Hack15.Identify Stars by Catalog Designations

Hack 15. Identify Stars by Catalog Designations

Learn about the stellar catalogs used by amateur astronomers.

Of the thousands of stars visible to the naked eyeand millionsvisible with optical aidonly a few hundred of the brightest have proper or common names, such as Vega or Sirius. Although proper names are convenient for referring to bright "guidepost" stars, astronomers also need unambiguous designations for dimmer stars.

It is possible, although awkward, to specify a particular star by giving its coordinates [Hack #17]. For example, we could say, the star located at 14h15m40.35s right ascension and +19°11'14.2" declination, but that gets old fast. As a more convenient alternative, astronomers have developed star catalogs, which assign each star a unique short identifier, such as a-Boötis or HIP 69673. (All three of these designations specify Arcturus, the brightest star in the constellation Boötes.)

Early astronomers began the work of cataloging the stars visible to the naked eye, dividing the stars by constellation and then assigning unique identifiers to each. Modernday astronomers have continued that practice, but they now treat the sky as a contiguous whole and catalog stars without respect to constellation. Surprisingly, the star naming syntax of some catalogs produced 300 to 400 years ago remains in common use. Here are the stellar catalogs you need to be familiar with.

2.6.1. The Bayer Catalog

The German astronomer Johann Bayer (15721625) published Uranometria, the first comprehensive star atlas, in Augsburgin 1603. Uranometria predates telescopes, so it contains only stars that are visible to the naked eye. Uranometria was unique for its time because it mapped far southerly stars, including those in south circumpolar constellations. In Uranometria, Bayer introduced his system of labeling the bright stars in each constellation with lowercase Greek letters, a system that is still used today.

Bayer designated the brightest star in each constellation alpha (a), the second brightest beta (b), and so on through the Greek alphabet, shown in Table 2-3, although he made numerous exceptions to the brightnessbased hierarchy. For example, Bayer sometimes assigned "brighter" letters to stars that happened to be nearer the head of the traditional mythological constellation figure, and lower letters to those stars near its feet. Bayer appended the genitive form of the name of the constellation [Hack #16] to assign each star a unique label. For example, Vega, the brightest starin the constellation Lyra, has the Bayer designation a-Lyrae.

Prominent multiple stars and some strings of stars share one Greek letter, with an Arabic numeral as a qualifier. For example, b-Cygni is a stunning double star. The primary, golden Albireo (3.08m) is b1-Cygni. The unnamed blue secondary (5.11m) is b2-Cygni. Similarly, a long string of stars off Bellatrix are assigned the Bayer designations p1 through p6-Orionis.

Table 2-3. Greek letters and their pronunciation

Greek letter Lower	Upper	Names	Pronunciation Common	Correct
a	A	alpha	AL-fuh
b	B	beta	BATE-uh	BEET-uh (VEET-uh in modern Greek)
g	G	gamma	GAMM-uh
d	D	delta	DELT-uh
e	E	epsilon	EPP-sih-lawn
z	Z	Zeta	ZATE-UH	ZEET-uh
h	H	eta	ATE-uh	EET-uh
q	Q	theta	THATE-uh	THEE-tuh
I	I	iota	eye-OTE-uh	YOTT-uh
k	K	kappa	CAP-uh
l	L	lambda	LAM-duh
m	M	mu	MEW	MEE
n	N	nu	NEW	NEE
x	X	xi	ZIE	KSEE (KS as in foX)
o	O	omicron	OH-mih-kron	AW-muh-kron
p	P	pi	PIE	PEE
r	R	rho	ROE
s	S	sigma	SIG-muh
t	T	tau	TOW (OW as in hOW)	TAF (AF as in cALF)
u	U	upsilon	UP-sih-lawn	EEP-sih-lawn
f	F	phi	FIE	FEE
c	C	chi	KYE	KHEE (KH as in Scottish loCH)
y	Y	psi	SIGH	puh-SEE
w	W	omega	oh-MEG-uh, oh-MEE-guh	aw-MEG-uh

Two characters represent the lowercase sigma. sis used at the beginning and in the middle of words. zis used only as the final letter in words. zappears rarely in Bayer designations instead of the correct s.

Bayer had no instruments for measuring magnitudes, and his catalog ranks stellar brightness incorrectly in many cases. For example, Bayer labeled Betelgeuse (0.50m) a-Orionis and the brighter Rigel (0.12m) b-Orionis. Similarly, Pollux (1.14m) is b-Geminorum, while the dimmer Castor (1.98m) is a-Geminorum.

Bayer also intentionally made exceptions for stars he "liked." For example, Bayer assigned Thuban (3.65m) as a-Draconis, despite the fact that Thuban is noticeably dimmer than b-, g-, d-,z-, and even h-Draconis. Bayer presumably honored Thuban for its historical importance rather than its brightness. Five thousand years ago, when the ancient Egyptians were building the first pyramids, Polaris was not the Pole Star. Thuban was.

Despite the fact that stellar magnitudes are now known precisely and Bayer's brightness rankings are known to be wrong, astronomers continue to use Bayer's original designations. It's important to understand Bayer designations because they are used widely in charts, handbooks, articles, and so on. Figure 2-5 shows the constellation Orion with its prominent stars labeled with Bayer designations.

Figure 2-5. Prominent stars in Orion, with Bayer labels

It's also important to know Greek alphabet symbols because symbols and names are used interchangeably. For example, if you are reading an article about the galaxy NGC 404 in the constellation Andromeda, it may mention that the galaxy is located only a few arcminutes from the bright star Mirach. But the article may refer to that star as beta Andromedae or b-Andromedae. If you don't know that bis the symbol for beta, you won't know which star the article refers to.

2.6.2. The Flamsteed Catalog

The Englishman John Flamsteed (16461719) was appointed the first Astronomer Royal of Britain in 1676. He created Historia Coelestis Britannica, the first great scientific reference work released under the aegis of the Greenwich Observatory. It contained Flamsteed's detailed observations and a catalog of 2,682 stars with their positions mapped with unprecedented accuracy. Flamsteed never traveled far south, so his catalog contained only stars located at declinations of -50° and higher. Despite this gap, Flam-steed's catalog quickly became the authoritative star atlas because of its accuracy.

To his extreme annoyance, Flamsteed was published involuntarily. He wanted to delay publication until he considered the work complete, but other scientists including Edmond Halley and Isaac Newton desperately needed Flamsteed's accurate data for their own work, and so eventually forced publication of the work over Flamsteed's objections. The first, unauthorized edition of 400 copies was published in 1712, prepared by Halley and funded by Prince George of Denmark. Flamsteed successfully sued and was able to reclaim and burn these unauthorized copies. The authorized version of Coelestis Britannica was finally published in 1725, six years after Flamsteed's death.

Flamsteed's original catalog did not label stars, but merely mapped them by declination and right ascension. The French astronomer Joseph-Jerome de Lalande decided that the absence of star labels made the catalog awkward to use, so he annotated his French edition of Coelestis Britannica, adding numeric labels to each of the stars mapped by Flamsteed. Those numbers, universally called Flamsteed numbers despite that fact that Flamsteed had nothing to do with them, are still used today.

Figure 2-6 shows part of the constellation Orion, with the prominent stars labeled with their proper names, Bayer designations, and Flamsteed numbers. Notice that the Flamsteed numbers increase from right to left, without regard to where the star is located vertically on this chart. For example, Rigel, at the bottom right of the chart, is assigned the Flamsteed number 19. Bellatrix, at the top center of the chart, is assigned Flamsteed 24. Betelgeuse, at the upper left, is assigned Flamsteed 58.

This is true because Lalande assigned numbers to Flamsteed's stars inorder of right ascension, from west to east [Hack #17]. Lalande started with the westernmost star in each constellation, assigning it the number 1. He then proceeded east within the constellation, numbering each star without regard to its declination. (The highest Flamsteed number is 140 Tauri.)

Figure 2-6. Prominent stars in Orion, with proper names, Bayer labels, and Flamsteed numbers

Flamsteed numbers are still commonly used today. Most amateur astronomers use the following rules to decide how to refer to a star.

• If the star has a proper name that is common usage, use it. Every amateur astronomer knows which star you mean when you mention Betelgeuse. (Most will also recognize the a-Orionis Bayer designation; few will recognize the Flamsteed designation 58 Orionis without referring to a chart.)

• If the star has no common proper name, use the Bayer designation if one exists. (Most astronomers recognize the Bayer designation s-Orionis; almost none recognize the Flamsteed designation 48 Orionis.)

• If the star has no common proper name or Bayer designation, use the Flamsteed number. Some famous stars, for example 61 Cygni, are best known by their Flamsteed numbers.

• If the star has no common name, Bayer designation, or Flamsteed number, use the designation from one of the catalogs described in the following section.

2.6.3. Modern Stellar Catalogs

As useful as the Bayer and Flamsteed catalogs were and still are, they don't go very deep. Bayer maps only naked eye stars, and Flamsteed maps stars only down to 8th magnitude. Even standard binoculars show stars to 9th magnitude or dimmer, and large amateur telescopes show stars to 17th magnitude or dimmer.

For comparison, a "beginner" star atlas like Norton's or Cambridge shows stars to 6th magnitude. Sky Atlas 2000.0, popular among intermediate and advanced amateur astronomers, shows 81,000 stars down to magnitude 8.5. Uranometria 2000.0 (the new version, not the classic) shows 280,000 stars down to magnitude 9.75. The Millennium Star Atlas, now sadly out of print, shows more than 1,000,000 stars down to magnitude 11.

As the science of astronomy developed, the need for deeper star catalogs quickly became apparent. There was also a need for specialty catalogs that focused on such matters as extremely precise magnitude measurements or detailed spectra. In the mid-19th century, various national and university observatories began developing such catalogs. (Amateur astronomers do not use these catalogs in their native or raw forms; instead, these catalog numbers are incorporated in planetarium software and in printed charts such as Sky Atlas 2000.0 and Uranometria 2000.0). Here are the most important catalogs to know about:

Bonner Durchmusterung (BD) Catalog

In 1859, the German astronomer F. W. A. Argelander, working at the Bonn Observatory, set out to map all stars down to magnitude 9.5. Cataloging stars by constellation becomes confusing below 8th magnitude, so for the Bonner Durchmusterung Catalog, Argelander decided to ignore constellations and map the stars by declination strips 1° wide, each of which incorporated the full 24 hours of right ascension. For each declination strip, Argelander assigned numeric designations to each star in right ascension order, from west to east. For example, Vega is designated BD+38°3238, which indicates that Vega is the 3,238th star in the 38° to 39° declination strip. The original BD catalog mapped stars to 2° south of the celestial equator; it was later extended by the SBD and Cordoba Durchmusterung (CD or CoD) catalogs. The consolidated BD catalogs contain 324,188 stars.

Revised Harvard Photometry (Harvard Revised, or HR)

In 1908, Harvard College issued the Revised Harvard Photometry Catalog, which is usually referred to as Harvard Revised or HR. This catalog lists only 9,110 stars down to magnitude 6.5, but it provides extremely precise magnitude information for each. HD catalogs stars by right ascension, from west to east, beginning at RA 0h00m00s, and numbers each cataloged star sequentially. For example, Caph (b-Cas) is located at RA 0h09m10.70s, and is designated HR 21, while Bellatrix (g-Orionis), much farther east at RA 5h25m07.90s is designated HD 1790.

Contrary to common belief, the positions of the stars are not fixed in the firmament. The declination and right ascension of all stars changes gradually and in lockstep because of precession, the slow wobbling of the earth on its axis. Also, each star is actually moving relative to other stars, although the amount of actual motion is exceedingly small relative to their extreme distance. If a star is close enough to us, it is possible to detect this actual change in position against distant background stars, which is called the proper motion for that star.

To take into account precession and proper motion, when astronomers specify stellar coordinates they also include the epoch for those coordinates, which specifies the date for which the locations were accurate. Early catalogs, such as HD and HR, used epoch J1900.0, which specified the coordinates of stars as of 1 January 1900. Epoch is usually listed by Julian date, indicated by the J prefix; other date formats are used so seldom that Julian is assumed unless otherwise specified.

Later catalogs, and updated versions of older catalogs, use epoch 1950.0 or 2000.0, which is often indicated in the name of the catalog, as in the names of the printed catalogs Sky Atlas 2000.0 and Uranometria 2000.0. Good planetarium programs and goto scopes use the date epoch, which is the epoch as of the current date. The difference can be surprisingly large. For example, the J2000 coordinates of Vega are RA 18h36m56.19s and Dec +38°46'58.8". The date epoch coordinates on 8 July 2005 are RA 18h37m07.31s and Dec +38°47'16.6". In other words, in only 5.5 years, the position of Vega has changed by 11.12s in RA and17.8" in declination.

Henry Draper Catalog (HD)

From 1911 until 1915, Annie Jump Cannon at the Harvard College Observatory compiled the Henry Draper Catalog (HD), which was published in the years after WWI. HD was named in honor of the astronomerHenry Draper, whose widow donated the funds needed to compile and publish it.

HD started out as a specialty catalog, with the goal of cataloging the spectral classes of 225,300 stars down to magnitude 10, but it became commonly used as a general catalog. Like HR, HD assigns numeric designations to stars by RA, from west to east, epoch 1900.0, beginning at the vernal equinox. Of course, HD catalogs many more stars, so, for example, Caph (HR 21)is HD 432, and Bellatrix (HR 1790) is HD 35468. In 1949, the Henry Draper Extension (HDE) catalog supplemented the original HD catalog, bringing the total number of cataloged stars to 359,083. There is no overlap in the numbering of the HD and HDE catalogs, so stars in the HDE catalog are often listed with the HD designation. HD numbers are used widely today to identify stars that have no Bayer or Flamsteed designation.

Although amateur astronomers seldom use them nowadays, you may encounter Smithsonian Astrophysical Observatory (SAO) catalog designations in older publications. In 1966, Harvard University SAO combined and consolidated a dozen older positional catalogs, adding proper motion datafor each. The SAO contains 258,997 stars down to magnitude 9. The original SAO used epoch 1950.0; the current version uses epoch 2000.0. SAO divides the sky into 10° declination strips and assigns numeric designations to each star within each declination strip. Astronomers quickly abandoned Benjamin Boss's 1937 General Catalog of 33,342 Stars in favor of SAO. In turn, SAO has now been largely abandoned in favor of modern catalogs like the Tycho2, Hipparchos, and HST catalogs described next.

Hubble Space Telescope (HST) Guide Star Catalog (GSC)

The most comprehensive deep star catalog available is the Hubble Space Telescope (HST) Guide Star Catalog (GSC), for which you may see stars listed under either abbreviation. GSC 1.1 lists positions with 1.7 arcsecond accuracy and magnitudes to within 0.5 accuracy for 18,819,291 objects from magnitude 9 to about magnitude 13.5, with many objects as faint as magnitude 15. Of the listed objects, about 15.2 million are stars and the remainder are mostly dim galaxies. GSC divides the sky into 9,537 regions and then assigns numbers to each object within a region.

The latest iteration of the HST GSC, called GSC 2.2, catalogs an incredible 435,457,355 objects, down to magnitude 19.5. The vast majority of these objects are so dim they are invisible in all but the largest amateur scopes.

Hipparcos Catalog (HIP)

From 1989 until 1993, the Hipparcos satellite, launched by the European Space Agency, mapped precise locations and parallax data for 118,218 relatively nearby stars down to magnitude 8.5 (complete to magnitude 7.3), with the goal of determining accurate proper motions for each. Although it originated as a specialty catalog, the Hipparcos Catalog (HIP) is now used as a general catalog. HIP assigns numeric designations to stars by RA from west to east beginning at RA 0h00m00s. For example, Rigel is HIP 24436 and Bellatrix, about 11 minutes more easterly, is HIP 25336. HIP is valued by amateurs for the accurate stellar distances it provides.

Tycho-2 Catalog (TYC)

The Tycho-2 catalog is also based on data gathered by the Hipparcos satellite mission and compiled by the Copenhagen University Observatory and the U.S. Naval Observatory. Tycho-2 maps about 2.5 million stars, with nearly complete coverage of 900,000 stars to magnitude 11 and 1.6 million additional stars to magnitude 12.

For more detailed information about stellar catalogs, visit the U.S. Naval Observatory Catalog Information and Recommendations page: http://ad.usno.navy.mil/star/star_cats_rec.shtml.

2.6.4. Specialty Catalogs

In addition to general or all sky catalogs, scores of specialty catalogs exist, some of which are very specialized indeed. Double star observers, for example, can choose among the Aitken Double Star Catalog (ADS), the Lick Observatory's Index Catalog of Visual Double Stars (IDS), and the U.S. Naval Observatory's Washington Double Star Catalog (WDS), to name only three. Similar catalogs exist for variable stars, nearby stars, carbon stars, and so on. Those interested in Lunar occultations of stars use the Zodiacal Catalog (ZD), which includes only stars that Luna can occult.

We don't have room to list (let alone describe) even a small fraction of the total number of specialty catalogs. If you develop an interest in a specialized category of stellar observing, you'll soon enough become familiar with the online and printed catalogs and other resources that target that specialty.