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A starburst galaxy is a galaxy forming stars at a very high rate (undergoing a starburst), i.e., so high that all its gas would be gone if this had been going on for the life of the galaxy, forming stars at a rate of on the order of 10 to 100 solar masses per year. By comparison, the Milky Way, which is not a starburst galaxy, is estimated to produce about one solar mass per year. The term quiescent, as in quiescent galaxy can refer to a galaxy that is not starburst, or in context, it can refer to a galaxy that isn't forming stars at all, i.e., not a star forming galaxy (SFG).
Such extreme star formation can be detected in a distant galaxy by the strength of spectral features associated with hot, short-lived stars, which are necessarily recent. In many cases, such stars are hidden by the dust in the clouds that formed them, but are detected by infrared radiation reradiated from these clouds due to energy from the hot stars. It is thought that starbursts are triggered by close encounters and mergers between galaxies. There is evidence that starbursts can form so many stars so close together that a gas blowout results from the pressure created by the EMR and heated gas, changing it into in a quenched galaxy (another term for a galaxy with minimal star formation). A post-starburst galaxy (PSB galaxy) is one which is thought to have made this transition, which sometimes is a green valley galaxy (i.e., not totally quenched).