Allison Thompson Quiz 12

How do stars form?
Stars form in dark clouds of dusty interstellar space. Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.

How massive are newborn stars?
Very massive to small.

What are the life stages of a low-mass star?
A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. Observation of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over. As the core contracts H begins fusion He in a shell around the core. Luminosity increases because the core thermostat is broken. the increasing fusion rate in the shell does not stop the core from contracting. Helium fusion does not begin right away because it requires higher temperature than hydrogen fusion larger charge leads to greater repulsion. The fusion of two helium nuclei doesn't work so helium fusion must combine three He. The thermostat is broken in low mass red giant because degeneracy pressure supports the core. Helium core fusion stars neither shrink nor grow because the core thermostat is temporarily fixed.

How does a low-mass star die?
After core helium fusion stops He fuses into carbon in a shell around the carbon core an h fuses into he shell around helium layer. Double shell fusion ends with a gust that ejects the H and He into space as a planetary nebula. The core left behind becomes a white dwarf. Fusion progresses no further in a low mass star because the core temperature never grows hot enough for fusion of heavier elements Degeneracy pressure supports the white dwarf against gravity

What are the life stages of a high-mass star?
High mass main sequence stars fuse H to He at higher rate using carbon nitrogen and oxygen as catalysts. A greater core temperature enables H nuclei to over come greater repulsion. Later life stages of high mass stars are similar to those of low mass stars How do high mass stars make the elements necessary for life? high temperature allow helium to fuse with heavier elements. Core temp in stars allow fusion of elements as heavy as iron. Advance nuclear burning. proceeds in serious of nested shells Iron is a dead end for fusion because nuclear reactions involving iron do not release energy Higher abundance of elements with even number of protons

How does a high-mass star die?
Iron builds up and the core collapses. Neutrons collapse forming a neutron star.

How does a star's mass determine the life story?
Low mass stars never get hot enough to fuse elements in there core. High mass stars live bright short lives.

How are the lives of stars with close companions different?
When a star in close binary system begins to swell in size at the end of hydrogen burning life, it can transfer its mass to its companion. This changes both histories of the stars.