NGC 253 (Sculptor Galaxy)

Though not a part of the Local Group, NGC 253 (Sculptor Galaxy) is one of the brightest galaxies in the sky     [Source]

A galaxy is an “island universe” typically harboring a supermassive black hole at its center. Galaxies are essentially the only places where stars exist, and they can contain as many as 100 trillion (1014) stars. Our galaxy, the Milky Way, is thought to consist of 300 billion stars and a black hole—known as Sagittarius A*—4 million times more massive than the Sun. From the vantage point of the Solar System, essentially everything that can be seen with the naked eye (except for Andromeda Galaxy, the Magellanic Clouds, and Triangulum Galaxy) is a part of the Milky Way Galaxy, be it planetary objects, comets, nebulae, star clusters, or individual star systems.

Our galaxy, the Milky Way, as seen from Monument Valley

Our galaxy, the Milky Way, as seen from Monument Valley     [Source]

Galaxies can be broken up into several classifications: ellipticals, spirals, barred spirals, lenticulars (a combination of the first two), or irregulars. Some of these interact and eventually merge with one another over long time scales. In fact, our own neighbor—Andromeda Galaxy—is currently headed on a collision course with the Milky Way. Galaxies that form an accretion disk around their central black holes are termed active galactic nuclei (AGN) and are responsible for quasars, blazars, BL Lacertae objects, relativistic jets, radio galaxies, and Seyfert galaxies.

Posts on galaxies:

* Quasars


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