Lecture 19

Question 1

What is the composition of the sun?

Answer 1

• 91% hydrogen
• 8.9% helium
• 0.1% other elements

Question 2

What are some of the first theories about how the sun produced its energy?

Answer 2

• Chemical burning: wrong since the sun will have run out of energy
• Shrinking: provided a more reasonable age for the sun

Question 3

How does Einstein’s equation explain the energy produced by the sun?

Answer 3

E=mc^2
The energy densities for nuclear reactions are much greater than chemical

Question 4

What is nuclear fission?

Answer 4

A large nucleus splits into smaller pieces

Question 5

What is nuclear fusion?

Answer 5

Smaller nuclei combine to create one large one- this is the process that occurs in stars

Question 6

Why can nuclear fusion only occur within stars?

Answer 6

• Electromagnetism pushes protons apart due to them both having positive charges
• At temperatures greater than 5 million K, nuclei have enough speed for the strong force to bind them together

Question 7

What is p-p chain fusion?

Answer 7

• 4 protons in
• 1 He, 2 photons, 2 positrons and 2 neutrinos out

Question 8

What is the mass deficit caused by?

Answer 8

When nuclei combine some of the mass is converted to energy, producing a particle with a smaller mass than the original

Question 9

What do neutrinos from the sun provide evidence of?

Answer 9

Neutrinos coming from the sun provide direct evidence for nuclear fusion occurring in the core of the sun

Question 10

What are neutrinos?

Answer 10

Extremely small, charged particles that travel close to the speed of light. Neutrinos (almost) do not react with other matter

Question 11

How does gravitational equilibrium affect the sun?

Answer 11

The gravity of the mass pulling inwards is countered by the pressure from fusion pushing out

Question 12

What affects does gravity have on the core of the sun?

Answer 12

• Increases its temperature to 15 million K
• High density
• High pressure

Question 13

What are some of the fusion products of the sun?

Answer 13

• helium
• neutrinos
• gamma rays
• positrons (annihilate electrons to produce more gamma rays)

Question 14

How does energy leave the sun?

Answer 14

Gamma ray photons leave the sun via radiation. As the interior of the sun is very dense the photons are continuously absorbed and re-emitted until they leave (this takes about 100,000 years)

Question 15

What is the convection zone?

Answer 15

• It is an area where the gas has cooled to only a few million K- photons can no longer be redirected and are absorbed
• Heat now travels via convection

Question 16

What is the photosphere?

Answer 16

• The visible surface of the sun where photons can leave- T=5800K

Question 17

What is granulation on the surface of the sun caused by?

Answer 17

Convection

Question 18

What are sunspots?

Answer 18

• Darker, cooler regions on the surface
• Caused by magnetic fields trapping gas in huge, bubbling loops

Question 19

Why do the field lines of the sun strangely arranged?

Answer 19

Differential rotation- the sun rotates differently at different latitudes and “winds itself up”
- 24 days at the equator and 39 days at the poles

Question 20

What is the chromosphere?

Answer 20

The incredibly low density layer just above the photosphere. It has a temperature of around 6000-10000K

Question 21

What are the differences between the photosphere and the chromosphere?

Answer 21

Photosphere:
- Temperature drops with altitude (6000-4000K)
- Low density
- Absorption lines
- Sunspots and granulation

Chromosphere:
- Temperature rises with altitude (4000- 20,000K)
- Very low density
- Emission spectrum
- Prominences and spiculae

Question 22

What is a prominence?

Answer 22

The loop standing off the surface of a sunspot

Question 23

What is the corona?

Answer 23

The extremely low density very hot (1 million K) layer of the sun. It is the origin of most of the sun’s x-rays

Question 24

How is solar wind formed by the corona?

Answer 24

• the corona is constantly evaporating- mainly protons and electrons
• the corona can “evaporate” as the gas is hot enough to escape the sun’s gravity

Question 25

How are particles from the solar wind affected by the Earth?

Answer 25

• most particles are deflected by Earth’s magnetic field
• the rest are directed along the magnetic field to the poles

Question 26

What is the heliosphere and how far does it extend?

Answer 26

The heliosphere is the extension of the solar wind and it extends to about 100AU past the sun