I have been looking lately at David Nash’s “HYG” star catalogue, which contains data collected from the Hipparcos, Yale, and Gliese catalogues. HYG lists each star’s StarID, Hipparcos number, HD number, HR number, Gliese number, Bayer-Flamsteed identifier, Proper Name, right ascension, declination, Distance, visual magnitude, absolute magnitude, Spectral class, and color index.
I am a bit disappointed that the catalogue omits Durchmusterung identifiers, and I’m a bit unsure how it indicates multiples. I also find it a pest that the Bayer-Flamsteed identifiers are given in abbreviated form, especially as ‘Tau’ means both tau and Taurus. But basically it is a great resource.
I do, however, have two questions.
How do I calculate luminosity from absolute magnitude?
What is a star’s color index, and what is its significance? Is it an alternative measure of temperature, like spectral class but more objective?
You’re spot on about the color index. There are several pre-defined filters that only look at certain parts of the spectrum. Say, for example, a blue filter and a red filter. You measure the apparent magnitude of the star through each of these filters (called the color magnitude), and subtract. Blue-Red (blue minus red) would be your color index. So, a hot star might have a blue magnitude of 5 and a red magnitude of 6, and thus a Blue-Red color index of -1. Meanwhile a cool star might have a blue magnitude of 2 and a red magnitude of 0, making for a color index of 2. You’re right that it’s more objective than temperature fitting.
I believe that the there most common filters are U, B, and V (for ultraviolet, blue, and visible) in the Johnson system. The most commonly-cited color indices are Johnson U-B and Johnson B-V.
Here are the formulae if you know the absolute magnitude in B or V:
L = 10^(0.4(5.48 - M[sub]B[/sub])) L[sub]SUN[/sub]
L = 10^(0.4(4.83 - M[sub]V[/sub])) L[sub]SUN[/sub]
If you just have the straight-up bolometric magnitude, I believe you use the formula for M[sub]V[/sub]. These formulae work because M[sub]B,SUN[/sub] = 5.48 and M[sub]V,SUN[/sub] = 4.83. You can replace them, though, with any star you know the luminosity and magnitude for. A good rule of thumb is that Vega is 0 in all filters, by original definition. You can see, incidentally, that the Sun’s B-V color index is 0.65, and Vega’s B-V color index is 0.