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| the range of wavelengths or frequencies over which electromagnetic radiation extends. |
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| the rate at which something occurs or is repeated over a particular period of time or in a given sample. |
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| the distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave. |
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| reach the top of (something such as a hill or wave). |
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| a unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607 × 1012 km (nearly 6 trillion miles). |
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| The Hertzsprung–Russell diagram, abbreviated H–R diagram or HRD, is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral classifications or effective temperatures. |
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| a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch or as a dark silhouette against other luminous matter. |
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| a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. |
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