Physics Exam (Chapter 9)

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What is the composition of the interstellar medium
about 75% hydrogen, 25% helium, 1% interstellar dust, with traces of heavier elements
How is a nebula different from the interstellar medium
A nebula can be seen with visible light
Why are these clouds of interstellar media stable
Gas pressure outward balances gravity inward. Turbulence and magnetic elds also help gas pressure balance against gravity
What is the source of energy in a protostar
Gravitational potential energy is transformed into heat as the protostar contracts
What forces are in balance in a star on the main sequence
Gas pressure outward balances gravity inward
What factors do gas pressure and gravity depend on
Gas pressure depends on temperature, which in turn, depends on the rate of nuclear fusion (for a main sequence star). Gravity depends on how large a star's mass is
What are the characteristics of a main sequence star
(a) It's the earliest stage of a star's life. (b) It is still contracting and heating up. (c) It is surrounded by a disk of gas and dust
What is the proton-proton chain? What type of nuclear reaction is it
4 protons fuse together to form helium and energy. Nuclear fusion
What is the difference between the proton-proton chain and nuclear fission
The proton-proton chain takes the smaller hydrogen nuclei and fuses them together to form the larger helium nucleus. Nuclear ssion takes a larger nucleus and splits it into smaller nuclei
Where does the energy come from in the proton-proton chain
The mass of a helium nucleus is smaller than the masses of the hydrogen nuclei that are fused together. This dierence in mass was converted into energy (E = mc2)
What is the CNO cycle
The CNO cycle is a nuclear fusion cycle involving carbon, nitrogen, and oxygen. It is a more ecient way of fusing hydrogen into helium and is used by stars more massive than our Sun
Why does the CNO cycle require a higher temperature than the proton-proton chain
There are more protons in the carbon, nitrogen, and oxygen nuclei than in hydrogen nuclei so these nuclei must be moving with higher speeds to overcome the electrical repulsion between the nuclei and come close enough for fusion to occur. Since the speed of a gas molecule depends on the temperature of a gas, higher speeds require higher temperatures
State the mass-luminosity relation
The more massive a star is, the more luminous it is because more mass leads to more gravity. More gravity requires more pressure to support the star's weight. More pressure is acheived by a higher fusion rate which makes the star hotter. Hotter gases emit more electromagnetic radiation (are more luminous) than cooler gases

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