The Life and Death of Stars

Question 1

What does the total time that a star spends as a main-sequence star depend on?

Answer 1

1) the amount of hydrogen in the core.

2) rate at which it is consumed

Question 2

Why do more massive stars have shorter lifetimes?

Answer 2

Although more massive stars have more fuel (hydrogen in their cores), they fuse it at much higher rates because they are so much hotter. Thus they use up their fuel.

Question 3

What is the Sun’s main-sequence lifetime?

Answer 3

10 to 12 billion years

Question 4

Describe the relationship between mass and main sequence lifetime.

Answer 4

As mass increases, main sequence lifetime decreases.

Question 5

OBAFGKM Classification

Answer 5

O stars are big and blue, and have shorter lives. M stars are smaller and redder, and have longer lives.

Question 6

Amount of solar masses for a low mass star

Answer 6

less than 8 solar masses

Question 7

Amount of solar masses for a high mass star

Answer 7

greater than 8 solar masses

Question 8

How do red giants get so huge? (EDIT)

Answer 8

Gravity pushes in, squeezes and contracts, while gas pressure pushes out and expands. Gravity increases when mass increases or when radius decreases. Gas pressure increases when temperature increatures….

Question 9

When is the end of main-sequence?

Answer 9

When there is no more hydrogen to fuse in the core.

Question 10

What happens when there is no more fusion in a star’s core?

Answer 10

Core temperature drops, then gas pressure in core drops. Gravity wins in the combat between internal pushing outward of gas pressure and therefore the core shrinks. Because the core shrinks, it heats up again even more than before and this hotter core heats up the surrounding shell of hydrogen (the hydrogen that was just outside the core). The hydrogen shell gets hot enough to fuse hydrogen, and because the temperature is so high, the fusion rate is huge. Gas pressure increases disproportionally, beating gravity, and pushes outer layers of the star so they expand dramatically. Because of expansion, they cool down. The result is a red giant.

Question 11

What happens to Earth when the sun is expected to start turning into a red giant in 5 billion years?

Answer 11

Oceans will boil 3-4 billion years from now. Then in 5-7 billion years the Sun will leave the main-sequence stage. About 700 million years later the red-giant Sun will envelop Earth and vaporize inner planets an evaporate the atmosphere of outer planets.

Question 12

What happens to the temperature of the core of a star if the core continues to shrink?

Answer 12

When core temperature reaches 100 million Kelvin, Helium fusion begins. Helium fusion requires higher temperatures than Hydrogen fusion because its larger charge (two protons in each nucleus) leads to greater repulsion. Fusion of two helium nuclei doesn’t work, so helium fusion must combine three helium nuclei to make carbon. Helium-burning stars neither shrink nor grow because balance is temporarily fixed. But this phase doesn’t last very long.

Question 13

What happens when the star’s core runs out of helium (i.e. when all the helium has been fused into carbon)?

Answer 13

Carbon only fuses above 600 million K, and since low mass star cores never get that hot, then these less than 8 solar masses stars turn into planetary nebulas.

Question 14

How does a planetary nebula form?

Answer 14

For stars less than 8 solar masses, gravity can’t hold outer layers of gas and dusts and thus they are expelled. As debris moves away, the hot, dense core becomes visible. Hot core emits UV radiation, which heats and ionizes surrounding gas. Gas begins to glow, forming “planetary nubula”. (The core is a white dwarf, the planetary nebula is the outer layers).

Question 15

What forms from the eventual fading of planetary nebulae?

Answer 15

When the shell spreads out far from the cooling star, it ceases to glow. Eventually, nebulae’s gases mix with interstellar medium. Later, new stars will be born from this interstellar medium.

Question 16

What happens to cores that are left behind after low-mass stars die?

Answer 16

They become white dwarfs.

Question 17

How are white dwarfs formed?

Answer 17

After a star expels its outer layers (planetary nebula), the core shrinks to about the size of Earth. This exposed core is no longer able to have fusion of any kind, so it slowly cools down. At first, it is bright (UV thermal radiation), but it becomes dimmer (and redder) as it cools.

Question 18

What makes white dwarfs stop shrinking?

Answer 18

Not gas pressure: white dwarfs don’t have any kind of fusion, so they can’t keep producing heat and so gas pressure is not enough to beat gravity. They stop shrinking from degeneracy pressure.

Question 19

What is degeneracy pressure?

Answer 19

A type of pressure that arises when electrons or neutrons are packed so tightly that the exclusion and uncertainty principles come into play. The exclusion principle says two (or more) electrons or neutrons cannot be in the same state at the same time. Therefore electrons/neutrons have to move faster to find empty states. This motion creates a pressure called degeneracy pressure.

Question 20

If gas pressure depends on temperature, what does degeneracy pressure depend on?

Answer 20

Density (i.e. how “squeezed” matter is)

Question 21

How dense are white dwarfs?

Answer 21

White dwarfs are a million times denser than water: one teaspoon of white dwarf material is as heavy as an elephant! Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space, forcing them to go faster.

Question 22

Is there a limit to how much mass a white dwarf can support?

Answer 22

Einstein’s theory of relativity says that nothing can move faster than light. When electron speeds in white dwarf approach speed of light, electron degeneracy pressure can’t increase any longer, so it can no longer support the star against collapse. A white dwarf more massive than 1.4 solar masses would collapse under its own gravity.

Question 23

What is the Chandrasekhar Limit?

Answer 23

The limit of a white dwarfs mass. A white dwarf more massive than 1.4 solar masses would collapse under its own gravity. (Increase of mass results in the decrease of the radius).

Question 24

What is the difference in the stages of life and death of a high-mass star versus a low-mass star?

Answer 24

Remember, low mass stars cannot reach the level of heat needed for carbon fusion after helium fusion. A high-mass star can because hydrogen fuses to helium in its core at a much faster rate. This results in a red supergiant: when hydrogen fuses to Helium in a shell around inert Helium core. Helium-burning supergiant: helium fuses to carbon in core.
High-mass stars can also fuse heavier elements because the core can continue to get hotter, allowing for fusion of heavier elements.

Question 25

What heavy elements can a high-mass star fuse?

Answer 25

Thermonuclear reactions in core and shells include carbon fusion, neon fusion, oxygen fusion, and silicon fusion. (The basic elements for life - C, N, O, etc - are all made in stars).

Question 26

What is the core of a red supergiant made of that cannot be fused?

Answer 26

Iron

Question 27

How do high-mass stars die?

Answer 27

High-mass stars die in violent cataclysm. At the star’s final stage, the iron core is surrounded by shells fusing lighter elements. Core is more massive than 1.4 solar masses, so electron degeneracy pressure cannot stop the collapse. So, the core keeps contracting and it contracts so much that protons and electrons combine into neutrons (they “neutronize”). The core collapses into a few km ball of neutrons in a fraction of a second. Neutrons then become degenerate, forming a neutron star.

Question 28

What happens from the collapse of a high-mass star?

Answer 28

The collapse releases enormous amounts of energy. A shock wave propels star’s upper layers into space and most matter is ejected at high speeds in a bright explosion. This is a supernova, specifically a type II supernova. Elements heavier than iron are formed during this violent explosion.

Question 29

Supernova remnants

Answer 29

the “nebula” left behind after a supernova explosion.

Question 30

What are the two types of supernova?

Answer 30

Type II (Massive star supernova): Iron core of massive star reaches the Chandrasehkar limit and collapses into a neutron star, causing explosion.
Type Ia (White dwarf supernova): Carbon fusion suddenly begins when a white dwarf that is in a close binary system reaches the Chandrasehkar limit, causing a total explosion.

Question 31

Describe the process of a Type Ia Supernova.

Answer 31

Dealing with a close binary star, a white dwarf’s gravity pulls matter off of its companion when it becomes a giant. Angular momentum, however, prevents the matter from falling straight in. Infalling matter forms an accretion disk around the white dwarf. Eventually matter falls onto the surface of the white dwarf, adding to its mass. If white dwarf accretes so much mass that it exceeds the Chandrasekhar limit, it begins to collapse. During collapse, carbon fusion ignites all at once and the white dwarf explodes.

Question 32

What happens to cores of supernovae?

Answer 32

They either become neutron stars or black holes.

Question 33

What is a neutron star?

Answer 33

The ball of neutrons left behind by a massive-star supernova (Type II). Degeneracy pressure of neutrons supports neutron star against gravity. They are small but very dense. Because of the shrinking, they rotate very fast and their magnetic fields are strong because of their concentration.

Question 34

What are pulsars?

Answer 34

Rapidly rotating neutron stars that appear like a lighthouse beacon. They have misaligned rotation and magnetic axes.

Question 35

What is a neutron star’s limit and what results when it exceeds that limit?

Answer 35

If the mass of a neutron star exceeds about 3 solar masses, neutron degeneracy pressure can no longer support it against gravity. Thus, a black hole is created.

Question 36

Summarize stellar corpses.

Answer 36

When stars die, they expel most of their outer layers and they leave behind inert cores. The nature of these cores depends on how much mass they have. A white dwarf forms from a core less than 1.4 solar masses. A neutron star from core of 1.4 to 3 solar masses. A black hole from a core more than 3 solar masses.