Our Evolving Universe
Self Test 5

1. Most of the stars formed in the 15 billion years or so since the universe began
   1. have now exploded
   2. have now become white dwarfs
   3. are now red giants
   4. are still on the main sequence
   
   
2. Most white dwarfs are composed
   1. mainly of hydrogen
   2. mainly of helium
   3. mainly of carbon
   4. mainly of iron
   
   
3. Neutron stars are
   1. larger than white dwarfs, because they are more massive
   2. smaller than white dwarfs, because they are made of nuclear matter rather than atomic matter
   3. younger than white dwarfs, because the massive stars from which they form do not live long
   4. more than one of the above
   
   
4. When a star forms an iron core, it collapses because
   1. iron is very dense so the iron core is too heavy to be supported by fusion
   2. iron is not radioactive, so nuclear reactions do not occur
   3. fusion of iron nuclei does not generate energy
   4. fusion of iron nuclei requires too high a temperature
   
   
5. Some heavy isotopes can only be generated by rapid addition of neutrons because
   1. the isotopes leading up to them are unstable and decay
   2. neutrons are unstable and decay
   3. the isotopes leading up to them are stable, so they use up all the neutrons
   4. none of the above

out of 5.

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