| definition |
A galaxy that produces huge amounts of energy at its center, which cannot be attributed
to normal processes from stars, interstellar medium, and their interactions. There
are several types of active galaxies: Seyfert galaxies, quasars, and blazars. All
of these objects show brightness variations, some as short as 3 hours. These fluctuations
indicate a relatively very small size for the central object, because an object cannot
vary in brightness faster than light can travel across it. For example, an object
that is one light-year in diameter cannot vary significantly in brightness over a
period of less than one year.
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