How do we know luminosity of a star from its spectral class

[billyharris72]

Hi all:

I wonder if someone can clarify this for me. My questions comes from going back over what little I know about how we can calculate the distance to stars other than using the parallax method.

I can get the following:

* Once we have a relative and absolute magnitude we can work out the distance using the distance modulus.

* The spectral class of a star gives us a good idea of its temperature based on absorption lines.

What is confusing is how we get from there to luminosity (and also to mass). It would be possible to establish these empirically (based on observations of stars whose distance can be calculated using parallax), and that would enable us to work out luminosity and mass for main sequence stars. But how do we know that a star is on the main sequence? If I establish that a star is Class A it is probably about 100 times more luminous than the sun, and about 5 times more massive. But how would I know from its spectrum that it is not a white dwarf? Ditto for classes K and M, where we could have red giants or red dwarfs.

I'm guessing that the answer has to do with other information contained in the spectrum, but struggling to find more detail. Can anyone enlighten me?

Thanks,

Billy.

[billyharris72]

Okay - managed to answer my own question with a bit of digging. Link below for anyone interested.

http://www.skyandtelescope.com/astronomy-equipment/the-spectral-types-of-stars/

 

[barkis]

It is a very deep and interesting study Billy..
Without the spectrum, a star is just a point of light, as mentioned in the link you supplied above,
but so much information can be pulled from a star's   spectrum's absorption lines,  all it's make up is revealed. Clever stuff, and I wish I 
was smart enough to understand it all. When you can stand up in front of an audience at a local society Meeting,  and expertly describe 
the processes chapter and verse  would  be great. Only a dream for moi though 

[Merlin66]

I would strongly recommend:

"The stars and their spectra" - An introduction to the Spectral Sequence,  by James B. Kaler, 2nd Ed. published by Cambridge.

It will answer all your questions........

[ollypenrice]

6 hours ago, Merlin66 said:

I would strongly recommend:

"The stars and their spectra" - An introduction to the Spectral Sequence,  by James B. Kaler, 2nd Ed. published by Cambridge.

It will answer all your questions........

 I thnk he's a great scientific writer for the general reader. I haven't read this one but I'm off to Amazon with the next click! Thanks Ken.

Olly

[robin_astro]

On 4/21/2016 at 17:12, billyharris72 said:

But how would I know from its spectrum that it is not a white dwarf? Ditto for classes K and M, where we could have red giants or red dwarfs.

I'm guessing that the answer has to do with other information contained in the spectrum, but struggling to find more detail. Can anyone enlighten me?

 

Hi Billy,

to determine if the star is on the main sequence you need to know that the spectroscopic luminosity class is v (As in G2v for the sun)  You can determine this from the width of the spectral lines (The width of the line depends on the density of the star so for example giants and supergiants with luminosity class i,ii have narrower lines compared with main sequence stars)  See here for more details 

http://spiff.rit.edu/classes/phys440/lectures/lumclass/lumclass.html

Cheers

Robin

[saac]

Those interested in exploring stellar evolution may be interested in this useful app.  We've been using it in the classroom recently and found it really easy to use and helpful in building a good understanding of the relative luminosities/spectral classification/stellar temperature via the Hertzsprung-Russell diagram. 

http://starinabox.lcogt.net/

 

Jim