Milky Way

The Milky Way.

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The **Milky Way** is the galaxy which contains the Solar System. The name "milky" is derived from its appearance looking like "milky patches" as a dim glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. From Earth, the Milky Way appears like a band because of its disk-shaped structure is viewed from within the Galaxy. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. In the past, astronomers thought that all of the stars in the universe were contained in the Milky Way. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edward Hubble showed that the Milky Way was just one of the billions of galaxies in the universe.

The Milky Way is a barred spiral galaxy some 100,000-120,000 light-years in diameter which contains approximately 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 the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A\*, likely a supermassive 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 form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.

## Appearance

The Milky Way is visible from Earth as a hazy band of white light some 30 degrees wide arcing across the sky. Although all the individual naked-eye stars in the entire sky are part of the Milky Way, the light in this band originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where light from distant stars is blocked by interstellar dust. The area of the sky obscured by the Milky Way is called the Zone of Avoidance.

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen. should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from any brightly lit urban or suburban location, but very prominent when viewed from a rural area when the Moon is below the horizon. "The new world atlas of artificial night sky brightness" shows that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.