ch 15 vocab Flashcards
galaxies that look like flat white disks with yellowish bulges at their centers. the disks are filled with cool gas and dust, interspersed with hotter ionized gas, and usually display beautiful spiral arms
spiral galaxies
the bright, prominent arms, usually in a spiral pattern, found in most spiral galaxies
spiral arms
portion of a spiral galaxy, contains an interstellar medium with cool gas and dust; stars of many ages are found in this
disk
the central portion of a spiral galaxy that is roughly spherical and shows above and below the plane of the galactic disk
bulge
the spherical region surrounding the disk of a spiral galaxy
halo
the gas and dust that fills the space between stars in a galaxy
interstellar medium
the process of galactic recycling in which stars expel gas into space, where it mixes with the interstellar medium and eventually forms new stars
star-gas-star cycle
an expanding shell of hot, ionized gas driven by stellar winds or supernovae, with very hot and very low density gas inside
bubble
particles such as electrons, protons, and atomic nuclei that zip through interstellar space at close to the speed of light
cosmic rays
gas composed mostly of hydrogen atoms, though in space it is generally mixed with helium and small amounts of other elements as well; it is the most common form of interstellar gas
atomic hydrogen gas
a spectral line from atomic hydrogen with wavelength 21 cm
21 centimeter line
tiny solid flecks of carbon and silicon minerals found in cool interstellar clouds; they resemble particles of smoke and form in the winds of red giant stars
dust grains
a colorful, wispy cloud of gas that glows because neighboring hot stars irradiate it with ultraviolet photons that can ionize hydrogen atoms
ionization nebulae
gravitationally driven waves of enhanced density that move through a spiral galaxy and are responsible for maintaining its spiral arms
spiral density waves
the stars that orbit within the disk of a spiral galaxy
disk population
