Chapter 20
black dwarf
a cold, dense, burned-out ember in space
helium flash
an explosive event in the post-main-sequence evolution of a low-mass star - when helium fusion begins in a dense stellar core, the burning is explosive in nature - it continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again
hydrogen shell burning stage
fusion of hydrogen in a shell that is driven by contraction and heating of the helium core - once hydrogen is depleted in the core of a star, hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star, and increasing the burning rate there
asymptotic giant branch
path on the H-R diagram corresponding to the changes that a star undergoes after helium burning ceases in the core - at this stage, the carbon core shrinks and drives the expansion of the envelope, and the star becomes a swollen red giant for a second time
hydrogen burning
process of nuclear fusion - when a star in main-sequence burns hydrogen into helium in its core
main-sequence turnoff
special point on the H-R diagram for a cluster, indicative of the cluster's age - if all the stars in the cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch - the point where stars are just beginning to evolve off is the main-sequence turnoff
subgiant
star on the subgiant branch of the H-R diagram
triple alpha process
the creation of carbon-12 by the fusion of three helium-4 nuclei (alpha particles) - helium-burning stars occupy a region of the H-R diagram known as the horizontal branch
electron degeneracy pressure
the pressure associated with the contact of the tiny electron spheres
planetary nebula
the result of the core becoming so hot that its ultraviolet radiation ionizes the inner parts of the surround cloud - it is a ring of three-dimensional shell of the glowing gas
subgiant branch
the section of the evolutionary track of a star corresponding to changes that occur just after hydrogen is depleted in the core, and core hydrogen burning ceases - shell hydrogen burning heats the outer layers of the star, which causes a general expansion of the stellar envelope
red-giant branch
the section of the evolutionary track of a star corresponding to intense hydrogen shell burning, which drives a steady expansion and cooling of the outer envelope of the star - as the star gets larger in radius and its surface temperature cools, it becomes this
blue stragglers
they are observed in many star clusters, lying on the main sequence, but in locations suggesting that they should have evolved into white dwarfs long ago, given the parent cluster's age - they are main sequence stars but did not form when the cluster did - formed through mergers of lower mass stars
horizontal branch
where core-helium-burning stars remain for a time before resuming their journey around the H-R diagram
