Lives in the Balance Life as a Low Mass Star

Star mass categories: Low-mass stars: born with less than about 2 M Sun Intermediate-mass stars: born with between about 2 and 8 M Sun High-mass stars: born with more than about 8 M Sun Similar in old age They are all very different in death (next chapter!)

The convection zones of stars of different masses vary In smaller stars, the convection zone is deeper in lower-mass stars In larger stars, the convection zone is inside the radiation zone. Almost all of the hydrogen in the convection zone ends up being fused. In the very smallest stars, the entire star is convective. It’s going to fuse ALL of its hydrogen.

During their time on the main sequence, stars’ cores slowly shrink. That raises the temperature of the core, increasing the rate of hydrogen fusion. That makes them brighter, and causes their outer layers to swell.

When they run out of hydrogen fuel, the core is no longer generating heat. The decrease in thermal pressure means that gravity is stronger than thermal pressure, and the helium core and hydrogen outside it begin to contract. That increases temperatures. Soon, the hydrogen shell around the helium core gets hot enough for hydrogen fusion to begin…

The hydrogen fusing shell releases a lot of energy, causing the star’s luminosity to grow, and the outer layers to expand. This is the red giant stage

Where is the energy generated that causes the outer layers of the star to expand? A) In the hydrogen fusing core B) In the helium fusing core C) In the hydrogen fusing shell D) The energy is due only to contraction

The contraction of the core continues because newly formed helium from the shell adds to the mass of the core As contraction continues, the core and shell get hotter and hotter. This is a run-away process. At the same time, the larger radius means that the outer layers are held by a smaller gravitational pull, and so much of the outer layers are blown away in the stellar wind. The lowest mass stars will stop contracting due to degeneracy pressure before anything else happens. They will then be called “helium white dwarfs” Contraction continues until degeneracy pressure stops it.

The core will continue to heat up due to the added helium from the fusing shell. Eventually it gets hot enough for the helium to begin fusing. Since the core is being held up by degeneracy pressure, it does not expand when core fusion starts. Instead, it just gets hotter, causing fusion to go faster, making it hotter still, etc. Finally it gets hot enough that thermal pressure exceeds degeneracy pressure, and the core starts to expand and cool. It will find a new equilibrium, at a total energy output lower than it had at its peak. The triple alpha process

It will now have hydrogen fusion going on in a shell around the core, and helium fusion going on the core. It will be resting on the “horizontal branch” in the H-R diagram.

Eventually the helium in the center of the core gets used up. The core will then be made of carbon, and it will begin collapsing again… Helium fusion can start in a shell around that inner core of carbon It never reaches a stable equilibrium, but instead proceeds in a series of thermal pulses

Those thermal pulses cause the star to blow out most of its outer layers into space The core will collapse until degeneracy pressure stops it. It will then be a dead white dwarf, surrounded by a…

Planetary nebula Dumbbell nebula Ring nebula

Eskimo nebula Cat’s Eye nebula

Ant nebula Spirograph nebula

Astro-Cash Cab! Tabitha Pauline Rebecca Derek

1) Put the following events, related to when a star runs out of hydrogen fuel in its core, into the correct cause  effect (time sequence) order: Hydrogen fusion in shell begins Outer layers of star expand Gravity causes the core and shell to contract

2) What is a red giant star fusing in its core while it sits on the horizontal branch?

3) True / False ALL stars will fuse helium in their cores at some point.

4) What fusion reactions are going on inside a star that is undergoing “thermal pulses” and creating a planetary nebula? (Choose ALL that are true) Hydrogen fusion in the core Helium fusion in the core Hydrogen fusion in a shell Helium fusion in a shell