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constellation an imaginary pattern of stars. spectrograph an instrument that separates light into colors and makes an image of the resulting spectrum apparent brightness the brightness of star as seen on Earth absolute brightness a brightness a star would have if it were at a standard distance from earth light-year the distance light travels in 1 year, about 9.5 million mllion kilometers parallax the apparent change in position of an object when seen from different places. Hertzsprung-russel diagram a graph relating the surfaces temperatures and absolute brightnesses of stars. main sequence a diagonal area on an H-R diagram that includes more than 90 percent of all stars. nebula a large cloud of gas and dust in space, spread out in an immense volume. protostar a crontacting cloud of gas and dust with enough mass to form a star. white dwarf the blue-white hot core of a star that is, left behind after its outer layers have expanded and drifted out in space. supernova the brilliant explosion of a supergiant star. neutron star the small, dense remains of a high-mass star after a supernova. pulsar a rapidly spinning neutron star that produces radio waves. black hole an object whose gravity is so strong that nothing, not even light, can escape.