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Milky Way

Milky Way Galaxy, galactic rotation, Serpent Bearer, Scutum, Cepheus

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Milky Way, the large, disk-shaped aggregation of stars, or galaxy, that includes the Sun and its solar system. In addition to the Sun, the Milky Way contains about 400 billion other stars. There are hundreds of billions of other galaxies in the universe, some of which are much larger and contain many more stars than the Milky Way.

The Milky Way is visible at night, appearing as a faintly luminous band that stretches across the sky. The name Milky Way is derived from Greek mythology, in which the band of light was said to be milk from the breast of the goddess Hera. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. All of the individual stars that are distinct in the sky lie within the Milky Way Galaxy.

From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horizon. It extends through the constellations Perseus, Cassiopeia, and Cepheus. In the region of the Northern Cross it divides into two streams: the western stream, which is bright as it passes through the Northern Cross, fades near Ophiuchus, or the Serpent Bearer, because of dense dust clouds, and appears again in Scorpio; and the eastern stream, which grows brighter as it passes southward through Scutum and Sagittarius. The brightest part of the Milky Way extends from Scutum to Scorpio, through Sagittarius. The center of the galaxy lies in the direction of Sagittarius and is about 25,000 light-years from the Sun (a light-year is the distance light travels in a year, about 9.46 trillion km or 5.88 trillion mi).


The Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, and the spiral arms trail in the same direction. The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system the period of rotation is more than 200 million years. The speed of the solar system due to the galactic rotation is about 220 km/sec (about 140 mi/sec).

Future Merger With Andromeda Galaxy

The Milky Way is estimated to be about 13.6 billion years old and must have begun to form shortly after the big bang 13.7 billion years ago. However, in a few billion years it will no longer exist as a separate galaxy. Calculations indicate that the Milky Way and the Andromeda Galaxy will eventually collide to form a single, giant elliptical galaxy.

The two galaxies are approaching each other at about 120 km (75 mi) per second. A series of close passes that may begin in about 2 billion years will first pull their spiral arms into long streams of stars. Complete merger of the galaxies could take place by about 5 billion years from now. The black holes at the center of each galaxy will combine as a single supermassive black hole, possibly resulting in a powerful quasar. Recent computer models suggest our Sun and its solar system will survive the merger but likely will be thrown into the distant outer halo of the new combined galaxy.

Article key phrases:

Milky Way Galaxy, galactic rotation, Serpent Bearer, Scutum, Cepheus, Ophiuchus, Andromeda Galaxy, Northern Cross, Cassiopeia, spiral arms, goddess Hera, Greek mythology, axis, black holes, solar system, galaxies, summer nights, big bang, Milky Way, Scorpio, southward, band of light, sky, light-years, Rotation, galaxy, universe, Calculations, breast, neighborhood, direction, streams, distance, Sun, speed, region, addition, night, years old, center, years

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