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28 Cards in this Set

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The establishment of the relationship between a parameter that is easily determined and a parameter that is more difficult to determine. For example, the periods of Cepheid variables have been calibrated to reveal absolute magnitudes, which can then be used to find distance. Thus astronomers say Cepheids have been calibrated as distance indicators. (p. 258)
Cepheid variable star
Variable star with a period of 60 days. Period of variation is related to luminosity. (p. 255)
dark halo
The low-density extension of the halo of our galaxy believed to be composed of dark matter. (p. 263)
dark matter
Nonluminous matter that is detected only by its gravitational influence. (p. 263)
density wave theory
Theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy. (p. 270)
differential rotation
The rotation of a body in which different parts of the body have different periods of rotation. This is true of the sun, the Jovian planets, and the disk of the galaxy. (p. 262)
disk component
All material confined to the plane of the galaxy. (p. 260)
Describes a galaxy whose spiral arms have a woolly or fluffy appearance. (p. 272)
galactic corona
The extended, spherical distribution of low-luminosity matter believed to surround the Milky Way and other galaxies. (p. 263)
The spherical region of a spiral galaxy, containing a thin scattering of stars, star clusters, and small amounts of gas. (p. 261)
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. (p. 255)
kiloparsec (kpc)
A unit of distance equal to 1000 pc or 3260 ly. (p. 260)
Local Group
The small cluster of a few dozen galaxies that contains our Milky Way Galaxy. (p. 267)
In astronomical usage, all atoms heavier than helium. (p. 264)
nuclear bulge
The spherical cloud of stars that lies at the center of spiral galaxies. (p. 261)
period-luminosity relation
The relation between period of pulsation and intrinsic brightness among Cepheid variable stars. (p. 256)
population I
Stars rich in atoms heavier than helium. Nearly always relatively young stars found in the disk of the galaxy. (p. 264)
population II
Stars poor in atoms heavier than helium. Nearly always relatively old stars found in the halo, globular clusters, or the nuclear bulge. (p. 264)
proper motion
The rate at which a star moves across the sky. Measured in seconds of arc per year. (p. 258)
rotation curve
A graph of orbital velocity versus radius in the disk of a galaxy. (p. 262)
RR Lyrae variable star
Variable star with period of from 12 to 24 hours. Common in some globular clusters. (p. 255)
Sagittarius A*
The powerful radio source located at the core of the Milky Way Galaxy. (p. 273)
self-sustaining star formation
The process by which the birth of stars compresses the surrounding gas clouds and triggers the formation of more stars. Proposed to explain spiral arms. (p. 272)
Shapley-Curtis debate
A 1920 debate between Harlow Shapley and Heber Curtis on the nature of spiral nebulae. Curtis argued that they are other galaxies, and Shapley argued they are internal to our own galaxy. (p. 259)
spherical component
The part of the galaxy including all matter in a spherical distribution around the center (the halo and nuclear bulge). (p. 261)
spiral arm
Long spiral pattern of bright stars, star clusters, gas, and dust. Spiral arms extend from the center to the edge of the disk of spiral galaxies. (p. 260)
spiral tracer
Object used to map the spiral arms - for example, O and B associations, open clusters, clouds of ionized hydrogen, and some types of variable stars. (p. 268)
variable star
A star whose brightness changes periodically. (p. 255)