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The Milky Way
Galaxies are systems of billions of stars, clusters, and nebulae, and are located many millions or billions of light-years from Earth. All the stars visible to the naked eye are located inside our own galaxy, the Milky Way.
The Milky Way
When viewed from the Earth, our own Milky Way galaxy appears to be a band of light across the sky. The stars in the Milky Way are so numerous and so dense that the ancients saw it as as a kind of “river” through the heavens. The first person to understand the true nature of the Milky Way was Galileo. When he turned his small telescope skyward in 1601, he was able to resolve the Milky Way into its millions of individual stars.
The center of the Milky Way is located in the direction of Sagittarius, which is a summer constellation in the northern hemisphere. Thus, the Milky Way is more spectacular in the summer. To see the Milky Way in its full glory, very dark skies are required, far away from city lights. Anyone observing the Milky Way will also notice dark “dust lanes”, which block the light of stars further behind them. Observers in the southern hemisphere will also see two smaller patches of light, the Magellanic Clouds (named after Ferdinand Magellan, who was the first European to see them). The Magellanic Clouds are actually two small, nearby satellite galaxies which orbit the Milky Way.
Panorama of the Milky Way centered on Sagittarius. Note the Magellanic Clouds to the lower right. (Axel Mellinger.)
The discovery of true size and shape of the Milky Way was one of the great advances of 20th-century astronomy. We now know that the Milky Way contains about 200 billion stars - more than the total number of humans who have ever lived!
At the center of the Milky Way is the galactic nucleus, with a radius of about 10,000 light years. The nucleus consists of mainly older stars, and is very densely packed. Orbiting around the nucleus, the remainder of the Milky Way’s stars, gas, and dust are spread out into a flat disk. The diameter of the disk is about 100,000 light years, but its thickness is only a few thousand light years. Thus, the disk is very thin compared to its diameter.
It is difficult to detect the spiral arms in our galaxy because: 1) we are inside the galaxy, and 2) the dark dust lanes block our view. Radio astronomers have traced out the spiral structure of the arms using interstellar hydrogen, which generates a characteristic radio emission with a wavelength of 21 cm. Most of the sources of this 21-cm emission are hydrogen gas clouds, which are abundant in the spiral arms. Our Sun is located in the Orion spiral arm, about two-thirds of the way out from the center.
The structure of the Milky Way galaxy.
The nucleus and disk are surrounded by the galactic halo, which is spherical and even larger than the galactic disk. The halo consists of old stars (some in globular clusters) and interstellar matter.