What is milky way? What are the components, characteristics, structure of milky way? Information about milky way galaxy.


The visible band of the Milky Way describes an almost perfect great circle in the sky, indicating that our sun and its planets are located close to the central plane of the galaxy. For many years astronomers did not question the presupposition that the sun lay at or very near the center of our galaxy, so it came as a shock to many when it was found that our sun lies far from the center. The U. S. astronomer Harlow Shapley made this determination about 1920 by studying the distribution of remote galactic objects, such as globular clusters of stars. Modern estimates place the distance from the sun to the galactic center—located in the direction of the constellation Sagittarius—at 33,000 light-years. The main body of the Milky Way galaxy is a flattened disk with an overall diameter of nearly 100,000 light-years.

The flatness of the equatorial disk of the galaxy suggests that it must be rotating fairly rapidly. This is indeed so. The observational evidence for galactic rotation strongly indicates an average circular velocity, for stars in the neighborhood of our sun, of about 160 miles (260 km) per second. The sun whirls around the galactic center at this rapid speed, but, with an orbital radius of 33,000 light-years, it takes our star fully 250 million years to complete a single circuit. The gravitational force that controls this motion is produced by the central star clouds of the galaxy, which must have a total mass of about 50 billion solar masses.

Components. The galaxy is a conglomerate of young and old stars that account for about 90% of the total galactic mass. Interstellar gas makes up the other 102, and there is a sprinkling of cosmic dust. The interstellar gas is composed primarily of hydrogen and helium atoms, but it also contains a mixture of other elements—carbon, nitrogen, oxygen, neon, iron, calcium, and so forth. The cosmic dust probably consists of tiny icelike particles—frozen combinations of carbon, nitrogen, and oxygen, with a little iron as well. The particles that produce obscuring clouds in the plane of the Milky Way in all probability have average diameters of a few hundred thousandths of a millimeter.

Structure. The thickness of the layer of interstellar gas is no more than about 1,000 light-years in the region of our sun, and probably does not exceed twice that amount elsewhere in the galaxy. This means that interstellar gas is probably confined to a central layer in the galaxy, with a thickness of about 1,000 light-years and an overall diameter of 100,000 light-years. This thin layer is also typified by cosmic dust and by the young blue-white supergiant stars that for the most part apparently have been formed from the interstellar gas within the past 10 million years. These young stars, together with the cosmic dust and gas, are often referred to as Population I.

A different picture is obtained when considering the distribution of stars as old as or older than our sun (about 4 to 5 billion years). The overall diameter of the area in which these stars exist is about 100,000 light-years, but the thickness of the area is much greater than that of the Population I material. The oldest of the stars lie more than 5,000 light-years above or below the central galactic plane; indeed, some outlying stars and clusters of stars suggest that an almost spherical halo of such objects surrounds our galaxy. The existence of this sparsely populated outer halo is supported by evidence from cosmic rays and from radio-astronomical studies of synchrotron radiation (emitted by free electrons spiraling about the weak lines of force of large-scale magnetic fields in the halo). These older stars in the halo and the galactic plane are referred to as Population II.

The spiral structure of the galaxy seems to be confined for the most part to a very flat ring in the galactic plane. The ring has an inner radius of about 12,000 light-years and an outer radius of 50,000 light-years, and is only about 1,000 light-years thick. This is the region of the galaxy where star formation still is taking place. Very different conditions exist in the halo, where little or no free interstellar gas is available for star formation. The galactic nucleus, about 20,000 light-years wide, is in many ways an enigma. It contains older stars resembling those of Population II and gives every sign of being a region where new stars no longer are being formed. Yet there are still considerable amounts of interstellar gas in the nucleus. The gas is in a highly turbulent state, and some features resemble sections of spiral arms. One, about 10,000 light-years from galactic center, is expanding toward our sun at a rate of 30 miles (50 km) per second. Much of the radio radiation from the nucleus originates in clouds of neutral atomic hydrogen and in small dense clouds containing the OH (hydroxyl) radical. Part of the radiation, however, is of the synchrotron variety. Some of the dozen or more radio sources detected in the nucleus seem to be gas clouds embedded with young stars.


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