a glowing cloud of gas or a cloud of dust reflecting or obscuring the light of nearby stars
interstellar reddening
the process in which dust scatters blue light out of starlight andm akes the stars look redder
interstellar dust
microscopic solid grains in the interstellar medium
emission nebula
a cloud of glowing gas excited by ultraviolet radiation from hot stars
HII region
a region of ionized hydrogen around a hot star
reflection nebula
a nebula produced by starlight reflecting off dust particles in the interstellar medium
dark nebula
a cloud of gas and dust of any size seen silhouetted against a brighter nebula or star field
interstellar absortion lines
narrow spectral lines produced as starlight travels through the thin gas of the interstellar medium
molecular cloud
a dense interstellar gas cloud in which atoms are able to link together to form molecules such as H2 and CO
shockwave
a sudden change in pressure that travels as an intense sound wave
star cluster
stars held together in a stable group by their combined gravity
association
group of widely scattered stars moving together through space
protostar
a collapsing cloud of gas and dust destined to become a star
evolutionary track
a path star follow on the H-R diagram as it gradually changed its surface temperature and luminosity
CNO cycle
a series of nuclear reactions that use carbon as a catalyst to combine four hydrogen atoms to make one helium atoms plus energy
triple-alpha process
the nuclear fusion process that combines three helium nuclei the make on carbon nucleus
conservation mass
one of the basic laws of stellar structure that states the total mass of the star must equal the sum of the masses of the shells
conservation of energy
one of the basic laws of stellar structure that states the amount of energy flowing out of the top of a shell must equal the amount cmoing in at the bottom plus whatever energy is generated within the shell
hydrostatic equilibrium
the balance between the weight of the material pressing downward on alayer in a star and the pressure in that layer
stellar model
a table of numbers representing the conditions in various layers within a star
brown dwarf
a star whose mass is too low to ignite nuclear fusion
zero-age main sequence (ZAMS)
the location of the H-R diagram where stars first reach stability as hydrogen-burning star
pressure-temperature thermostat
the relation between gas pressure and temperature, the stability of a star depends upon this