Milky Way - astronomy.

Milky Way - astronomy. I INTRODUCTION 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). II STRUCTURE Galaxies have three common shapes: elliptical, spiral, and irregular. Elliptical galaxies have an ovoid or globular shape and generally contain older stars. Spiral galaxies are disk-shaped with arms that curve around their edges, making these galaxies look like whirlpools. Spiral galaxies contain both old and young stars as well as numerous clouds of dust and gas from which new stars are born. Irregular galaxies have no regular structure. Astronomers believe that their structures were distorted by collisions with other galaxies. Astronomers classify the Milky Way as a large spiral or possibly a barred spiral galaxy, with several spiral arms coiling around a central bulge about 10,000 light-years thick. Stars in the central bulge are close together, while those in the arms are farther apart. The arms also contain clouds of interstellar dust and gas. The disk is about 100,000 light-years in diameter and is surrounded by a larger cloud of hydrogen gas. Surrounding this cloud in turn is a spherical halo that contains many separate globular clusters of stars mainly lying above or below the disk. This halo may be more than twice as wide as the disk itself. In addition, studies of galactic movements suggest that the Milky Way system contains far more matter than is accounted for by the visible disk and attendant clusters--up to 2,000 billion times more mass than the Sun contains. The Milky Way is the most massive member of a group of about 40 galaxies called the Local Group, which includes another large spiral galaxy known as the Andromeda Galaxy and many dwarf galaxies such as the Magellanic Clouds. Although the Andromeda Galaxy appears to have more stars and visible matter in its disk than the Milky Way does, recent research indicates that the Milky Way has more dark matter and is the more massive of the two. Dark matter is a still unidentified substance that does not give off or reflect detectable electromagnetic radiation but can be measured by its gravitational effects. Dark matter is thought to surround most galaxies as a kind of invisible halo that influences how stars orbit within the galaxy. The mass and density of the dark matter around the Milky Way was determined by studying 12 dwarf galaxies that orbit our galaxy. III TYPES OF STARS The Milky Way contains both the so-called type I stars, brilliant, blue stars; and type II stars, giant red stars. Blue stars tend to be younger because they burn furiously and use up all of their fuel within a few tens of millions of years. Red stars are usually older, and use their fuel at a slower rate that they can sustain for tens of billions of years. The central Milky Way and the halo are largely composed of the type II population. Most of this region is obscured behind dust clouds, which prevent visual observation. Astronomers have been able to detect light from this region at other wavelengths in the electromagnetic spectrum, however, using radio and infrared telescopes and satellites that detect X rays (see Radio Astronomy; Infrared Astronomy; X-Ray Astronomy). Such studies indicate compact objects near the galactic center, probably a massive black hole. A black hole is an object so dense that nothing, not even light, can escape its intense gravity. The center of the galaxy is home to clouds of antimatter particles, which reveal themselves by emitting gamma rays when they meet particles of matter and annihilate. Astronomers believe the antimatter particles provide more evidence for a massive black hole at the Milky Way's center. Observations of stars racing around the center also suggest the presence of a black hole. The stars orbit at speeds up to 1.8 million km/h (1.1 million mph)--17 times the speed at which Earth circles the Sun--even though they are hundreds of times farther from the center than Earth is from the Sun. The greater an object's mass, the faster an object orbiting it at a given distance will move. Whatever lies at the center of the galaxy must have a tremendous amount of mass packed into a relatively small area in order to cause these stars to orbit so quickly at such a distance. The most likely candidate is a black hole. Surrounding the central region is a fairly flat disk comprising stars of both type II and type I; the brightest members of the latter category are luminous, blue supergiants. Imbedded in the disk, and emerging from opposite sides of the central region, are the spiral arms, which contain a majority of the type I population together with much interstellar dust and gas. One arm passes in the vicinity of the Sun and includes the great nebula in Orion. See Nebula. IV ROTATION 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). V 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. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.