SJAA Ephemeris June 2008 | SJAA Home
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Last month we explored the history of cataloging deep sky objects. This month we will look at how astronomers catalog the positions of stars. The cataloging system for stars is much more overlapped system than with Deep Space Objects. The positions of bright stars are cataloged in several overlapping catalogs. A star might also be in one or specialty catalogs based on some property. On the surface this might all seem very confusing, but there is a definite method to it.
In this article I will be discussing Positional catalogs whose purpose is to map the locations all of the stars down to some level of depth (dimness). The technically correct name is Astrometric Survey1. In a future article I will discuss Specialty Catalogs that list stars as belonging to a particular group (e.g double stars, carbon stars, etc)2.
The International Astronomical Union (IAU)3 recognizes the “proper” names for about 250 stars4. You may know this group for its work on Pluto. It is the only world body authorized to assign names to astronomical objects. The IAU is no longer assigning names to stars.
Most of the traditional star names were derived from Arabic, but some are from Latin or Greek or have been Latinized from Arabic or other cultures. Names beginning with “Al” are definitely from Arabic sources5. Scientists have generally moved away from names since there are many overlapping traditions of which stars have which names6. For example, is Almach or Almaach or Almak or Alamak the correct spelling for Gamma Andromedae? Is the correct name for Alpha Persei Mirfak or Algenib or Alcheb? These seem like minor points until a name comes up in your DSC7 hand controller that does not match your paper chart. It has happened to me!
What about all of the services that “sell” new star names? We talked about that last month. The IAU has a wonderful piece on this practice on their site8. Needless to say if you want to spend $50 for a pretty piece of paper that is your own business. As PT Barnum said “This way to the Egress”!
Johann Bayer (1572-1625) and John Flamsteed (1646-1719) introduced more formal systems of identifying or cataloging stars. Bayer’s system assigns a Greek letter to each star in a constellation. He labeled the brightest9 alpha, second beta, and so on. Altogether Bayer’s system cataloged 1,564 of the brightest stars10 in his star atlas Uranometria. Bayer’s system had the advantage that each star had a unique and simple name. The obvious disadvantage was that the alphabet was limited (and you have to know Greek). He did extend beyond the Greek alphabet, but today we only retain at most 26 stars in each constellation. Bayer’s system remains the most common formal designation of bright stars.
Flamsteed’s system was to number the stars11. This avoided the problems with Bayer’s approach. Flamsteed’s numbers were assigned by right ascension, but today the assigned stars are not strictly by RA due to precession12. His system catalogs about 3,000 stars13.
The Bayer and Flamsteed systems overlap. Generally the Bayer designation is used in preference to the Flamsteed.
The stars that Bayer and Flamsteed cataloged were the stars that one might see visually in a perfect dark sky. As technology improved there was a need for a more complete catalog that listed the positions, spectral type, movement, etc of dimmer stars. Successive catalogs have improved the positions of stars and provided accurate information on their movements.
This catalog containing 225,000 stars was published between 1918 and 192414 by Harvard. It catalogs stars down to about 9 magnitude or about what an 9x50 finder would see. I occasionally see HD numbers used. For example, Stellarium uses this as its preferred catalog. Other more modern catalogs have gained more support.
The Smithsonian Astrophysical Observatory Catalog was published in 196615. It contains about 260,000 stars and like the HD catalog is limited to about mag 9. This catalog is more popular. For example, this is the preferred designation in the Meade handset.
This is a product of the ESA satellite of the same name16. The satellite, which operated for four years from November 1989 to March 1993 returned high quality scientific data. The HIP catalog contains 118,218 stars charted with high precision17. It maps stars down to about mag 818.
The Tycho-2 catalog contains about 2.5 million stars to about magnitude 1120. This catalog was also derived from the ESA Hipparcos satellite.
The Hubble Guide Star Catalog is exactly what it says it is21. Hubble Space Telescope can use this list of stars the as guide sources (to correct for movement during an exposure). The catalog contains objects in the range of 7-16. Thus it is deeper than the Tycho without being as large as the USNO. The GSC used Hipparcos satellite data but also used their own data from Schmidt Survey plates.
The USNO catalog is the mother of all catalogs. The Naval Observatory compiled the catalog by examining a complete set of photometric plates. B1.0 contains more than 1 billion entries22 of both stars and DSOs. That is over 80 GBytes of data! A2.0 is just ½ billion and contains every detectable star down to declination -30º. The next release UCAC3 will be available soon. It has about 60 million entries23 making its size more realistic for common use. The main feature is the accuracy of the positions. UCAC3 is speced to chart the positions to 20 milliarcseconds24.
As a practical matter the B1.0 catalog is so large that it is not useful except in research projects. Planetarium programs that rely on DVDs25 can exploit the smaller USNO catalogs to give an almost photographic representation of the sky.
Every star you observe will be in one or more of these catalogs. The very brightest stars will be named and be in each. The chief difference of the catalogs is their depth and improved accuracy.
So what should you use? For the brightest stars I use either the name or Bayer letter. Using a DSC will quickly teach you star names (at least according to the conventions used by the DSC maker). When I had a Meade LX200 I used SAO most often since the Meade tools prefer that designation.
I configure my SkyMap Pro Planetarium program to display the popular name, then Bayer letter, then Flamsteed number. If the star has none of these it reverts to the Tycho then GSC catalogs. SMP does not support the USNO. An information popup will list other catalogs entries. The combined catalogs contain all of the stars I am likely to see visually with a 15” telescope.
Uranometria lists the name, Bayer, and Flamsteed number. The rest of the stars are plotted, but not identified. Their locations are plotted using Hipparcos and Tycho data.
7 Digital Setting Circle
9 Although as measuring techniques improved it was discovered that the alpha star is not always the brightest. See: http://en.wikipedia.org/wiki/Bayer_designation#Is_Alpha_Al ways_the_Brightest_Star.3F
16 http://heas arc.gsfc.nasa.gov/W3Browse/all/hipparcos.html for more reading