Death of stars
1) What kinds of nuclear reactions occur within the Sun as it ages? The Sun’s core first
fuses Hydrogen into Helium, and then it will fuse Helium into Carbon. Then it will not
have enough pressure to fuse the carbon, so it will die out.
2) Where did the carbon atoms in our bodies come from? All atoms in the universe other
that hydrogen and helium are produced in stars so the carbon atoms in our bodies come
from stars.
3) What is a planetary nebula and what does it have to do with planets. A planetary
nebula is the expelled layers of gas and dust around a low-mass (less than 4 solar masses)
star in the late stages of their life. These stars expel a lot of material into the space
around them. Through a telescope they look like a ghostly planet. Otherwise they have
no connection to planets.
4) What is a white dwarf? When stars with low masses reach the end of their lives they
expel a lot of their mass and what remains is an object that is mostly carbon and oxygen.
A white dwarf can have up to 1.4 solar masses packed into a volume the size of the Earth
or even smaller.
5) What is a neutron star? A neutron star is the end stage of the life of a large star. A
neutron star can have several solar masses packed into an object that is only 10 or 20
miles in diameter.
6) Why do neutron stars spin very fast? Because of conservation of angular momentum.
Just as an ice skater spins faster when she pulls her arms in, a star spins faster when it
collapses inward. A star can go from a million miles in diameter to a few miles in
diameter (neutron star) and this can cause rotation times of less than a tenth of a second.
7) What is a pulsar? A pulsar is a neutron star that is producing radio wave emissions
from its poles due to its high rate of spin. The spinning causes the radio waves to be
directed in a circular motion so far away we see a “pulse.”
8) What is a black hole? A black hole is what happens in the core collapse of very large
stars (more than about 8 solar masses) in their final stages when their core collapses.
Gravity becomes so strong that nothing can stop the collapse of the core. Even light
cannot escape a black hole, so it is impossible to see anything in it.
9) Why do high mass stars go through more evolutionary changes than low-mass stars.
Because there is enough mass in high mass stars to cause the material in the core to fuse a
few more times. High mass stars can fuse Carbon into Neon, and then neon into silicon.
10) What happens to a high mass star to turn it into a supernova? The mass in its core is
so great that after it goes through all levels of core fusion, the gravity is overpowers the
pressure and the core collapses. As the core falls in the electrons and protons combine to
form neutrons which suddenly resist any further compression. This causes all the
material falling in to “core bounce” and the star undergoes a massive explosion.
11) How luminous can a supernova be relative to the Sun? A supernova can outshine ten
billion suns for a brief time.
12) What happens that causes a star to leave the main sequence on an H-R diagram?
13) Where do low-mass stars at the end of their lives appear on an H-R diagram?
14) Where are red giants on an H-R diagram?