What Is the Milky Way Galaxy?
The Milky Way, or Galaxy, is the whole concourse of stars and other bodies which can be seen stretched across the heavens. It includes our own sun and its planets, as well as all stars visible to the naked eye. But the name is commonly restricted to the luminous band or belt where most classes of stars are concentrated.
The spiral arms of the Milky Way are rich in hot, bright stars; inter stellar clouds of gas (mainly hydrogen) and dust. The first evidence of spiral arms was obtained in 1951 by the American astronomer W. W. Morgan, who identified three. Our own system of sun and planets appears to be situated towards the inner edge of one of the arms, which is about 1,300 light-years away.
The Andromeda nebula, a vast mixture of gaseous and solid matter, is visible as a small luminous patch in our sky. But it is comparable in size to the Milky Way and seems remarkably similar to our own galaxy. The Palomar telescope, 200 inches in diameter, situated on Mount Palomar in California, has perhaps 1,000 million galaxies within the scope of its vision.
The descriptive “milky” is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within.
Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars. There are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm.
The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a super massive black hole. Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second.
The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed “dark matter”. The rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference.
The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.