Why Do M-Type Stars Exhibit More Diverse Spectral Lines Than O-Type Stars?

[shirin]

As we look to the spectra of stars from O to M type, we see that as temprature decreses, the spectral lines become more crowded. Why M-stars have more various elements than O-stars? Is that because of mass or age or what?

 

[Astronuc]

The composition would be determined from the gas cloud from which a star forms. Since heavy elements are formed during stellar evolution, there would have to be older evolved stars and perhaps supernovae in the vicinity of the gas clouds. Or some main sequence stars will evolve into red giants.

Bear in mind that M's can be any size from red dwarves to red supergiants.

http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/starform.htm
http://abyss.uoregon.edu/~js/ast122/lectures/lec13.html

Like many of the stars of Orion, it was probably formed a few million years ago from the molecular clouds observed in Orion but has evolved rapidly due to its unusually high mass. The star is a member of the "Orion OB1 Association," where massive young objects with over 10 times the Sol's mass can be found in abundance (more on OB associations and stellar nurseries).

from http://www.solstation.com/x-objects/betelgeuse.htm

See also - http://en.wikipedia.org/wiki/Stellar_classification#Class_M
http://en.wikipedia.org/wiki/Betelgeuse
http://en.wikipedia.org/wiki/Alpha_Herculis
http://en.wikipedia.org/wiki/Proxima_Centauri

http://en.wikipedia.org/wiki/Main_sequence

In roughly 5 billion years, the Sun will cool and expand outward to many times its current diameter (becoming a red giant), before casting off its outer layers as a planetary nebula, and leaving behind a stellar corpse known as a white dwarf.

http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System
http://astrosun2.astro.cornell.edu/academics/courses//astro201/evol_sun.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/herrus.html#c6

See also - Molecular Cloud Formation and the Star Formation Efficiency in M~33
http://arxiv.org/abs/1007.0702

http://abyss.uoregon.edu/~js/glossary/star.html


See lectures - http://abyss.uoregon.edu/~js/ast122/

 

[Ken G]

I think the simple answer is basically "the lower the temperature, the more the bound electrons (as opposed to being free), and the more the lines." Free electrons produce a flat continuum opacity, not lines. There's probably more complexity to the issue though-- for example, if you compare hot stars to cool stars, you find that cool stars are convective near their surface, which means you have hot gas rising and cool gas falling back down. This activity also produces a hot chromosphere above the layers where the continuum forms. All this complexity produces a wide range of temperatures which also adds to the diversity of lines in the cooler stars.