[DOWNLOAD] "Deep-Sky." by Sky Guide Africa South " eBook PDF Kindle ePub Free
eBook details
- Title: Deep-Sky.
- Author : Sky Guide Africa South
- Release Date : January 01, 2009
- Genre: Astronomy,Books,Science & Nature,
- Pages : * pages
- Size : 147 KB
Description
Beyond the Sun, Moon and planets lie the stars of our Galaxy. Some are grouped together in clusters, others are spread out along the plane of the Galaxy along its spiral arms, a region rich in nebulae (vast clouds of gas & dust). Far beyond our Galaxy we encounter other galaxies, clusters of galaxies and quasars. It is such objects--galaxies, star clusters, nebulae, quasars and clusters of galaxies--that deep-sky observers study. Galaxies take on a wide variety of forms, based on their structure, they can be categorized as spiral, elliptical, or irregular. Spiral galaxies are probably the most familiar; they are flat disks with a bright central bulge from which the arms unwind. Spirals contain large amounts of gas and dust, with both young and old stars. Seen through all but the largest amateur telescopes, spirals appear circular if they are orientated face-on. At least a 30-cm telescope is needed to show some spiral structure. If the galaxy is orientated edge-on, it appears cigar-shaped, sometimes with a dark dust lane bisecting it. The brightest spiral seen from Earth is the Andromeda Galaxy (Messier 31), and even though it never rises very high as seen from Southern Africa, it is still an easy naked-eye object from a dark-sky site. The brightest southern spiral is NGC 253 in Sculptor. Barred spirals are similar to spirals, except that their arms curve away from a bar-like feature extending through the galactic centre. A prominent southern barred spiral is NGC 1365 in Fornax. Elliptical galaxies range from spherical to rugby ball shaped, and have no arms. Unlike spirals, they are composed of only old stars, with little interstellar matter. Through a telescope, ellipticals appear round to spindle-shaped. Messier 87 in Virgo is a good example of a bright elliptical galaxy. Irregular galaxies have no particular shape--the Small Magellanic Cloud is an example. They contain mostly young stars, with much interstellar matter. The dwarf galaxies are much smaller than the others and are particularly difficult to see.