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association
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a loose group of young stars whose spectral types, motions, and positions in the sky indicate a common origin
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globular cluster
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one of about 150 large, spherical star clusters (each with hundreds of thousands of stars) that form a spherical halo around the center of our Galaxy
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helium flash
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a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star
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main-sequence turnoff
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location in the H–R diagram where stars begin to leave the main sequence
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nucleosynthesis
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the building up of heavy elements from lighter ones by nuclear fusion
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open cluster
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a comparatively loose cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of our Galaxy; sometimes referred to as a galactic cluster
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planetary nebula
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a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the central star)
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triple-alpha process
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a nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus
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zero-age main sequence
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a line denoting the main sequence on the H–R diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy from nuclear reactions, but whose chemical composition has not yet been altered substantially by nuclear reactions