Chapter 12 Quiz: Olivia Ward

1. How do stars form?
   
   Stars are formed in molecular clouds. When a cloud fragment collapses under gravity, it rapidly rotates, turning into a protostar. This protostar is surrounded by a spinning disk of gas and may fire jets of matter outward.
   
2. How massive are newborn stars?
   
   The masses of newborn stars vary, but are no more massive than 300 MSun or less massive than 0.08 MSun. Degeneracy pressure prevents gravity from making the core hot enough for hydrogen fusion under 0.08 MSun, making the object a brown dwarf.
   
   
3. What are the life stages of a low-mass star?
   
   A low mass star spends majority of its life fusing hydrogen in its core in order to generate energy. When the core hydrogen is exhausted, the core beings to shrink, while the star expands and becomes a red giant. Helium flash initiates helium fusion (which turns helium into carbon) in the core, but only if the core is hot enough. The core shrinks again. Helium and hydrogen fusion occur in shells around he carbon core. This causes the outer layers to expand again.
   
   
4. How does a low-mass star die?
   
   A low-mass star expels its outer layer into space as a planetary nebula. This leaves its exposed core behind as a white dwarf that is supported by degeneracy pressure.
   
5. What are the life stages of a high-mass star?
   
   A high-mass star lives a short life and fuses hydrogen into helium through the CNO cycle. After the star exhausts the hydrogen in its core, it begins hydrogen shell fusion and then experiences a series of stages, fusing heavier elements.
   
   
6. How do high-mass stars make the elements necessary for life?
   
   In a high-mass star's final stages of life, its core becomes hot enough to fuse carbon and other heavy elements. The different types of fusion reactions produce many different elements, including all of the elements necessary for life. These elements are released into space when the star dies.
   
   
7. How does a high-mass star die?
   
   A high-mass star dies in an explosion called a supernova. The supernova occurs after fusion begins to collect iron in the core, because iron fusion cannot release energy. Gravity overcomes the degeneracy pressure, and the core collapses and the star explodes. This explosion scatters newly produced elements into space and leaves behind a neutron star or black hole.
   
8. How does a star's mass determine the life story?
   
   A star's mass determines its temperature, and therefore, how it lives it life. A low-mass star never gets hot enough to fuse carbon or heavier elements. They end their lives by expelling their outer layers and leaving white dwarfs behind. High-mass stars live short lives and produce several elements due to fusion, but die in supernovae.
   
9. How are the lives of stars with close companions different?
   
   When a star in a close binary system begins to swell in size at the end of its main-sequence stage, it can transfer mass to its companion, which can influence the lives of both stars.
   
10. Choose a supernova from Wikipedia, and describe it.
    
    Lupus is a constellation int eh southern sky. The ancient Greeks considered it to be an arbitrary animal, killed, or about to be killed, for Centaurus. The constellation does not have extremely bright stars.  It was one of the 48 constellations listed by Ptolemy.