Faint band of light crossing the night sky, consisting of stars in the plane of our Galaxy. The name Milky Way is often used for the Galaxy itself. It is a spiral galaxy, 100,000 light years in diameter and 2,000 light years thick, containing at least 100 billion stars.
The Sun is in one of its spiral arms, about 27,000 light years from the centre, not far from its central plane. An extra arm to the Milky Way was discovered by astronomers in 2004. The arc of hydrogen is a few thousand light years thick and over 77,000 light years long, running along the outermost edge of the Milky Way.
The densest parts of the Milky Way, towards the Galaxy's centre, lie in the constellation Sagittarius. In places, the Milky Way is interrupted by lanes of dark dust that obscure light from the stars beyond, such as the Coalsack nebula in Crux (the Southern Cross). It is because of these that the Milky Way is irregular in width and appears to be divided into two between Centaurus and Cygnus.
The Milky Way passes through the constellations of Cassiopeia, Perseus, Auriga, Orion, Canis Major, Puppis, Vela, Carina, Crux, Centaurus, Norma, Scorpius, Sagittarius, Scutum, Aquila, and Cygnus. NASA's Rossi X-Ray Timing Explorer spacecraft was launched in 1995 to explore the Milky Way.
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