Megamaser be very different from those that produce galactic water masers. Observations of water megamasers have been used to make accurate measurements of distances to galaxies in order to provide constraints on the Hubble constant. The word maser derives from the acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation". The maser is a predecessor to lasers, which operate at optical wavelengths, and is named by the replacement of "microwave" with "light". Given a system of atoms or molecules, each with different energy states, an atom or molecule may absorb a photon and move to a higher energy
Sound amplification by stimulated emission of radiation one quantum well to another neighbouring well it produces a phonon. External energy pumping (e.g. a light beam or voltage) can help to the excitation of an electron. Relaxation of an electron from one of the upper states may occur by emission of either a photon or a phonon. This is determined by the density of states of phonons and photons. Density of states is the number of states per volume unit in an interval of energy (E, E+dE) that are available to be occupied by electrons. Both phonons and photons are bosons and thus, they obey Bose–Einstein statistics. This