H-R MS relationship or data for abs. mag. and ST

ramparts · Jun 7, 2005

I need to be able to estimate a star's distance given its B-V and/or spectral type (including integer type), but I'm making a program for this, so I can't extrapolate rough values for an object just by looking at the graph. Instead, I need either a table giving the approx. absolute magnitude for each spectral type, or an equation relating B-V (or any other kind of value using B and V) to absolute magnitude. Anyone have any idea?

someone_else · Jun 7, 2005

I sure would love to help you, but I'm not even sure what language you're speaking! <img src="/images/icons/wink.gif" />

Saiph · Jun 7, 2005

Good question. You may wish to try Nasa's ADS at http://adswww.harvard.edu/ and search through those papers. They often have tables to accompany their figures on such material. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

ramparts · Jun 8, 2005

That might be the best way, but it would be such a pain! I wouldn't know where to begin searching! But yeah, that might be best.... As for the last post, that's a pretty good idea, too, but I don't know about it. Now, I could probably just download the Hipparcos data off Vizier, import it into Excel, and have my way with it, but then there's the problem of all the non-main sequence stars on there that would complicate the calculations a bit. Maybe I'll try doing that right now, though, just for fun at least

It would just be so nice if there were some sample data or a relationship already available somewhere........but I might as well check out ADS, then. Thanks <img src="/images/icons/wink.gif" />

Saiph · Jun 8, 2005

ADS you can search, so do some digging to figure out the terms you want, and search the abstracts for those phrases. Things like "Absolute Magnitude" and "color index" or more precicesly the term "E(B-V)". If you use Sinbad or other star catalogue you can obtain spectral types (and color index), and calculated distances / absolute magnitudes as well as the apparent...that's more than enough information there. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

Saiph · Jun 8, 2005

Or you could use an Astronomica Ephemeris, as that has lots of handy tables and such (it's like an astronomer's almanac). THere should be some available in a university's science library. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

thalion · Jun 11, 2005

As others have already posted, it will be very difficult to determine distance or absolute magnitude based on spectral type or B-V index alone. The best way to get around this is to just ignore non-MS stars, and work with the MS only. If you want to include all stars though, you'll have to include the luminosity class as well, which can only be determined by careful study of the width of the absorption lines. For what it's worth, I've long read that giants and supergiant stars often have a slightly higher (that is, redder and cooler) B-V index than MS stars of the same spectral type, though this seems to break down near the top of the MS. For instance, check out the following color indices for M0 and G5 stars of various luminosity classes, adapted from Norton's Star Atlas (19th ed.), p. 142: M0 V B-V = +1.40 (dwarf) M0 III B-V = +1.56 (giant) M0 Iab B-V = +1.67 (supergiant) G5 V B-V = +0.68 (dwarf) G5 III B-V = +0.86 (giant) G5 Iab B-V = +1.02 (supergiant)

Saiph · Jun 12, 2005

a lot of that is due to how stars are not perfect blackbodies. Anyway, ramparts asked for the spectral type quote]including integer type which is the Ia, Ib, II, etc that distinguish between giant branches and the MS. Thalions quotation of values from Nortons star atlas does show a source of such information though. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

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