use SIMBAD to convert from
Hipparcos catalog names to more familiar names.

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Top Document: [sci.astro] Stars (Astronomy Frequently Asked Questions) (7/9)
Previous Document: G.04 What fraction of stars are in multiple systems?
Next Document: G.06 Which nearby stars might become supernovae?

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