| Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope —and How to Find Them —Guy Consolmagno, Dan M. Davis, Karen Kotash Sepp, Anne Drogin, Mary Lynn Skirvin— (Seasonal Objects: Spring) | Galaxies are the basic units of the universe. | | After the universe was created it seems to have fragmented into discrete lumps of matter, each with enough mass to make billions of stars. | | A lump would condense into a galaxy, with a cloud of globular clusters swarming erratically around the galactic center, and a disk of stars orbiting the center. | | Galaxies come in three general forms: elliptical galaxies, irregular galaxies, spiral galaxies. | | Elliptical galaxies look like large, somewhat flattened globular clusters (a collection of tens of thousands of stars in a ball within the galaxy). | | In spiral galaxies, stars are organized into two or more arms that twist around their galactic center. Spirals are the most beautiful of galaxies. | | The Whirlpool Galaxy, and our own Milky Way are examples of spiral galaxies. The Whirlpool's companion may be an elliptical galaxy. | | Galaxies are observed to be clumped together into clusters, which may be anywhere from a dozen to hundreds of galaxies, each tied by the gravity of others into a cloud moving together through space. | | Andromeda and its companions, the Triangulum Galaxy, and the Milky Way and its companions (the Magellanic Clouds) are all part of the Local Group. The galaxies in Ursa Major and in Leo are examples of members of other groups. | | All the clusters appear to be moving away from each other, implying that they are the fragments of the Big Bang that took place some twelve to fifteen thousand billion years ago. | | The clusters themselves are associated into clusters of clusters called superclusters. | | We don't know whether these superclusters are independent entities, like raisins in a pudding, or connected together like the stuff in a sponge around “bubbles” of empty space. | |
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