The Whirlpool galaxy, M51
by Rob Gendler
Galaxies are gigantic cities of stars, typically measuring 100,000 light years in diameter and containing over 100 billion stars. The Sun and all the other stars we see in the sky belong to one such galaxy, the Milky Way. Through a telescope one can see many other galaxies—such as the Whirlpool galaxy, in the constellation Ursa Major, the big bear. This long-exposure photo shows the blue-white light of bright giant stars recently formed along the spiral arms of the Whirlpool galaxy, the yellow glow of older stars near its center, and the reds of emission nebulae. The object to the right is another galaxy passing nearby—a relationship discussed in one of the closing scenes of Seeing in the Dark. Like the beautiful picture of the Orion Nebula, this image was carefully "constructed" using several different photographs, each taken in a different color of light.