A galaxy that is observed to be undergoing an unusually high rate of formation of stars. It is often defined as a galaxy that, if it continues to form stars at the currently observed rate, will exhaust its entire supply of star-forming material, the interstellar gas and dust, in a time period that is very short compared to the age of the universe. For a typical starburst galaxy, this gas and dust exhaustion time scale is less than 10⁸ years, that is, less than 1% of the age of the universe. Since such a galaxy must shortly run out of star-forming material, the high star formation rate currently observed not only must end soon but also must have started relatively recently or the gas supply would have run out long ago. It follows that such galaxies must be undergoing a passing burst of star formation. The rates of star formation that are estimated to occur in starbursts can exceed 100 times the mass of the Sun per year for the most energetic bursts observed in the nearby universe. This may be compared to about 2–3 solar masses per year in the Milky Way Galaxy. However, there are large uncertainties in such estimates because there is only weak evidence on the range of masses of stars that form in starburst events. There are theoretical limits to the total rate of star formation that can occur in a starburst. One limit is derived from the mass of all the available interstellar material turning into stars during the time taken for that material to fall inward under gravity to form a typically sized galaxy. Another limit is related to the expectation that when large numbers of newly formed massive stars reach supernova stage, they will blow away much of the remaining star-forming material, thus inhibiting the burst.