Our Evolving Universe
Self Test 6

1. On the HR diagram, pre-main-sequence stars are
   1. below the main sequence
   2. above and to the right of the main sequence
   3. to the left of the main sequence
   4. not visible at all, because they have not started fusion yet
   
   
2. The luminosity of pre-main-sequence stars comes from
   1. chemical reactions in the dense gas
   2. nuclear fusion
   3. conversion of gravitational energy
   4. nothing: pre-main-sequence stars are not luminous
   
   
3. Newly formed stars are higher in heavy elements because
   1. supernovae and planetary nebulae have enriched the interstellar gas from which they form
   2. the heavy elements have not yet been used up in nuclear reactions
   3. the heavy elements have not yet sunk to the core of the star, so can still be detected on the surface
   4. none of the above: actually young stars are lower in heavy elements
   
   
4. Binary systems form when
   1. a collapsing gas cloud is spinning so fast it fragments
   2. a star forms very close to an existing star
   3. a star approaches too close to a more massive star
   4. dust grains in the disc around a young star stick together to make a larger body
   
   
5. A brown dwarf is
   1. the dead remnant of a low mass star
   2. a young star which has not yet started fusion
   3. a "star" which does not have enough mass to start fusion
   4. a type of massive planet similar to Jupiter

out of 5.

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