# Evolution of low-mass stars

1. May 18, 2009

### Maria76

Let's say we have a star of 0.7 solar masses on the main sequence. Can somebody describe what happens to it when it comes off the main sequence?

Thanks
Maria

2. May 19, 2009

### Chronos

They may [according to current models] bypass the red giant phase and evolve more or less directly into white dwarfs due to their inability to fuse helium. This is almost surely true for any star less than .5 solar masses. At .7 solar masses, the issue is less clear. It may enter the red giant phase then rapidly shed its outer atmosphere before settling in as a white dwarf. It is entirely possible, however, the universe is not yet old enough for a star of this size to have died of old age.

3. May 19, 2009

### Nabeshin

Very likely the case. The Lifetime proportional to mass^3.5 approximation gives a main sequence lifetime of ~24 billion years for a star of .7 solar masses.

4. May 19, 2009

### Orion1

WMAP Universe age:
$$t_u = 1.373 \cdot 10^{10} \; \text{y}$$

$$t_{L} = 1.1 \cdot 10^{10} \; \text{y}$$

$$\tau_{ms} = t_{L} \left( \frac{M_{\odot}}{M_s} \right)^{2.5}$$

$$\boxed{\tau_{ms} \geq t_u}$$

Integration by substitution:
$$t_{L} \left( \frac{M_{\odot}}{M_s} \right)^{2.5} \geq t_u$$

Main sequence minimum stellar mass threshold currently available for red giant phase branch ascention:
$$\boxed{M_s \leq M_{\odot} \left( \frac{t_u}{t_{L}} \right)^{-0.4}}$$

$$\boxed{M_s \leq 0.915 \cdot M_{\odot}}$$

Red Giant phase mass threshold:
$$0.23 \cdot M_{\odot} \leq M_s \leq 10 \cdot M_{\odot}$$

Reference:
http://en.wikipedia.org/wiki/Age_of_the_universe#WMAP"
http://en.wikipedia.org/wiki/Main_sequence#Evolutionary_tracks"
http://www.eso.org/public/outreach/press-rel/pr-2007/images/phot-29-07-normal.jpg" [Broken]

Last edited by a moderator: May 4, 2017
5. May 24, 2009

### qraal

A better source for low mass stars are the papers by Laughlin, Adams and Bodenheimer - "The End of the Main Sequence" and its follow-up papers - but Wikipedia is more or less correct. A 0.7 solar mass star will eventually burn its helium and it will form a Red Giant, but there hasn't been time for such stars to evolve to such a late stage in the history of the present Universe. There do exist many white-dwarfs of that mass, and smaller, but that's because red-giants undergo a lot of mass loss - a 10 solar mass star will probably return about ~8.6 solar masses of material back to the interstellar medium. Our Sun is likely to return about ~0.46 solar masses, leaving a carbon/oxygen white-dwarf corpse of about 0.54 solar masses.

I do wonder if we can't engineer that final remnant as a gigantic fusion reactor, slowly trickling mass onto it until we creep up to the Chandrasekhar Limit. We could control the stages of collapse into neutronium by encouraging the formation of heavier elements, after carefully fusing the final mass into iron, then collapsing it into heavier and heavier elements before the final (hopefully not too quick) deconfinement of quarks in the core. If we could collapse it into a quark star some 6.96 km in radius (0.000001 solar radii) we can ultimately extract a total of ~1.87 trillion years worth of gravitational energy (counting the energy in years of present solar luminosity.)

6. May 26, 2009

### Chronos

I flinch at the .23 solar mass limit. I see no way helium fusion of any significance can occur in a star with such low mass.

7. May 26, 2009

### qraal

The lowest I've seen is about 0.3 solar masses, which is what the Adams/Laughlin/Bodenheimer paper says as well.

8. May 26, 2009

### Orion1

hydrogen burning shell...

At 0.23 solar masses it does not burn helium, the helium remains inert within a purely radiative core and burns a hydrogen shell surrounding the radiative helium core resulting in a red giant star.

Reference:
http://arxiv.org/PS_cache/astro-ph/pdf/9701/9701131v1.pdf" [Broken]

Last edited by a moderator: May 4, 2017