Chapter 12 Jane Lucas

Jane Lucas
Astronomy 100-Week 13 – Chapter 12
Star Stuff
Star Birth
Life as a Low-Mass Star
Life as a High-Mass Star
Stellar Lives
Star Stuff - All elements heavier than hydrogen or helium came from within the stars.  During their lives, stars fuse hydrogen into helium and, if they are big enough, fuse helium into heavier elements - carbon, oxygen, etc. All elements up to and including iron are produced within the stars. The most massive stars explode as supernovae at the end of their lives, and the resulting in extreme temperatures and pressures produce even heavier elements. All these elements are blown out into space and form new stars as well as planets, moons, asteroids, etc. We, our planet, and the rest of the solar system were formed out of these elements and are referred to as star stuff.
Star Birth – A star is born when gravity causes a cloud of interstellar gas to contract to the point at which the central object becomes hot enough to sustain nuclear fusion in its core.  They are born in cold, relatively dense molecular clouds.  Newborn stars come in a range of masses.  
Life as a Low-Mass Star – Mass is the key factor in any star’s life.  Therefore, all low-mass stars go through life stages.  It spends most of its life generating energy by fusing hydrogen in its core.  When the core hydrogen is exhausted, the core begins to shrink while the star as a whole expands to be 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 core 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.  The low-mass star 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 planetary nebula causing the low-mass star to die and end as white dwarfs.  
Life as a High-Mass Star – High-mass stars also go through life stages.  Hydrogen fusion inside a high-mass star proceeds through a different set of steps than hydrogen fusion in a low-mass star.  High-mass stars also have a shorter life, fusing hydrogen into helium via the CNO cycle.  In a high-mass star, gravity compresses the hydrogen core to a higher temperature than we find in lower-mass stars. 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 this fusion makes the star swell in size to become a supergiant.  In the final stages of life, a high mass star’s core becomes hot enough to fuse carbon and other heavy elements.  The variety of fusion reactions produces a wide range of elements that are released into space when the star dies.  The high-mass star dies when the iron core collapses, leading to a supernova.
Stellar Lives – Mass determines how high a star’s core temperature can rise and therefore determines how quickly a star uses its fuel and what kinds of elements it can make.  Stars with close companions can exchange mass, altering the usual life stories of stars.  

In conclusion to Chapter 12 we learned all about star stuff.  We learned how a star is born and how it dies.  We learned all about the low-mass stars and high-mass stars. Low-mass stars live long lives and die with the ejection of planetary nebula in which white dwarfs are left behind.  High-mass stars live fast and die young, exploding dramatically as supernovae and leaving behind neutron stars or black holes.  Mass is the key determination in the life of a star.