Speach on galaxy
Agalaxyis a massive,gravitationally boundsystem consisting ofstars,stellar remnants, aninterstellar mediumof gas anddust, anddark matter, an important but poorly understood component.The word galaxy is derived from theGreekgalaxias(γαλαξÎας), literally "milky", a reference to theMilky Way. Examples of galaxies range fromdwarfswith as few as ten million (107) starsto giants with a hundredtrillion(1014) stars,each orbiting their galaxy's owncenter of mass.
Galaxies contain varying numbers ofstar systems,star clustersand types ofinterstellar clouds. In between these objects is a sparseinterstellar mediumof gas, dust, andcosmic rays.Supermassive black holesreside at the center of most galaxies. They are thought to be the primary driver ofactive galactic nucleifound at the core of some galaxies. The Milky Way galaxy is known to harbor at least one such object.[5]
Galaxies have been historically categorized according to their apparent shape, usually referred to as their visual morphology. A common form is theelliptical galaxy,which has anellipse-shaped light profile.Spiral galaxiesare disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known asirregular galaxiesand typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merger, sometimes induce significantly increased incidents ofstar formationleading tostarburst galaxies. Smaller galaxies lacking a coherent structure are referred to asirregular galaxies.
There are probably more than 170billion (1.7 × 1011) galaxies in theobservable universe.Most are 1,000 to 100,000parsecsin diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs).Intergalactic space(the space between galaxies) is filled with a tenuous gas of an average density less than oneatomper cubic meter. The majority of galaxies are organized into a hierarchy of associations known asgroups and clusters, which, in turn usually form largersuperclusters. At thelargest scale, these associations are generally arranged intosheets and filaments, which are surrounded by immensevoids.
On December 12, 2012, astronomers, working with theHubble Space Telescope, reported that themost distant known galaxy,UDFj-39546284, is now estimated to be even farther away than previously believed. The galaxy, which is estimated to have formed around "380 million years"]after theBig Bang(about 13.8 billion years ago),[11]and has a z (redshift) of 11.9,[is approximately 13.42 billionlight yearsfrom Earth.
