Jessica Horn: Chapter 12 Quiz

1. How do stars form?
   Stars are born in cold, relatively dense molecular clouds. As a cloud fragment collapses under gravity, it becomes a rapidly rotating protostar surrounded by a spinning disk of gas in which planets may form. The protostar may also fire jets of matter outward along its poles.
   
2. How massive are newborn stars?
   Newborn stars come in a range of masses but cannot be more massive than about 300MSun or less massive than 0.08MSun. Below this mass, degeneracy pressure prevents gravity from making the core hot enough for efficient hydrogen fusion, and the object becomes a "failed star" knows as a brown dwarf.
   
3. What are the life stages of a low-mass star?
   A low-mass star spends most of its life generating energy by fusing hydrogen in its core. When core hydrogen is exhausted, the core begins to shrink while the star as a whole expands to become a red giant, with hydrogen shell fusion around an inert helium core. When the core becomes hot enough, a helium flash initiates helium fusion in the core, which fuses helium into carbon  the star shrinks somewhat in size and luminosity during this time. The core shrinks again when helium cor fusion ceases, while both helium and hydrogen fusion occur in shells around the inert carbon core and cause the outer layers to expand once more.
   
4. How does a low-mass star die?
   A low-mass star like the Sun never gets hot enough to fuse carbon in its core, because degeneracy pressure stops the gravitational collapse of the core. The star expels its outer layers into space as a planetary nebula, leaving its exposed core behind as a white dwarf, which is supported by degeneracy pressure.
   
5. What are the life stages of a high-mass star?
   A high-mass star lives a much shorter life than a low-mass star, fusing hydrogen into helium via the CNO cycle. After exhausting its core hydrogen, a high-mass star begins hydrogen shell fusion and then goes through a series of stages fusing successively heavier elements. The furious rate of the fusion makes the star swell in size to become a supergiant.
   
6. How do high-mass stars make the elements necessary for life?
   In its final stages of life, a high-mass star's core becomes hot enough to fuse carbon and other heavy elements. The variety of different fusion reactions produces a wide range of element, including all the elements necessary for life, that are then released into space when the star dies.
   
7. How does a high-mass star die?
   A high-mass star dies in a cataclysmic explosion call a supernova, scattering newly produced elements into space and leaving behind a neutron star or black hole. The supernova occurs after fusion begins to  pile up iron in the high-mass star's core. Because iron fusion cannot release energy, the core cannot hold off the crush of gravity for long. In the instant that gravity overcomes degeneracy pressure, the core collapses and the star explodes. The expelled gas may be visible for a few thousand years as a supernova remnant.
   
8. How does a star's mass determine the life story?
   A star's mass determines how it lives its life. Low-mass stars never get hot enough to fuse carbon or heavier elements in their cores and end their lives by expelling their outer layers and leaving white dwarfs behind. High-mass stars live short but brilliant lives, ultimately dying in supernova explosions.
   
9. How are the lives of stars with close companions different?
   When one star in a close binary system begins to swell in size at the end of its main-sequence stage, it can begin to transfer mass to its companion. This mass exchange can then change the remaining life histories of both stars.
   
10. Choose a supernova from Wikipedia, and describe it. The supernova I selected is Kepler's Supernova, also known as Supernova 1604. According to Wikipedia, Kepler's Supernova was a supernova that occurred in the Milky Way, in the constellation Ophiuchus. It is the most recent supernova to have been unquestionably observed by the naked eye in our own galaxy, occurring no farther than 6 kiloparsecs  or about 20,000 light-years from Earth. Visible to the naked eye, Kepler's Star was brighter at its peak than any other star  in the night sky, and all the planets other than Venus, with apparent magnitude −2.5. It was visible during the day for over three weeks. No further supernovae have since been observed with certainty in the Milky Way, though many others outside our galaxy have been seen since S Andromedae.