Lecture 19 Flashcards
What is the composition of the sun?
- 91% hydrogen
- 8.9% helium
- 0.1% other elements
What are some of the first theories about how the sun produced its energy?
- Chemical burning: wrong since the sun will have run out of energy
- Shrinking: provided a more reasonable age for the sun
How does Einstein’s equation explain the energy produced by the sun?
E=mc^2
The energy densities for nuclear reactions are much greater than chemical
What is nuclear fission?
A large nucleus splits into smaller pieces
What is nuclear fusion?
Smaller nuclei combine to create one large one- this is the process that occurs in stars
Why can nuclear fusion only occur within stars?
- 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
What is p-p chain fusion?
- 4 protons in
- 1 He, 2 photons, 2 positrons and 2 neutrinos out
What is the mass deficit caused by?
When nuclei combine some of the mass is converted to energy, producing a particle with a smaller mass than the original
What do neutrinos from the sun provide evidence of?
Neutrinos coming from the sun provide direct evidence for nuclear fusion occurring in the core of the sun
What are neutrinos?
Extremely small, charged particles that travel close to the speed of light. Neutrinos (almost) do not react with other matter
How does gravitational equilibrium affect the sun?
The gravity of the mass pulling inwards is countered by the pressure from fusion pushing out
What affects does gravity have on the core of the sun?
- Increases its temperature to 15 million K
- High density
- High pressure
What are some of the fusion products of the sun?
- helium
- neutrinos
- gamma rays
- positrons (annihilate electrons to produce more gamma rays)
How does energy leave the sun?
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)
What is the convection zone?
- 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
What is the photosphere?
- The visible surface of the sun where photons can leave- T=5800K
What is granulation on the surface of the sun caused by?
Convection
What are sunspots?
- Darker, cooler regions on the surface
- Caused by magnetic fields trapping gas in huge, bubbling loops
Why do the field lines of the sun strangely arranged?
Differential rotation- the sun rotates differently at different latitudes and “winds itself up”
- 24 days at the equator and 39 days at the poles
What is the chromosphere?
The incredibly low density layer just above the photosphere. It has a temperature of around 6000-10000K
What are the differences between the photosphere and the chromosphere?
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
What is a prominence?
The loop standing off the surface of a sunspot
What is the corona?
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
How is solar wind formed by the corona?
- 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
How are particles from the solar wind affected by the Earth?
- most particles are deflected by Earth’s magnetic field
- the rest are directed along the magnetic field to the poles
What is the heliosphere and how far does it extend?
The heliosphere is the extension of the solar wind and it extends to about 100AU past the sun
