A globular cluster is a spherical collection of stars. Globular clusters are very tightly bound by gravity,with a high concentration of stars towards their centers. Their name is derived from Latin globulus—a small sphere. Globular clusters are occasionally known simply as globulars.
A quasar is an extremely luminous active galactic nucleus (AGN),powered by a supermassive black hole,with mass ranging from millions to tens of billions of solar masses,surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous;the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Usually,quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.
NGC 6240,also known as the Starfish Galaxy,is a nearby ultraluminous infrared galaxy (ULIRG) in the constellation Ophiuchus. The galaxy is the remnant of a merger between three smaller galaxies. The collision between the three progenitor galaxies has resulted in a single,larger galaxy with two distinct nuclei and a highly disturbed structure,including faint extensions and loops.
Seyfert galaxies are one of the two largest groups of active galaxies,along with quasars. They have quasar-like nuclei with very high surface brightnesses whose spectra reveal strong,high-ionisation emission lines,but unlike quasars,their host galaxies are clearly detectable.
Messier 87 is a supergiant elliptical galaxy with several trillion stars in the constellation Virgo. One of the most massive galaxies in the local universe,it has a large population of globular clusters —about 15,000 compared with the 150–200 orbiting the Milky Way —and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs,traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
A supermassive black hole is the largest type of black hole,with its mass being on the order of millions to billions of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse,leaving behind spheroidal regions of space from which nothing can escape,not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example,the Milky Way has a supermassive black hole in its Galactic Center,corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.
Messier 61 is an intermediate barred spiral galaxy in the Virgo Cluster of galaxies. It was first discovered by Barnaba Oriani on May 5,1779,six days before Charles Messier discovered the same galaxy. Messier had observed it on the same night as Oriani but had mistaken it for a comet. Its distance has been estimated to be 45.61 million light years from the Milky Way Galaxy.
In astronomy,a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them,but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges:bulges that are like ellipticals and bulges that are like spiral galaxies.
Centaurus A is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta,in New South Wales,Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance. NGC 5128 is one of the closest radio galaxies to Earth,so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky,making it an ideal amateur astronomy target. It is only visible from the southern hemisphere and low northern latitudes.
The Sombrero Galaxy is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus,being about 9.55 megaparsecs from our galaxy,within the local supercluster. It has a diameter of approximately 15 kiloparsecs,three-tenths the size of the Milky Way. It has a bright nucleus,an unusually large central bulge,and a prominent dust lane in its outer disk,which is viewed almost edge-on. The dark dust lane and the bulge give it the appearance of a sombrero hat. Astronomers initially thought the halo was small and light,indicative of a spiral galaxy;but the Spitzer Space Telescope found that the dust ring was larger and more massive than previously thought,indicative of a giant elliptical galaxy. The galaxy has an apparent magnitude of +8.0,making it easily visible with amateur telescopes,and is considered by some authors to be the galaxy with the highest absolute magnitude within a radius of 10 megaparsecs of the Milky Way. Its large bulge,central supermassive black hole,and dust lane all attract the attention of professional astronomers.
APM 08279+5255 is a very distant,broad absorption line quasar located in the constellation Lynx. It is magnified and split into multiple images by the gravitational lensing effect of a foreground galaxy through which its light passes. It appears to be a giant elliptical galaxy with a supermassive black hole and associated accretion disk. It possesses large regions of hot dust and molecular gas,as well as regions with starburst activity.
A low-ionization nuclear emission-line region (LINER) is a type of galactic nucleus that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms,such as O,O+,N+,and S+. Conversely,the spectral line emission from strongly ionized atoms,such as O++,Ne++,and He+,is relatively weak. The class of galactic nuclei was first identified by Timothy Heckman in the third of a series of papers on the spectra of galactic nuclei that were published in 1980.
NGC 4261 is an elliptical galaxy located around 100 million light-years away in the constellation Virgo. It was discovered April 13,1784 by the German-born astronomer William Herschel. The galaxy is a member of its own somewhat meager galaxy group known as the NGC 4261 group,which is part of the Virgo Cluster.
David Roy Merritt is an American astrophysicist. Until 2017 he was a professor at the Rochester Institute of Technology in Rochester,New York. He received in 1982 his PhD in Astrophysical Sciences from Princeton University with thesis advisor Jeremiah P. Ostriker and held postdoctoral positions at the University of California,Berkeley and the Canadian Institute for Theoretical Astrophysics in Toronto. Merritt's fields of specialization include dynamics and evolution of galaxies,supermassive black holes,and computational astrophysics.
NGC 4293 is a lenticular galaxy in the northern constellation of Coma Berenices. It was discovered by English astronomer William Herschel on March 14,1784,who described it as "large,extended,resolvable,6 or 7′long". This galaxy is positioned to the north-northwest of the star 11 Comae Berenices and is a member of the Virgo Cluster of galaxies. It is assumed to lie at the same distance as the Virgo Cluster itself:around 54 million light years away. The galaxy spans an apparent area of 5.3 ×3.1 arc minutes.
NGC 7469 is an intermediate spiral galaxy in the constellation of Pegasus. NGC 7469 is located about 200 million light years away from Earth,which means,given its apparent dimensions,that NGC 7469 is approximately 90,000 light years across. It was discovered by William Herschel on November 12,1784.
NGC 6951 is a barred spiral galaxy located in the constellation Cepheus. It is located at a distance of about 75 million light-years from Earth,which,given its apparent dimensions,means that NGC 6951 is about 100,000 light-years across. It was discovered by Jérôme Eugène Coggia in 1877 and independently by Lewis Swift in 1878.
NGC 2273 is a barred spiral galaxy located in the constellation Lynx. It is located at a distance of circa 95 million light years from Earth,which,given its apparent dimensions,means that NGC 2273 is about 100,000 light years across. It was discovered by Nils Dunér on September 15,1867.
NGC 4800 is an isolated spiral galaxy in the constellation Canes Venatici,located at a distance of 95 megalight-years from the Milky Way. It was discovered by William Herschel on April 1,1788. The morphological classification of this galaxy is SA(rs)b,indicating a spiral galaxy with no visual bar at the nucleus (SA),an incomplete ring structure (rs),and moderately-tightly wound spiral arms (b). The galactic plane is inclined to the line of sight by an angle of 43°,and the long axis is oriented along a position angle of 25°. There is a weak bar structure at the nucleus that is visible in the infrared.
