Fall 2000

This  feature in our newsletter, inspired by the questions we have received over the years from interested readers, will answer your questions about astronomical issues. If you have a question about astronomy, please forward it to us at mira@mira.org.

I am looking for a table or a catalog of the number of stars in the Galaxy vs. apparent visual magnitude and spectral class (B, A, F, G, K, M). I don’t need the voluminous catalogs like Hipparcos or Tycho1 with position data, just the number of stars. I have a small table from the old book by Russell, Dugan and Stewart2 (1945) with the number of stars from m = 2.24 to m = 8.24 (42 entries in all). What I’d like is a more comprehensive table (with perhaps ~ 300 - 400 entries) for m = 0 to m = 28. Do you know of such a table? Thanks.

I suspect you need the answer to a slightly different question than the one you posed. Almost all the stars in the Milky Way Galaxy are not visible to us because of interstellar obscuration. Also, what we see is not the best estimate of what is there, since the Galactic core and bulge are much older that the nearby stars one sees. Since you’re asking about apparent magnitude, are you really interested in something like the surface density as a function of total contribution to various colors? This is actually a harder question to address.

What I am trying to do is to compute the spectral irradiance (watts per m^2 per micrometer) for all the stars in the hemisphere at the top of the atmosphere. I realize that the hemispherical distribution changes depending on my position on Earth and Greenwich Mean Time. Someone did this (Applied Optics, Vol. 1, No. 4, July 1962) and I would like to duplicate his work using a larger and perhaps better table of star populations. The idea is to find the peak of the Planck formula3  in terms of temperature and visual magnitude. Then use the number of stars associated with each spectral class and the corresponding temperature (this is approximate) one can integrate over magnitude to find the total irradiance. Failing this, if someone has a good, even approximate, model of the spectral radiance from the Milky Way I’ll take that. I thought that by now (year 2000) someone might have a simple model so that I could enter the coordinates of my Earth position and the GMT and it would provide an estimate of the spatially averaged flux from the Milky Way. I can convert from Galactic coordinates4  to ecliptic or equatorial coordinates. I don’t have time to consider the details of analyzing thousands of stars, each with its own magnitude and color index. All I think I need is a slightly better table of star populations than the one I now have (vintage 1945).

Hmmm . . . well, there are lots of ways to do this. The number of stars per square degree per each magnitude (photographic and visual) as a function of Galactic latitude is available. Also, integrated starlight, in terms of the number of equivalent 10th-magnitude stars is tabulated as a function of Galactic latitude in visual and photographic magnitudes. This is tabulated in Allen III (Allen, C.W., Astrophysical Quantities, 3rd ed., Athlone Press, 1973). Of course, the luminosity function is available but is per volume of space rather than per apparent area. There are also tables of log N(V,I) [the logarithm of the number of stars per square degree in the V and I color bands per increment in magnitude] averaged over longitude as function of Galactic latitude and vice versa. Also, apparently the Pioneer 10 made sky brightness measurements of blue and red around the sky. (BTW, the Large Magellanic Cloud5  has a huge effect on the color!) For more detail (short of reality) one could construct your distribution from the standard model of our Galaxy6 but that is probably more effort than you want to get into. In any case, the standard model values are also available in Allen IV. Allen IV (Astrophysical Quantities, 4th ed.), ed: Cox, was just published by Springer. It is nowhere near as good as the old Allens, but they are long out of print7 . I hope this helps.

(A few days later, Bob Turner was busy in the MIRA Priscilla Bok reference library. –ed)
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1 Catalogs of stellar positions and magnitudes taken from a satellite. 

2 The standard astronomy textbook of the mid-twentieth century.  

3 A formula that describes the brightness of a glowing object at different colors.  

4 A longitude and latitude-like coordinate system where the "equator" is along the plane of the Milky Way.  

5  An irregular galaxy that is a satellite to our own galaxy and is visible to the naked eye in the southern hemisphere.  

7 The series of editions of Astrophysical Quantities were the standard reference books for astronomers for 50 years. It was so popular that, long after C.W. Allen died, a new edition of the book has been released composed of reference sections by a wide range of contributors.

Last updated 1/11/01 et