Stellar Evolution

Question 1

What are high mass stars?

Answer 1

> 8 MSun

Question 2

What are low mass stars?

Answer 2

< 2 MSun

Question 3

Why are star clusters useful for study?

Answer 3

Star clusters are particularly useful because they contain stars of different mass that were born about the same time

Question 4

When does a star leave the main sequence?

Answer 4

A star remains on the main sequence

as long as it can fuse hydrogen into helium in its core

Question 5

What is the lifetrack of a star after the main sequence?

Answer 5

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

Question 6

What is meant by a broken thermostat?

Answer 6

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

Question 7

What is the issue with the beryllium isotope?

Answer 7

Beryllium isotope is unstable decays in 2.6 10 -16 seconds!

Question 8

What is the triple alpha process?

Answer 8

Triple alpha process produces stable Carbon nucleus. 3 (4/2He) = 12/6 C + alpha particle (energy). Stable.

Question 9

What is the binding energy of nucleon graph?

Answer 9

Relates mass per nucleon particle and atomic mass. Up to iron, elements can be fused to create energy. Down to iron, elements can undergo fission to produce energy. Fe has lowest mass per nuclear particle.

Question 10

Describe the graph of abundance of elements.

Answer 10

10^12 abundance of Hydrogen compared to 10 Li. Elements with even atomic numbers are higher in abundance because of helium + alpha particle. Odd number elements are because of a proton difference.

Question 11

Why is there more nitrogen on Earth than oxygen, despite oxygen being more abundant in the universe?

Answer 11

Oxygen reacts and will form oxides or be tied up in water. Rust doesn’t evaporate.

Question 12

What is a helium flash?

Answer 12

Thermostat is broken in low-mass red giant
because degeneracy pressure supports core. Core temperature rises rapidly when helium fusion begins. Helium fusion rate skyrockets until thermal pressure takes over and expands core again. Star bloats.

Question 13

What is the lifetrack of a star after a helium flash?

Answer 13

Models show that a red giant should

shrink and become less luminous after helium fusion begins in the core. Horizontal branch on H-R diagram

Question 14

What is double-shell burning?

Answer 14

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

Question 15

What are thermal pulses?

Answer 15

This double-shell burning stage never reaches
equilibrium—fusion rate periodically spikes upward in
a series of thermal pulses. With each spike, convection dredges carbon up from core and transports it to surface

Question 16

What are planetary nebulae?

Answer 16

Double-shell burning ends with
a pulse that ejects the H and He into
space as a planetary nebula. The core left behind is a white dwarf. As you bloat star, escape velocity drops, photosphere can eject entire atmosphere.

Question 17

How does a low mass star die?

Answer 17

With a whimper. Stars die slowly, gas disperses slowly. White dwarf becomes a black dwarf. Degeneracy pressure supports core.

Question 18

What is the speed that gas is ejected in a planetary nebula?

Answer 18

30km/s

Question 19

What is a white dwarf?

Answer 19

The inert carbon core of a low-mass star after it has ejected its atmosphere. Supported by degeneracy pressure.

Question 20

What is Earth’s fate? What will happen to the Sun’s luminosity and radius?

Answer 20

Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth. Sun’s radius will grow to near current radius of Earth’s orbit

Question 21

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

Answer 21

H fusion in core (main sequence). H fusion in shell around contracting core (red giant). He fusion in core (horizontal branch). Double-shell burning (red giant)

Question 22

What is the CNO cycle?

Answer 22

High-mass main sequence stars fuse
H to He at a higher rate using carbon,
nitrogen, and oxygen as catalysts. Greater core
temperature enables H nuclei to overcome
greater repulsion. 4 protons = 1 He nucleus + 3 gamma rays, 2 neutrinos, 2 positrons

Question 23

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

Answer 23

Main Sequence: H fuses to He in core
2. Red Supergiant: H fuses to He in shell around He core
3. Helium Core Burning: He fuses to C in core while H
fuses to He in shell
4. Multiple Shell Burning: Many elements fuse in shells
5. Supernova leaves neutron star behind

Question 24

What elements did the Big Bang make?

Answer 24

75% H, 25% He. Stars made everything else

Question 25

What is helium capture?

Answer 25

High core temperatures allow helium to fuse

with heavier elements. So He can make oxygen, neon, magnesium, etc.

Question 26

What is advanced nuclear burning?

Answer 26

Core temperatures in stars with >8MSun allow

fusion of elements as heavy as iron. Ex. 12C + 16O = 28Si. Allows elements like Si, S, Ca, Fe (56). 28Si + 28Si = 56Fe.

Question 27

What is multiple shell burning?

Answer 27

Advanced nuclear burning proceeds in a series of nested shells. Outer to inner: hydrogen, helium, carbon, oxygen, neon, magnesium, silicon, inert iron core.

Question 28

What is the evidence for helium capture?

Answer 28

Higher abundances of elements with even numbers of

protons.

Question 29

What causes a supernovae explosion?

Answer 29

Iron builds up in core until degeneracy

pressure can no longer resist gravity. Core then suddenly collapses, creating supernova explosion

Question 30

What occurs during a supernova?

Answer 30

Core degeneracy pressure goes away
because electrons combine with protons,
making neutrons and neutrinos. Neutrons collapse to
the center, forming a neutron star. Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au and U

Question 31

What is a supernova remnant? Give an example.

Answer 31

Energy released by collapse of core
drives outer layers into space. The Crab Nebula is
the remnant of the supernova seen in A.D. 1054

Question 32

What occurs to the duration, temperature and density of each stage of a high-mass star?

Answer 32

Increases in temperature, density and decreases in duration.

Question 33

What is the end life of an intermediate mass star?

Answer 33

Intermediate mass stars can make elements heavier than carbon but end as white dwarfs

Question 34

What is the Algol puzzle?

Answer 34

The binary star Algol consists of a 3.7 MSun main
sequence star and a 0.8 MSun subgiant star. Stars in Algol are close enough that matter can flow from
subgiant onto main sequence star. As it reached the end of its life and started to grow, it began to
transfer mass to its companion (mass exchange)