How do main sequence stars of low mass compare to those of high mass in terms of their lifetimes on the main sequence Their temperature Their color Their luminosity?

Answer 1

Main sequence stars of low mass spend a lot more time on the main sequence than do more massive stars. Less massive stars are also much less luminous than high mass stars.

Using the empirical mass-luminosity relation we can show this to be true.

L/Lsun~M/Msun3.5

Where L is luminosity and M is the mass of the star.

Consider a very low mass of M=0.1

Using this relation we get L =0.000316, a tiny fraction of the sun's luminosity!

Then we take a more massive star, say M=10

We get L = 3162 times more luminous than the sun!

The amount of material available (mass) and the rate of energy release (luminosity) can also be used to determine how long a star will remain stable on the main sequence with the equation:

MS Lifetime = M/Msun / L/Lsun x 1010 Years

We'll use the same two examples from above, M=0.1 and M=10

Low Mass Star = 0.1 / .000316 x 1010 years = 3.16x1012 years

High Mass Star = 10 / 3162 x 1010 years = 31.6 x 106 years

As we can plainly see, a high mass star shines much brighter than a low mass star, and stays on the main sequence a much shorter time. This implies that the higher the mass of the star, the faster it goes through its fuel. There are many factors contributing to this, including gravitational collapse and heating, increased surface area for energy output (since high mass stars are bigger than low mass stars), and fusion of materials in the core. Please note that internal fusion IS NOT responsible for luminosity--it IS responsible, however, for how long a star can remain on the main sequence.

A star has a much higher internal temperature than the temperature of its outer shell. Generally, the core of a star can be in the millions of degrees Kelvin, where the Effective Temperature (Teff) at the surface of the star is generally in the thousands to tens of thousands of degrees K.

For the sake of the question you asked, I'm going to assume you were referring to Teff, since it is related directly to the 'color' of the star.

As a generalization, we can think of stars as perfect radiators, or blackbodies. Practically this means that the hotter a star is, the more blue it appears, and the cooler a star is, the redder it appears. As you may have already guessed, the more massive stars are generally 'blue,' and the lower mass stars are generally 'red,' with yellow and white stars in between.

From Wikipedia:

Stellar

ClassRadiusMassLuminosityTemperatureExamples[24]R/R☉M/M☉L/L☉KO51840500,00038,000Zeta PuppisB07.41820,00030,000Phi1 OrionisB53.86.580016,400Pi Andromedae AA02.53.28010,800Alpha Coronae Borealis AA51.72.1208,620Beta PictorisF01.41.767,240Gamma VirginisF51.21.292.56,540Eta ArietisG01.051.101.266,000Beta Comae BerenicesG21.001.001.005,920Sun[note 2]G50.930.930.795,610Alpha MensaeK00.850.780.405,15070 Ophiuchi AK50.740.690.164,64061 Cygni A[25]M00.630.470.0633,920Gliese 185[26]M50.320.210.00793,120EZ Aquarii AM80.130.100.0008-Van Biesbroeck's star[27]

The Stellar Classes on this chart indicate several things about the star, including its brightness, color, and spectrum. O type stars are the biggest and hottest, and the most Blue. B are usually considered blue-white, A considered white, F called yellow-white, G are yellow (our sun is a G2 star), K are orange, and M are red.