Once a star begins its life as a red giant, where does fusion take place and what is being fused?

• Helium fused in the core

• Hydrogen fused in the core

• Helium fused in a shell around the core

• Hydrogen fused in a shell around the core

Why are red giants red?

• Their increased luminosity makes the star expand

• The star cools off because fusion has stopped

• These stars always were red

• When helium burns, it burns red

Answer explanation

Red giants are red because the hydrogen fusion in the shell combined with the heat released by the shrinking helium core increases the luminosity, which increase in luminosity causes the star to expand. The outer layers of this now expanded star can cool and that makes the star turn red.

What mass range of stars undergoes a helium flash and why?

• Stars greater than 2 solar masses; their cores become degenerate

• Stars greater than 2 solar masses; their cores do not become degenerate

• Stars less than 2 solar masses; their cores become degenerate

• Stars less than 2 solar masses; their cores do not become degenerate

Answer explanation

Because the cores of these lower mass stars are degenerate, the higher temperature from fusion doesn't expand the core, so then there is a runaway nuclear fusion event that outputs a lot of energy, leading to the helium flash.

Which of these is NOT happening in an asymptotic giant branch (AGB) star?

• Oxygen is inert in the core

• Helium is burning in a shell

• Carbon is inert in the core

• Carbon is burning in a shell

Answer explanation

In an AGB star, hydrogen and helium are burning in shells around an inert carbon/oxygen core.

What's left behind after an AGB star blows its outside layers away?

• A white dwarf

• A neutron star

• A black hole

• Nothing

Which of these events can form elements heavier than iron?

• Fusion in the cores of stars

• Supernova explosions

• Cosmic ray fusion

• The Big Bang

Answer explanation

According to Lecture 15, slide 25, many heavier elements are made from the explosions of massive stars (supernovae), but also in neutron star mergers and in the death throes of low mass stars.

Why don't stars that fuse iron in their cores remain stable?

• Elements that massive consume energy during fusion

• No star is massive enough to fuse iron

• Iron fusion releases so much energy the star explodes

• Iron isotopes made in stars are too unstable for fusion

Answer explanation

The fact that iron and heavier elements consume energy during fusion is a key part of the iron catastrophe that leads to supernovae. Because of that consumption, the burning of iron accelerates the collapse of the star.

Which of these is the densest?

• White dwarf stars

• Red dwarf stars

• O type stars

• Neutron stars

Answer explanation

Given in the lecture, one teaspoon of neutron star would weigh about as much as 1.4 billion people.

Neutron stars are very hot, but extremely tiny. Therefore, they have low luminosities. How do we find them?

• We detect their x-ray emissions

• We use polarized light to detect their magnetic fields

• Their star quakes release a lot of light

• They periodically shine beams of light at us

Answer explanation

We call a neutron star a pulsar when its jets of energy point toward earth at times during its rotation. Seeing those jets is the main way we detect neutron stars.

How massive should a main sequence star be to turn into a black hole?

• 3M_sun < M < 8M_sun

• 8M_sun < M < 25M_sun

• M > 25M_sun

• 1M_sun < M < 3M_sun