What Is A Megamaser?

A maser is similar in nature to a laser, the difference being that a maser occurs in the microwave section of the electromagnetic spectrum. The word maser is actually an acronym for Microwave Amplification by Stimulated Emission of Radiation. To understand what a maser is and in turn what a megamaser is, it is essential to understand what stimulated emission means. 

What Is Stimulated Emission?

                        Arp 220 is a galaxy that is currently emitting a megamser, NASA

Atoms are composed of a nucleus that is orbited by electrons. These electrons exist in various energy levels around the nucleus. When incoming particles of light (photons) impact electrons, the electrons will change energy levels. In the case of stimulated emission, a photon will impact an electron and the electron will then emit two more photons that have the same frequency and direction as the original photon. The release of two photons causes the electron to fall to a lower energy level. Interestingly, this process can only occur when there are more electrons in higher energy levels than in lower energy levels, which is referred to as a population inversion. Furthermore, a population inversion can only occur if an external process is providing a source of energy. Thus, stimulated emission is a process by which atoms release photons, and it can only happen when energy is being provided. The two photons emitted by an electron are microwave photons and, if enough are emitted, a maser forms. 

Masers In Space

As these two galaxies collide, they are emitting a megamaser, NASA

Laboratories on Earth are able to produce masers by inducing stimulated emission. Masers, however, are also produced by natural events in the cosmos. Masers are made by unique molecules and elements, such as water, hydroxyl, and methanol. Objects which contain an abundance of these and other chemicals will generally emit a maser. For example, oxygen-rich stars emit masers, as do gas giant planets such as Jupiter. Masers emitted by stars, nebulae, and planets are tiny compared to megamasers. Megamasers are gigantic masers that are typically produced by some of the most extreme events in the cosmos. One such event that can produce a megamaser is a galactic collision. A collision between two or more galaxies typically results in a massive increase in star formation, resulting in an increase in microwave radiation emitted. The combined microwaves from so many stars form a gigantic beam of microwave radiation that moves outwards from the colliding galaxies. The amount of energy contained within a megamaser is so high that it can even become brighter than an entire galaxy containing billions of stars. Interestingly, since particular molecules produce megamasers, they can be used to identify the composition of objects such as stars, planets, nebulae, and galaxies. 

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