Star Names And Catalogs

Star Names and Catalogs The brightest stars have proper names. For example, the brightest star in the sky is Sirius. Many of the stars with you are familiar with have have names like Vega, Deneb, Altair, Betelgeuse, Rigel, Regulus, Polaris, etc. Those names have mainly been handed down to us from the ancient Greeks or Arabs, from many hundreds or thousands of years ago. There are about 10,000 stars bright enough to be seen with the naked eye. With telescopes, we can see hundreds of thousands, if not millions, more. That is far too many star to require proper names for each one! Hundreds of years ago, astronomers devised different systems for naming stars; those systems are still commonly used today.

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Bayer (Greek) Letters

While only the brightest stars have proper names, a larger set of stars have Bayer or Greek letters. Around the year 1600 Johannes Bayer, in what is now Germany, applied lower case Greek letters to the stars in each constellation, more or less in order of brightness. The Latin possessive form of the constellation is appended to the Greek letter. For instance, the brightest star in Canis Major (Sirius) is Alpha Canis Majoris; the second-brightest star in Orion (Rigel) is Beta Orionis, etc. Constellation possessives are often written with a three-letter abbreviation, e.g. Alpha Canis Majoris is abbreviated α CMa, and Beta Orionis is β Ori. All stars with proper names generally also have Bayer letters. For reference, here is a table of the letters in the Greek alphabet: α - Alpha β - Beta γ - Gamma δ - Delta ε - Epsilon ζ - Zeta η - Eta θ - Theta ι - Iota κ - Kappa λ - Lambda μ - Mu ν - Nu ξ - Xi ο - Omicron π - Pi ρ - Rho σ - Sigma τ - Tau υ - Upsilon φ - Phi χ - Chi ψ - Psi ω - Omega Not every constellation uses every letter in the Greek alphabet. And with only 24 letters, Bayer quickly ran out of letters in larger constellations. He increased the possible number of names by applying superscripts to stars that fall near one another (a string of stars in Orion became π1 Ori, π2 Ori, π3 Ori, and so on). But the system was still limited. Bayer therefore followed the lower-case Greek alphabet with lower-case Roman letters, then with upper case Roman letters. These are rarely used nowadays, however, and only a few vestiges like “h Persei” (actually a star cluster in Perseus) and G Scorpii survive. Flamsteed Numbers To organize more of the naked eye stars, the eighteenth century John Flamsteed numbered the stars in each constellation by right ascension, from west to east. For example, 1 Lyrae would be the westernmost star in Lyra, 2 Lyrae the next, and so on. 3 Lyrae also happens to be Vega, or α Lyrae. All of the stars with a Bayer letter also have a Flamsteed number. However, if a star has both, it’s more common to use the Bayer letter.

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The Yale Bright Star (HR) Catalog

The first of the 20th century star catalogs was the “Yale Bright Star Catalogue,” which serially numbers 9096 naked-eye stars from west to east beginning at right ascension zero hours. Though now published at Yale, it derives from a catalogue produced at Harvard in 1908, so the star catalog numbers take on the name “HR” for “Harvard Revised.” Vega = α Lyrae = 3 Lyrae = HR 7001. For naked eye stars, HR numbers are still in common use today.

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The Durchmusterung (BD, CD, CP) Catalogs

With the advent of astro-photography in the 1850s, stars below naked-eye visibility needed catalogue numbers, too. The most famous general catalogue for fainter stars, the “Bonner Durchmusterung” (the Bonn Survey), was compiled in Germany in the nineteenth century, and lists stars through around tenth magnitude. That is about 50 times fainter than the naked eye can see alone. It divides the sky into declination bands one degree wide, and then serially numbers the stars from west to east according to their right ascension (for the year 1855). The catalogue name incorporates the declination. Vega, for example, is BD+38°3238, which means “the 3238th star in the declination strip between 38 and 39 degrees north”. The BD covers stars from the north celestial pole to 2 degrees south of the celestial equator. The rest of the southern hemisphere is covered by the “Cordoba Durchmusterung” (the Cordoba, Argentina, Survey), or CD; and the “Cape Photographic Durchmusterung”, or CP. Canopus (HR 2326) is CD-52°914, or the 914th listed star between declination 52 and 53 degrees south. BD, CD, and CP are sometimes combined as “DM” for “Durchmusterung.” Precession has now moved many stars out of their original declination strips.

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The Henry Draper (HD) Catalog

The most commonly used catalog for fainter stars is the Henry Draper (HD) Catalogue. This catalog was first published in 1924, and was originally created to list the spectral classes of over 300,000 stars. It has been updated and extended over the subsequent years. The current HD catalog (1985) includes position, magnitude, and spectral type data for 272150 stars. The HD catalog serially numbers stars through roughly tenth magnitude according to their right ascension in the year 1900. Vega is HD 172167; Canopus is HD 45348.

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The Smithsonian Astrophysical Observatory (SAO) Catalog

In the 1960s, over ten positional catalogues were combined into the Smithsonian Astrophysical Observatory (SAO) star catalogue. It serially numbers over 259,000 stars (according to right ascensions for the year 1900) to ninth magnitude, in 10 degree declination strips from north to south. Vega is SAO 067174; Canopus is SAO 234480. Though it enjoyed some popularity, the SAO catalog is no longer in common use.

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The Hipparcos (HIP) and Tycho (TYC) Catalogs

The Hipparcos satellite was launched in 1989 to measure the distances, brightness, and motions of the stars with unprecedented accuracy. The resulting Hipparcos Catalogue lists data for 118,218 stars, most of which are brighter than magnitude 8 (although several thousand fainter stars of astrophysical interest were also included). The Hipparcos catalog, which emerged from this mission, was the most accurate source of basic star data available for a long time. (In 2016, the data from the Gaia mission were published, with over one billion stellar positions, and distances to more than two million stars.) Vega is HIP 91262, Canopus is HIP 30438. The Hipparcos satellite also measured the positions of many fainter stars to a lesser degree of accuracy. This secondary set of measurements formed the Tycho catalog. The Tycho catalog includes 1,058,331 stars, most brighter than 11th magnitude. An updated version of the Tycho catalog (Tycho-2) was released in 2000. This new version combined Hipparcos mission data with measurements from the 19th-century photographic catalogs. Separated by more than 100 years in time, these observations yielded (after Hipparcos) the most accurate source of stellar proper motion data available. Tycho-2 contains over 2.5 million stars with precise positions and proper motions, and magnitude information at multiple wavelengths. The Tycho-1 and Tycho-2 catalogs use the same star-numbering scheme as the original Hubble Guide Star Catalog (see below).

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The Hubble Guide Star Catalog (GSC)

The Hubble Guide Star Catalog was created in the late 1980s to support the Hubble Space Telescope mission. It was by far the largest star catalog created up to its time. It listed positions for nearly 19 million objects, of which about 15 million were stars. The rest were faint galaxies, photographic plate defects, artifacts of the image processing system, etc. The GSC divides the sky into 9357 zones, and numbers the objects sequentially within each zone. For example, GSC 3118-1106 is a 13th-magnitude star near Vega in Lyra. In 2001-2002, the original photographic material generating the GSC was reprocessed and combined with Tycho-2 data. The resulting Guide Star Catalog 2 contains data for an even larger number of objects - almost 1 billion! - and has much more accurate position and brightness measurements than the original GSC. The GSC2 uses a completely different numbering scheme, however. In the GSC2, the 13th magnitude star 3118-1006 has the number N0220212-290.