Astr 309 -- Celestial Names and Magnitudes

I. Star Names

Most bright stars have a proper name like Vega, Aldebaran, or Zubenelgenubi. Many of the names are ancient Arabic, and in some cases may indicate where in its parent constellation it lies -- for example, Zubenelgenubi means "southern claw" of the scorpion.

As catalogs got longer, particularly after the developement of the telescope, it grew cumbersome to have proper names for every star, and astronomers looked to a shorthand. This was easier to group into regions of the sky, and took up less room on star charts.

Bayer Designation: John Bayer published his Uranometria star catalog in 1603 using a system of Greek lower-case letters and abbreviated constellation names (using a 3-letter abbreviation). Estimating by eye, Bayer assigned the brightest star in each constellation the letter alpha (), the second-brightest beta (), gamma (), delta (), etc. Later, when equipment became available for measuring star brightnesses more accurately it was discovered that some of the letters are out of order (eg, beta Cnc is actually brighter than alpha Cnc), but it is still a useful system often used on star charts. Notice that the brightness ranking is within the constellation only -- for example, alpha Cnc is fainter than gamma And. Examples are given in the table below.

Proper Name Bayer Flamsteed BD BSC HD
Vega Lyr 30 Lyr BD +38 3238 HR 7001 HD 172167
Aldebaran Tau 87 Tau BD +16 629 HR 1457 HD 29139
Zubenelgenubi Lib 9 Lib BD -15 3966 HR 5531 HD 130841

Flamsteed Designation: In 1725 John Flamsteed published a star catalog labeling stars with a number and the 3-letter constellation abbreviation, with numbers increasing fom west to east across each constellation. For example, "1 Cyg" is the westernmost star in the constellation Cygnus, and "7 Cyg" lies farther east. In many constellations, more stars have a Flamsteed Designation than have a Bayer Designation (astronomers say Flamsteed's catalog "goes deeper" -- includes fainter stars), so on detailed star charts you may see the brighter stars marked with Greek letters, and the fainter stars marked with numbers.

As telescopes became larger and more powerful, ever-larger catalogs containing the myriad of fainter stars were published. The Bonner Durchmusterung (BD), first published in the 1850s-60s, contained over 300,000 stars visible from Bonn, Germany. It was supplemented with stars in the southern hemisphere observed from Cordoba, Argentina and Cape Town, South Africa (published as the Cordoba Durchmusterung (CD) and Cape Photographic Durchmusterung (CPD)). Stars were cataloged in one degree tall declination (like latitude) strips around the sky, and numbered increasing from west to east. If you saw a star labeled "BD +05 1234" you would know it was cataloged in Germany (BD), lies in the +5 degree declination strip, and is the 1234th star eastward along the strip. Similarly, you might find "CP -44 482" or "CPD -67 282." These catalogs listed the stars' positions and magnitudes (brightness), and included an atlas (e.g., an image of part of a page). Sometimes you may see a star listed as "DM +03 125" indicating it is part of the three-part Durchmusterung series but not being specific over which catalog (BD, CD or CPD) it appeared in

Other star names you may encounter refer to catalogs such as the Henry Draper Catalog ("HD 1234", over 225,000 stars which includes stellar spectral types, compiled by Annie Jump Cannon in 1918-24), the Bright Star Catalog ("HR 12" for Harvard Revised Photometry, containing extensive information on brighter stars, has a revised edition in progress by Dorrit Hoffleit), the Smithsonian Astrophysical Observatory Catalog ("SAO 113271", 1966), the Hipparcos Catalog ("HIP 4234", ultra-precise positions from the European Space Agency's Hipparcos satellite, 1990s), the Tycho-2 Catalog (revised Hipparcos positions for over 2.5 million stars), and the US Naval Observatory's USNO B1.0 (over a billion stars and galaxies!). A broader list of catalogs old and new, and information about them can be found at Wikipedia.


II. Names of Extended Objects

Messier Catalog: While searching for comets, French astronomer Charles Messier compiled a list of fuzzy objects that did not move from night to night like comets. Published in 1781, the final list contained over 100 objects which contain some of the brightest and most popular extended objects viewable from the Northern hemisphere. Objects are simply listed as "M1" or "M31." There are many on-line lists of Messier objects, e.g., from SEDS.

New General Catalog: A much larger list of faint, fuzzy objects was gradually compiled during the 19th and early 20th centuries, resulting in the New General Catalog containing almost 8000 objects (e.g, "NGC 908"), and later supplemented with the First and Second Index Catalogs ("IC 444") containing over 5000 more objects, many of which are in the under-represented Southern hemisphere. As with the Messier list, there are on-line NGC lists like that from SEDS.


III. Magnitudes

The magnitude or brightness system we use today has its roots in a star catalog made by the Greek astronomer Hipparchus around 150 BC. Hipparchus ranked star brightnesses into six groups: magnitude 1 included the brightest stars, magnitude 2 contained slightly fainter stars, etc., and magnitude 6 included stars just barely bright enough to see. Thus, the larger the number, the fainter the star.

As telescopes came into use, fainter stars were discovered and the magnitude system was continued to 7, 8, 9, etc. By the mid-1800's, more precise methods became available for measuring star brightnesses, and the magnitude groupings were extended into a continous scale, with decimal numbers (eg, 4.2 magnitudes, or 8.1 mag). It was also realized that the eye does not respond linearly to light, but geometrically. That is, a difference in magnitudes corresponds to a ratio in energy fluxes (number of photons received per second in a specific wavelength region) coming from the two stars. In 1856, Norman Pogson proposed the mathematical defininition we still use today:


where m1 and m2 are the magnitudes of the two stars (say, 4.25 and 8.13) and F1 and F2 are their energy fluxes. Using this equation, we can calculate the energy flux ratios for several typical magnitude differences:



0.0 1.0


-0.5 1.585
-1.0 2.51
-2.0 6.31
-5.0 100
-10.0 10,000
-20.0 100,000,000

For example, if m1=4.25 and m2=8.13, then m1-m2 = -3.88 mag. From the table, we know the energy flux ratio is between 6.31 and 100, and if we calculate it using the right-most equation above, we find F1/F2 = 35.6. That is, star 1 is 35.6 times brighter than star 2 (consistent with its smaller magnitude value). If m1-m2 is positive, F1/F2 will be a fraction (between 0 and 1) -- you can take the inverse to find out how many times brighter star 2 is than star 1.

Using these equations and modern brightness-measuring equipment, astronomers have determined the apparent magnitude of a wide range of astronomical sources. You can see they span a huge range of magnitudes and an immense range of energy fluxes:

Object mag
Sun -26.7
full Moon -12.7
Venus (brightest planet at maximum brightness)


Sirius A (brightest star) -1.45
Polaris (North Star) +2.3
faintest naked eye stars


faintest stars with 8-inch telescope +12
faintest stars with 20-inch telescope +15
faintest stars yet observed with Hubble Space Tel. +30



* Chris Kitchin, Telescopes and Techniques: An Introduction to Practical Astronomy, 2003, Springer-Verlag Publishing, pp. 162-173





Anderw Layden Fall 2006 || Back to the A309 main page