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