| A | B |
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| core | the central region of the sun where nuclear fusion occurs |
| radiation zone | a region of closely-packed plasma outside teh core of the sun, where energy is transferred by the absorption of reradiation of light |
| convection zone | the region inside the sun where thermal energy is transferred outward mainly by convection currents |
| photosphere | the thininnermost layer of the sun's atmosphere, which is the visible surface of the sun |
| chromosphere | the middle layer of the sun's atmosphere, just outside the photosphere |
| corona | the outermost layer of the sun's atmosphere, extending far above the chromosphere |
| solar wind | a stream of electrically charged particles that flows from the sun outward through the solar system |
| sunspots | areas of gas in the sun's photosphere that is cooler than the surrounding gases |
| prominences | huge loops of gas that erupt from sunspot regions and extend upward from the photosphere into the chromosphere and sometimes into the corona |
| solar flare | a dramatic eruption on the sun's surface, usually near sunspot's that produces X-rays and sends charged particles into space at speeds of 1000 Km/s or more |
| star | a large, glowing ball of gas in space that generates energy through nuclear fusion in its core |
| light-year | the distance that light travels in a vacuum in a year, about 9.5 trillion kilimeters |
| parallax | an apparent change in position of an object with respect to a distant background when viewed from different locations |
| apparent brightness | the brightness of a star as it appears from Earth |
| absolute brightness | a description of how bright a star really is; a characteristic property of a star that does not depend on the star's distance from Earth |
| absorption lines | a set of dark lines that show frequencies at which light has been absorbed from a star's bright spectrum |
| H-R diagram | the Hertzsprung-Russell diagram, a graph of the surface temperature versus absolute brightness of a sample of stars |
| main sequence | a diagnol band on the H-R diagram, which represents about 90% of all stars |
| supergiants | very large, massive, bright star ranging in size from 100 to 1000 times the diameter of the sun |
| giants | lareg, massive, bright stars that are somewhat smaller and fainter than a spuergiant |
| white dwarf | the small, dense but dim remains of a low- or medium-mass star |
| nebula | a large cloud of gas and dust spread out over a large volume of space |
| protostar | a contracting nebula with enough mass to form a star |
| planetary nebula | a glowing cloud of gas surrounding a dying low-mass star |
| supernova | an enormous explosion in which the byproducts of a supergiant star's lifetime of fusion are flung into space |
| neutron star | the dense core left after a high-mass star has exploded as a supernova |
| pulsar | a spinning neutron star that appears to give off strong pulses of radio waves |
| black hole | an object whose surface gravity is so great that not even elctromagnetic waves cane escape from it |
| constellation | a group of stars that appears to form a pattern as seen from Earth |
| star system | a group of two or more stars held together by gravity |
| binary star | a star system with two stars that revolve around each other |
| globular cluster | a large sperical-shaped group of older stars that usually lacks sufficient amounts of gas and dust to form new stars |
| galaxy | a huge group of stars, star systems, star clusters, dust, and gas bound together by gravity |
| spiral galaxy | a galacy like the Milky Way with a bulge of stars at the center and arms extending ouutward like a pinwheel |
| barred- spiral galaxies | spiral galaxies inw hich spiral arms extend outward from a bar running through the center of teh galaxy |
| elliptical galaxies | spherical or oval-shaped galaxies with no trace of spiral arms and very little gas or dust between stars |
| irregular galaxies | galaxies with a disorganized appearance |
| quasars | enormously bright centers of a distant young galaxy |
| red shift | a shift toward the red wavelengths of light from stars or galaxies moving away from Earth |
| hubble's Law | the direct variation of the speed at which a galaxy is moving away from Earth with its distance from Earth |
| big bang theory | the theory of the beginning of the universe in an instant, billions of years ago, in an enormous explosion |
| dark matter | matter that does not emit radiation that astronomers can detect |
