The universe is expanding with most galaxies rushing away from each other, yet their distribution is uneven. Galaxies tend to exist in clusters, bound together gravitationally and constantly influencing each other both distally and locally.
Island Galaxies
A galaxy is any large and distinct collection of stars and dust whose shape is due mainly to the collective gravitational influence of all material within it. The term island galaxy describes the isolated location in interstellar space where the majority of galaxies outside the Milky Way can be found. In general galaxies are found in obvious clusters.
Galaxies are classified in optical light, based on the character of the central bulge or nucleus where star population is densest and the composition of the surrounding flat disc. Most galaxies rotate like giant wheels resulting from the accretion process from the formation of the galaxy. The slow steady rotation creates spiral arms of stars at the outermost edges.
The Spiral Galaxy M83
Deep sky objects in the dark night sky are fuzzy patches that lie outside the Solar System, they include star clusters, nebulae and galaxies. A telescope is essential for observing most deep sky objects, yet the Seven Sisters of the Pleiades and the Orion Nebula are crystal clear naked eye objects.
M83 is a prominent member of a galactic group that includes Centaurus A and NGC 5253. Several bright supernova explosions have been recorded in M83. Fortunately for life on Earth these stellar explosions are too far distant to be a risk to life. An X-ray photograph shows hot gas in the spiral arms and superheated gas at the core where sources of intense energy, probably in the form of neutron stars and black holes, are more densely packed.
The Hubble Space Telescope closer inspection of M83's spiral arms identify a range of globular clusters and several dozen supernova remnants. Radio observations map out the signals from hydrogen gas, showing how the spiral arms of M83 extend far deeper into space than their optically visible traces would ordinarily suggest with a blue whorl marking the visible portion.
M83 is 15 million light-years distant from Earth. The accompanying two galaxies, Centaurus A and NGC 5253 are 11 million light-years distant from Earth and are found in the constellation Centaurus.
Centaurus A and NGC 5253
Centaurus A is a large galaxy with very active star formation. NGC 5253 has many small blue stars and little dust yet new star formation is ongoing.
Sources
Michael Manning - ' The best lack all conviction while the worst are full of passionate intensity' W.B.Yeats
