Milky Way galaxy



The Milky Way is the home galaxy of mankind. This name derives from its appearance as a dim "milky" glowing band arching across the night sky on Earth, in which the naked eye cannot distinguish individual stars. The term "Milky Way" is a translation of the Classical Latin via lactea, from the Hellenistic Greek γαλαξίας κύκλος (pr. galaxías kýklos, "milky circle"). The Milky Way appears like a band because it is a disk-shaped structure being viewed from inside.

Astrography
The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200–400 billion stars. It may contain at least as many planets, with at least half as many meeting the condition for life, and an estimated 10 billion of those orbiting in the habitable zone of their parent stars.

The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years (30 kiloparsecs) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick.

The Solar System is located within the disk, around two thirds of the way out from the Galactic Center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm.

The stars in the inner ≈10,000 light-years are organized in a bulge and one or more bars. The very center is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole. The Galaxy rotates differentially, faster towards the center and slower towards the outer edge. The rotational period is about 200 million years at the position of the Sun.

The Galaxy as a whole is moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster.

The most prominent of the Milky Way's satellite galaxies - a pair of galaxies called the Magellanic Clouds - are producing a warp in the hydrogen gas layer of the Milky Way as they make their looped orbit through dark matter. A giant gaseous intergalactic streamer is flowing from them to the Milky Way.

The local Milkyway galaxy escape velocity at Sol's altitude is 525 km/s.

History & Evolution
The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Galaxy. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.

Since the first stars began to form, the Milky Way has grown through both galaxy mergers and accretion of gas directly from the Galactic halo. The Milky Way is currently in the process of stripping material from its two nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high velocity clouds like the Smith Cloud.

It's among the reddest and brightest spiral galaxies that are still forming new stars and it's just slightly bluer than the bluest red sequence galaxies.

Future
The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster. The Local Group, although it is moving away from the Virgo Supercluster at 967 km/s, the velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.

By the time the Triangulum galaxy is approaching Andromeda again, the Milky Way and Andromeda will have moved much closer together. The Andromeda–Milky Way collision is a predicted galaxy collision that will take place in approximately 4 billion years' time.

Human geography
Currently humankind is inhabiting the Milky Way galaxy through the use of sords. The human sphere of influence within the galaxy limited to star systems having a sord. It is currently divided into nations, the Humankind Empire of Abh, Federation of Hania, People's Sovereign Union of Planets, Republic of Greater Alkont, and the United Mankind.