Life as a Low-Mass Star What are the life stages of a low-mass star? How does a low-mass star die?

Life as a Low-Mass Star

• What are the life stages of a low-mass star?• How does a low-mass star die?

Before we begin lets have some revision questions on how stars are born (i.e. the previous lecture).

What is the first step in the formation of aprotostar?

A. Conservation of angular momentum causes galactic material to make a disk.

B. Gravity causes planetesimals to begin to accumulate.

C. Gravity causes a cloud of gas and dust to begin to contract.

D. Nuclear fusion heats material and causes it to glow.

Why do we think that clouds of gas and dust form stars?

A. We see young star clusters with gas and dust around them.

B. Infrared and microwave telescopes let us see inside dust clouds and see protostars.

C. Computer models predict that if a cloud has enough mass it will contract from the pull of gravity, heat up, and form a star.

D. The Hubble Telescope lets us watch stars form before our eyes.

E. A, B, and C

What is the energy source that heats a contracting protostar?

A. Friction

B. Pressure, as the gas and dust are compressed

C. Gravitational potential energy released as the material is pulled inward

D. Fusion

E. Kinetic energy

Why do protostars rotate rather fast and end up surrounded by disks of material?

A. The galaxy is rotating, so all the stars that form are rotating as well.

B. If a cloud spins even a little bit, the spin increases as it contracts.

C. The conservation of angular momentum causes this to occur.

D. All of the above.E. B and C

Why can’t a cloud with less than 0.08 times themass of the Sun become a star?

A. It will break into pieces.

B. Gravity will be too weak, and it will come apart.

C. It will never get hot enough for fusion to start.

D. It will become a star, but a small and faint one.

Life as a Low-Mass Star

• What are the life stages of a low-mass star?• How does a low-mass star die?

What are the life stages of a low-mass star?

A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.

Main-Sequence Lifetimes and Stellar Masses

Thought Question

What happens when a star can no longer fuse hydrogen to helium in its core?

A. Its core cools off.B. Its core shrinks and heats up.C. Its core expands and heats up.D. Helium fusion immediately begins.

Life Track After Main Sequence

• Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.

Broken Thermostat

• As the core contracts, H begins fusing to He in a shell around the core.

• Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting.

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion.

The fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon.

Helium Flash

• The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core.

• The core temperature rises rapidly when helium fusion begins.

• The helium fusion rate skyrockets until thermal pressure takes over and expands the core again.

Helium core-fusion stars neither shrink nor grow because the core thermostat is temporarily fixed.

Life Track After Helium Flash

• Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.

Life Track After Helium Flash

• Observations of star clusters agree with these models.

• Helium core-fusion stars are found in a horizontal branch on the H-R diagram.

How does a low-mass star die?

Thought Question

What happens when a star’s core runs out of helium?

A. The star explodes.B. Carbon fusion begins.C. The core cools off.D. Helium fuses in a shell around the core.

Double Shell Fusion• After core helium fusion stops, He fuses into

carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer.

• This double shell–fusion stage never reaches equilibrium—the fusion rate periodically spikes upward in a series of thermal pulses.

• These pulse cause the star to periodically throw of its outer layers.

Planetary Nebulae

• Double shell–fusion ends with a pulse that ejects the H and He into space as a planetary nebula.

• The core left behind becomes a white dwarf.

Planetary Nebulae

• Double shell–fusion ends with a pulse that ejects the H and He into space as a planetary nebula.

• The core left behind becomes a white dwarf.

End of Fusion• Fusion progresses no further in a low-mass star

because the core temperature never grows hot enough for fusion of heavier elements (some He fuses to C to make oxygen).

• Degeneracy pressure supports the white dwarf against gravity.

• Degeneracy pressure is due to the fact that all electron states fill up and it gets harder to contract\compress the star as the electrons cannot get closer together.

Life stages of a low-mass star such as the Sun

The Death Sequence of the Sun

Life Track of a Sun-Like Star

What are the life stages of a high-mass star?

When does a star leave the main sequence?

A. After a few million years

B. After a few billion years

C. It depends on its mass

D. When the hydrogen fuel in its core is used up

E. C and D

What happens to nuclear fusion when thehydrogen in a star’s core runs low?

A. It stops.B. It shifts from the core to a shell around the core.C. Other elements start to fuse.D. All of the above.

In a red giant star, three helium atoms (4He) can fuse together to … ?

A. Make more energy for the star

B. Give the star additional life

C. Create the element carbon (12C)

D. A and C

E. A, B, and C

After the Sun becomes a red giant star and makes carbon in its core, why will it not make heavier elements?

A. It will have run out of fuel

B. It will be near the end of its life and doesn’t have time

C. It will not be massive enough to make it hot enough for further reactions

D. The heavier elements will all go into a planetary nebula

E. A and B

After the Sun becomes a red giant, it will shedmuch of its atmosphere in … ?

A. A post-stellar nebula

B. A planetary nebula

C. A stellar nebula

D. A supernova explosion

• Now work on the Lecture Tutorial, Luminosity, Temperature and Size.

Recall we can increase the Luminosity of a blackbody by increasing its temperature or increasing its size (radius). Stars behave in the same way.