5.5 Astrophysics (Evolution of Stars)

Question 1

Define planet

Answer 1

An object in orbit around a star with a mass large enough for its own gravity to give it a round shape, that undergoes no fusion and that has cleared its orbit of (most) other objects.

Question 2

Define dwarf planets

Answer 2

Planets where the orbit has not been cleared of other objects.

Question 3

Define planetary satellites

Answer 3

A body in orbit around a planet (natural or manmade)

Question 4

Define asteroids

Answer 4

Objects which are too small and uneven in shape to be planets, with a near circular orbit around the sun.

Question 5

Define comets

Answer 5

A small irregular body made of dust, ice and rock. They orbit stars in highly eccentric elliptical orbits.

Question 6

Define solar system

Answer 6

A planetary system consisting of a star and at least one planet in orbit around it.

Question 7

Define galaxy.

Answer 7

A collection of stars, interstellar dust and gas bound together by their mutual gravitational attraction.

Question 8

What are the two types of galaxy?

Answer 8

Elliptical
Normal/Barred spiral

Question 9

Each galaxy contains around how many stars?

Answer 9

100 billion.

Question 10

Define nebula

Answer 10

A gigantic cloud of dust and gas in space.

Question 11

What do all stars begin as?

Answer 11

A nebula.

Question 12

Describe the process where a nebula turns into a protostar. [4]

Answer 12

• Over millions of years, the gravitational attraction between dust and gas particles pulls them together to form clouds.
• As they come closer together, the gravitational collapse accelerates, and some regions become denser and pull in more dust and gas.
• The GPE of the particles is converted to thermal energy.
• The resultant sphere of very hot, dense dust and gas is a protostar.

Question 13

Does fusion occur within a protostar?

Answer 13

No.

Question 14

Protostars are dense. What two quantities are increased as a result of this?

Answer 14

Pressure and temperature.

Question 15

For a star to form from a protostar, what must happen? [2]

Answer 15

The temperature and pressure must be high enough for hydrogen gas nuclei in the protostar to overcome the electrostatic forces of repulsion, and undergo nuclear fusion.

This nuclear fusion produces helium nuclei, producing a star.

Question 16

When a star is formed, it initially remains in stable equilibrium. What is this equilibrium known as?

Answer 16

The main phase of the star.

Question 17

What is the main phase of a star? [2]

Answer 17

A stable equilibrium.
The gravitational forces of the particles act to compress the star, but radiation pressure from photons emitted in fusion and gas pressure from nuclei in the core counteract this, keeping the size of the star almost constant.

Question 18

What is the difference between a larger star and a smaller star? [2]

Answer 18

Larger stars are hotter, and so undergo fusion faster, using up available hydrogen nuclei more quickly.
This means they have a shorter main phase than smaller stars.

Question 19

What happens after the main phase of a star depends on a certain factor. What is this factor?

Answer 19

The mass of the star’s core.

Question 20

Define the core of a star.

Answer 20

The central region of the star where fusion occurs.

Question 21

What is a solar mass?

Answer 21

1M☉ is the mass of our sun (2x10^30)

Question 22

Low mass stars are classed as having what solar mass?

Answer 22

Between 0.5M☉ and 10M☉

Question 23

Why do low mass stars remain in the main sequence for longer?

Answer 23

They have smaller, cooler cores.

Question 24

How are red giants formed from low mass stars?

Answer 24

When the hydrogen supplies are low, the gravitational forces inwards overcome the radiation and gas pressures, causing the star to collapse inwards. It evolves into a red giant.

Question 25

Why does fusion occur within the outer shell of a red giant, but not the core?

Answer 25

The core is too cool for helium to fuse. However, the pressure in the outer shell is great enough for fusion to occur there.

Question 26

Describe the evolution of a red giant into a white dwarf.

Answer 26

As helium nuclei run low, the red giant evolves into a white dwarf. The outer shells begin to drift off into space as a planetary nebula, and the core remains as a very dense white dwarf.

Question 27

What is the temperature of a white dwarf?

Answer 27

Around 3000K

Question 28

Does fusion occur within a white dwarf?

Answer 28

No.

Question 29

How is heat dissipated from a white dwarf?

Answer 29

Photons produced earlier in the evolution of the star leak out, dissipating heat.

Question 30

What prevents the core of a white dwarf from collapsing?

Answer 30

Electron degeneracy pressure (caused as two electrons cannot exist at the same point, so there is repulsion as they are close together)

Question 31

A white dwarf remains stable so long as the core’s mass is below what?

Answer 31

1.44M☉ (or the Chandrasekhar limit)

Question 32

A star will take a different evolution path when its mass is in excess of what?

Answer 32

10M☉

Question 33

Describe the evolution of a high mass star into a red supergiant.

Answer 33

As hydrogen supplies deplete, the core begins to contract, making the temperature high enough for helium fusion to occur. Helium fusion will produce heavier elements. This releases even more energy, which causes the outer layers of the star to expand and cool, forming a red supergiant.

Question 34

High mass stars burn hydrogen quickly. How does this affect their lifetimes?

Answer 34

It reduces it.

Question 35

Describe features of a red supergiant.
[Teacher Verify]

Answer 35

A massive star with a low surface temperaure, giving it a reddish appearance. It has layers of increasingly heavy elements produced from fusion, and surrounding lighter layers undergoing fusion reactions. It has an inert iron core. The outer layers expand and cool, until it undergoes a supernova explosion

Question 36

How does a red supergiant become unstable?

Answer 36

The instability of a red supergiant is caused by the exhaustion of fuel for fusion reactions in its core, which leads to the collapse of the core and the production of an iron core. The iron core triggers a type 2 supernova explosion, which ejects the materials in the outer shells out into space.

Question 37

What happens to the core of a red supergiant after a supernova explosion.

Answer 37

If the remaining core mass is between 1.44M☉ and about 3M☉, it collapses to form a neutron star. If the remaining core mass is above 3M☉, the gravitational forces are so strong that the escape velocity of the core is greater than the speed of light, and it forms a black hole from which not even light can escape.

Question 38

What is the chandrasekhar limit?

Answer 38

1.44M☉. It is the maximum possible mass of a white dwarf. At any mass beyond this, the gravitational forces would be too strong compared to the electron degeneracy pressure, and it would collapse.