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Milky Way Galaxy

The Milky Way seen in the night skyThe Milky Way seen in the night sky Every star you see in the night sky is part of the Milky Way Galaxy (although on a clear night, you can glimpse one or two galaxies that lie beyond the Milky Way). The Milky Way Galaxy, the galaxy to which our own star, the Sun, belongs, is a vast, disc-like spiral of stars. It closely resembles the Andromeda Galaxy, which lies 2.25 million light years away. Named after the misty band of stars in the night sky—actually our side-on view of one of its spiral arms—it contains about 200 billion stars and measures about 100,000 light years across. It spins at 250 km/sec (150 miles/sec), taking more than 200 million years to complete one full circle.

Milky Way GalaxyMilky Way Galaxy
Milky Way Galaxy in profileMilky Way Galaxy in profile

Side-on view

Seen from the side, the Milky Way Galaxy looks like a pair of fried eggs stuck together back-to-back. The “yolks” form the nucleus, while the “whites” form the spiral-shaped disc surrounding it. 

Structure

The Milky Way Galaxy has a bulge at its centre, called the nucleus, where older, red stars are concentrated. Four giant arms spiral out from the nucleus: the Crux-Centaurus Arm, the Perseus Arm, the Orion Arm and the Sagittarius Arm. Younger blue stars are found in these arms, along with clouds of gas and dust, where new stars are forming. The Sun is situated on one of the spiral arms about 25,000 light years—roughly halfway out—from the nucleus. Here are mostly yellow and orange young-to-middle aged stars. 

Milky Way GalaxyMilky Way GalaxyThe galactic disc is surrounded by a halo of old stars and globular clusters (dense, ball-shaped collections of stars), that form an enormous sphere measuring about 200,000 light years in diameter.




It takes the Galaxy more than 200 million years to complete one full circle.

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