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variable star
a star whose brightness changed periodically
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Cepheid variable star
- variable star with a period of up to 60 days.
- the period of variation is related to lumionsity
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instability strip
the region of the H-R diagram in which stars are unstable to pulsation. A star passing through this strip becomes a variable star
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RR Lyrae variable star
variable star with a period of from 12-24 hours. Common in some globular clusters
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Period-luminosity relation
the relation between period of pulsation and intrinsic brightness among Cepheid variable stars
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proper motion
the rate at which a star moves across the sky. This rate is measured in seconds of arc per year
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calibrated
the establishment of the relationship between a parameter that is easily determined and a parameter that is more difficult to determine
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disk component
all material confined to the plane of the galaxy
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kiloparsec
a unit of distance equal to 1000 pc or 3260 ly
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spiral arm
long spiral pattern of bright stars, star clusters, gas and dust that extend from the center to the edge of the disk of spiral galaxies
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spherical component
the part of the galaxy including all matter in a spherical distibution around the center (the halo and nuclear bulge)
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halo
the spherical region of a spiral galaxy, containing a thin scattering of stars, globular star clusters, and small amounts of gas
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nuclear bulge
the spherical cloud of stars that lies at the center of spiral galaxies
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rotation curve
a graph of orbital velocity versus radius in the disk of a galaxy
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dark halo
the low-density extension of the halo of our galaxy believed to be composed of dark matter; also called the galactic corona
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galactic corona
the extended, spherical distribution of the low-luminosity matter believed to surround the Milky Way and the other galaxies; also called the dark halo.
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dark matter
nonluminous matter that is detected only by its gravitational influence
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population I
stars rich in atoms heavier than helium. Nearly always relatively young stars found in the disk of the galaxy
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population II
stars poor in atoms heavier than helium
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metal
in astronomical usage, all atoms heavier than helium
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spiral tracer
a celestial object used to map spiral arms.
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density wave theory
theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy
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self-sustaining star formation
the process by which the birth of stars compresses the surrounding gas clouds and triggers the formation of more stars
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