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Definition
| Variable stars with pulsation periods of 1 to 60 days whose period of variation is related to their luminosity. |
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Definition
| The region of the H-R diagram in which stars are unstable to pulsation. A star evolving through this strip becomes a variable star. |
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| period-luminosity relation |
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Definition
| The relation between period of pulsation and intrinsic brightness among Cepheid variable stars. |
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Definition
| The rate at which a star moves across the sky, measured in seconds of arc per year. |
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Definition
| To make observations of reference objects, checks on instrument perofrmance, calculations of units conversions, and so on, needed to completely understand measurements of unknown quantities. |
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Definition
| All material confined to the plane of the galaxy. |
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| A unit of distance equal to 1,000 pc or 3,260 ly. |
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Definition
| Long spiral patern of bright stars, star clusters, gas, and dust. Spiral arms extend from the center to the edge of the disk of spiral galaxies. |
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Definition
| The part of the galaxy including all matter in a spherical distribution around the center (the halo and central bulge). |
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Definition
| The spherical region of a spiral galaxy, containing a thin scattering of stars, star clusters, and small amounds of gas. |
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Definition
| The spherical cloud of stars that lies at the center of spiral galaxies. |
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Definition
| A graph of orbital velocity versus radius in the disk of a galaxy. |
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Definition
| The low-density extension of the halo of our galaxy, believed to be composed of dark matter. |
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Definition
| Nonluminous matter that is detected only by its gravitational influence. |
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Definition
| 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. |
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Definition
| Theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy. |
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Term
| self-sustaining star formation |
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Definition
| The process by which the birth of stars compresses the surrounding gas clouds and triggers the formation of more stars. |
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Definition
| Stars rich in atoms heavier than helium, nearly always relatively young stars found in the disk of the galaxy. |
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Definition
| Stars poor in atoms heavier than helium, nearly always relatively old stars found in the halo, globular clusters, or the central bulge. |
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Definition
| In astronomical usage, all atoms heavier than helium. |
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Term
| monolithic collapse model |
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Definition
| An early hypothesis says that the galaxy formed from the collapse of a single large cloud of turbulent gas over 13 billion years ago. |
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Term
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Definition
| The powerful radio source located at the core of the Milky Way Galaxy. |
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