As we have now begun working towards deeper and deeper fields, we have realised that we can go no further without discussing something of incredible importance: stellar evolution. There are two main types of stellar evolution: high mass and low mass. We’ll be discussing low-mass evolution this time—the stars that don’t go bang.
First, let’s define what a “low mass” star is. The technical definition means any sun 4 M⊙ (M⊙ stands for “mass of the Sun”, and the dot-in-circle thing is the symbol for sun) or less. While these stars are actually not all that small, they are very low mass in comparison with the high-mass stars that do go bang.
These stars start out as any other—as part of a collapsing cloud of gas that becomes a protostar (Figure 1). Any protostars with masses less than about 0.08 M⊙ never get hot enough to begin nuclear fusion. As we mentioned last week, these are known as brown dwarfs (Figure 2), and anything smaller than a brown dwarf is considered a planet.
For all other stars in the protostar stage, their core temperature will eventually hit the critical temperature to begin nuclear fusion—10 million K. At this point, the star quickly becomes stable and enters into the main sequence (Figure 3). For a low-mass star, the Main Sequence stage takes billions or even trillions of years, and is where the star spends most of its lifetime. Hydrogen fusion through the P-P chain or the CNO cycle is constant and efficient, and for the most part, the star is very stable.
Now this is where the differences in high mass and low mass stars begin. Low mass stars of less than 0.5 M⊙ will never begin helium fusion after their hydrogen fusion supply runs out. These are red dwarfs (Figure 4), and they simply cool off and fade away after billions and billions of years, leaving behind just a white dwarf. The reason they can stay on the main sequence so long is because hydrogen fusion proceeds at a slower rate since the star has less mass. Also, especially for lower-mass red dwarfs, heat transport by convection keeps helium from building up at the core, allowing the star to burn a larger proportion of its hydrogen. Red dwarfs are the most common kind of star in the universe, but are very hard to detect because they emit so little radiation. Our nearest star neighbour, Proxima Centauri, is a red dwarf.
Slightly higher-mass stars of 0.5-10 M⊙ will actually become red giants instead of dying a slow, boring death like the extremely-low mass stars. What happens in these stars is that after fusion in the core stops fusing hydrogen to helium, the star contracts, allowing the temperature and pressure to rise high enough for a hydrogen-burning shell to ignite. This is the subgiant branch. The helium core keeps growing until it reaches something called the Schönberg-Chandrasekhar Limit, which causes the core to contract rapidly. After this, the gravitational energy released from the core causes the immediate outer layer of the star to expand a bit and the temperature drops a bit as well. The helium core continues to contract, and it heats the hydrogen shell above it, where fusion speeds up. This increased rate of fusion means more energy is released, which makes the star become anywhere from 1,000-10,000 times brighter. The layers also expand a lot due to the radiation pressure. This, friends, is the stage that we call red giant (Figure 5). During the red giant branch, a convection zone forms near the surface and eventually reaches down nearly to the core, transporting material from the interior to the surface—this is called the first dredge-up (the second and third dredge-ups occur later, during the helium-burning phases).
This is where things start to become really interesting—that is, if you’re a larger star. Once hydrogen fusion goes on long enough, and temperature (about 108 K) and pressure become high enough, there will be a brilliant helium flash to start helium to carbon fusion—if the mass is low enough (under 1.4 M⊙). If not, then helium fusion will begin fairly quietly, for only in lower-mass stars does the helium core become degenerate before fusion ignites. Fusion occurs in the core through the triple alpha process, although the hydrogen shell is still burning around it. This allows the star to move onto the Horizontal Branch, where its equilibrium is mostly restored and it burns relatively quietly. However, helium runs out faster than hydrogen did. Now the star enters the Asymptotic Giant Branch, with an inert carbon-oxygen core and two shells burning helium and hydrogen. The AGB is dubbed “Asymptotic” because the star now has a higher temperature and its path on the HR diagram seems to use the original path on the Red Giant Branch as an asymptote.
However, low-mass stars don’t have enough mass to create the pressures and temperatures required to fuse carbon, so even as the core contracts, no further fusion ignites there. Helium fusion is very, VERY dependent upon temperature—a small difference can lead to a large difference in energy output. Along with strong stellar winds, these thermal pulses cast off the outer layers of the star, which will become a planetary nebula (Figure 6).
Now, all that is left of the original star is the nebula and a white dwarf (Figure 7), which is just a compressed core of hot carbon supported by electron degeneracy pressure, with a little bit of other elements like oxygen or neon. Eventually the white dwarf will cool off and become a black dwarf when it no longer radiates a significant amount of heat. However, it takes so long for white dwarfs to cool off that astronomers have not yet observed a single black dwarf, and they are really only theoretical objects right now.
Thus ends the sad life of a low-mass star.
TL;DR: The life of a low mass star starts in a gas cloud which then becomes a protostar. If it’s a really small star, it turns into a brown dwarf. If it is larger, it starts nuclear fusion and becomes a main sequence star. Billions of years after this, the main sequence star will turn into a red giant. Larger stars will start helium fusion after the red giant phase, and eventually expel their outer layers. These outer layers turn into planetary nebulae, and the core becomes a white dwarf that eventually cools to form a black dwarf.
http://cosmos.phy.tufts.edu/~zirbel/ast21/handouts/StellarEvolution.PDF (the last page of this has some really nice HR diagrams with evolution tracks)
Carroll and Ostlie, An Introduction to Modern Astrophysics, 2nd ed. (p. 447-451, 457-474)