![]() The realization that we live in a galaxy that is one among many parallels major discoveries about the Milky Way and other nebulae. Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey, in which M109 is cataloged as SDSS J115735.97+532228.9. All the well-known galaxies appear in one or more of these catalogs but each time under a different number.įor example, Messier 109 (or "M109") is a spiral galaxy having the number 109 in the catalog of Messier. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC ( New General Catalogue), the IC ( Index Catalogue), the CGCG ( Catalogue of Galaxies and of Clusters of Galaxies), the MCG ( Morphological Catalogue of Galaxies), the UGC ( Uppsala General Catalogue of Galaxies), and the PGC ( Catalogue of Principal Galaxies, also known as LEDA). Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. SDSS stands for Sloan Digital Sky Survey, J for Julian epoch, and 1152+3313 for right ascension and declination respectively. Nomenclature Galaxy cluster SDSS J1152+3313. Instead, they became known simply as galaxies. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Galaxies were initially discovered telescopically and were known as spiral nebulae. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The group is part of the Virgo Supercluster. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. Most galaxies are gravitationally organized into groups, clusters and superclusters. The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter. For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy (with diameter of about 152,000 ly), its nearest large neighbor, by 780,000 parsecs (2.5 million ly.) Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). It is estimated that there are roughly 200 billion galaxies ( 2 ×10 11) in the observable universe. It has a comoving distance of 32 billion light-years from Earth, and is seen as it existed just 400 million years after the Big Bang. As of March 2016, GN-z11 is the oldest and most distant galaxy observed. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. Many are thought to have supermassive black holes at their centers. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Supermassive black holes are a common feature at the centres of galaxies. ![]() Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. The word is derived from the Greek galaxias ( γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ![]() NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.Ī galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |