what is active galactic nuclei?
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They were first defined as a class of objects at the center of galaxies that were very small and very bright, often observable as radio sources and optical sources. There are several classes of AGN depending on their spectral characteristics (Liners, Seyfert Type 1 and Type 2, BL Lac objects, blazars, etc.)  Robert "Ski" Antonucci wrote a great review article on AGN that educated me a great deal in what was becoming a popular interpretation that unifies all this menagerie (some AGN look VERY different from others) into one phenomenon:  a supermassive black hole accreting gas a fairly good clip.  The different spectral characteristics rely upon the orientation of the disk of gas around the black hole relative to us, just how much gas is flowing into the black hole, and the environment around the black hole.\n\nQuasars are believed to be AGN that are both consuming gas at a high rate and happen to oriented axis on towards earth (we are looking right down the bipolar jets associated with such objects.)\n\nInterestingly we have found supermassive black holes in the center of every galaxy we have been able to examine closely enough (including our own!)  These are always about 0.1% the mass the parent galaxy.  In most cases they are not being fed much.  When they are, you have an AGN.\n\nI believe this is still the accepted theory, though I have been out of touch with the field for a 3-4 years now.
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All matter and energy, including the earth, the galaxies, and the contents of intergalactic space, regarded as a whole. \n\nThe earth together with all its inhabitants and created things. \nThe human race. \nThe sphere or realm in which something exists or takes place.
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We can only give a rough guess as to how many galaxies there are, and it's billions at least. All but the closest ones visible as galaxies to the naked eye have names. Our local galaxy is the Milky Way, which you can see in a dark place on a clear night as a streak of stars running across the sky. A cluster is a group of galaxies.\n\nSome galaxies we have discovered with Hubble and other new telescopes seem to crashing into each other, and the Milky Way may be on its way to having a galactic interloper in the far future.
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a black hole is an area of space-time with a gravitational field so intense that its escape velocity is equal to or exceeds the speed of light.
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an accretion disk is a disk of matter from the smaller star that is being sucked into the other star by gravity.
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Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects, as well as the formation and development of the universe.\n\nhttp://en.wikipedia.org/wiki/Astronomy\n\n----------------------------------------------------------------------------\n\nGeology (from Greek γη- (ge-, "the earth") and λογος (logos, "word", "reason"))is the science and study of the solid matter of a celestial body, its composition, structure, physical properties, history and the processes that shape it. It is one of the Earth sciences.\n\nhttp://en.wikipedia.org/wiki/Geology\n\n---------------------------------------------------------------------------\n\nChemistry (from Greek χημεία khemeia[1] meaning "alchemy") is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as molecules, crystals, and metals. Chemistry deals with the composition and statistical properties of such structures, as well as their transformations and interactions to become materials encountered in everyday life. Chemistry also deals with understanding the properties and interactions of individual atoms with the purpose of applying that knowledge at the macroscopic level. According to modern chemistry, the physical properties of materials are generally determined by their structure at the atomic scale which is determined by the properties and energies of the interactions.\n\nhttp://en.wikipedia.org/wiki/Chemistry\n\n-----------------------------------------------------------------------------\n\nPhysics (from the Greek, φύσις (phúsis), "nature" and φυσική (phusiké), "knowledge of nature") is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space and time. That is, physics deals with the elementary constituents of the Universe and their interactions, as well as the analysis of systems which are best understood in terms of these fundamental principles. Physics is a study of the inorganic, physical world, as opposed to the organic world of biology, physiology, etc. Physics overlaps with chemistry concerning the electro-chemical interactions of substances.\n\nhttp://en.wikipedia.org/wiki/Physics\n\n--------------------------------------------------------------------------------\n\nMeteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. Meteorological phenomena are observable weather events which illuminate and are explained by the science of meteorology. Those events are bound by the variables that exist in Earth's atmosphere. They are temperature, pressure, water vapor, and the gradients and interactions of each variable, and how they change in time. The majority of Earth's observed weather is located in the troposphere.\n\nMeteorology, climatology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology comprise the interdiscplinary field of hydrometeorology.\n\nhttp://en.wikipedia.org/wiki/Meteorology
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Humans found out that they live in a small living planet in the middle of a very-small galaxy. our galaxy is in the middle of a small super-cluster too. And they dont know more about this.
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The answer is (A) most quasars happened a long time ago, so we can only see those that are very distant.\n\nQuasars are the precursors of galaxy formations. They are gigantic blackholes ejecting enormous amount of energy at their poles. These massive blackholes are now at the centers of all galaxies, including our own the Milky Way. So the quasars haven't disappeared, they've just turned into the galaxies that we see now.
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Hawkings radiation is the radiation that a black hole gives off. It is why Black Holes have not become so big as to distroy everything.  It also is one of the things which might eventually lead to the combining of general relativity and Quantum Theory.
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The largest galaxies that we know of are giant elliptical galaxies in the centres of clusters of galaxies. They are sometimes called cD galaxies (for giant diffuse galaxies) or BCGs (for brightest cluster galaxies). These galaxies are about 10 times more luminous that a typical galaxy (like the Milky Way) and about 100 times more massive. They can have diameters of more than 6 million light years (compare to about 100 thousand light years for the Milky Way). A good example is the central galaxy in the cluster Abell 2029. \n\ncD galaxies are thought to grow by accreting any galaxy that comes anywhere near them, and in the centre of a cluster this can be a lot of galaxies over the age of the universe - so their mass can grow by a large factor. They usually have an area around them in which no smaller galaxies are found (as they've all been 'eaten') and a diffuse halo of gas and dust which is thought to be the debris of the smaller galaxies.
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Check your terminology! A spiral galaxy is an immense collection of stars, planets along with huge clouds of dust and gas. Our sun and its planets are inside a spiral galaxy called the Milky Way.\n\nGod is spelled B-I-G B-A-N-G\nPBrax@netscape.com
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The Sun sudden bursts of energy contains Gamma rays and X-rays, plus energized particles (protons and electrons). This forms the Solar wind. Flare activity picks up as sunspots increase.\n\nhttp://www.space.com/scienceastronomy/solarsystem/sun-ez.html
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Nebula, in the most basic sense, is any observable cloud of gas.  You have objects like the Orion nebula (a star forming region), the Ring Nebula (a "planetary" nebula that is the result of a low-mass star's death), and the Crab Nebula (gas expelled during the supernoave death of a higher mass star).\n\nA black hole is, at its most basic, a region of space where the local escape velocity exceeds the speed of light.
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The Virgo Supercluster.\nWikipedia knows all.
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no, but i can tell you what we think it is:\n\nit is a star whose gravity is too great for the nuclear fusion in its core to negate.  it collapses in on itself and goes supernova.  then it is a neutron star.  if the gravity is still too great, it collapses some more and becomes a black hole.  its gravity is so strong that light itself cannot escape it (thus it is 'black').  they are suspected to be at the center of many galaxies, especially quasars and other active galaxies.  one is believed to be at the center of our own galaxy, the milky way.\n\nonce again, no one has really seen one, just the indirect evidence that one exists (strange star orbits, gas being sucked into nothing, strange bursts of some kind of rays (can't remember what kind) coming out of the poles, etc.\n\neverybody is pretty convinced that they exist.  its just kinda hard to prove.\n\nps the idiot down there who said black holes are dead galaxies has no clue what he is talking about. please ignore him.  and its not the size of an atom, its an infinitely small point, the swartzchild point.
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"The Local Group (also called M31 group or the Andromeda group) is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 30 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy. The galaxies of the Local Group cover a 10 million light year diameter (see 1 E23 m for distance comparisons). The group belongs to the Virgo Supercluster."
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In astronomy, an object so massive that nothing, not even light, can escape its gravitation. Black holes were given their name because they absorb all the light that falls on them. The existence of black holes was first predicted by the general theory of relativity. Supermassive black holes have been found in the centers of many galaxies. Stellar black holes are thought to arise from the death of very massive stars. Astronomers expect to find many stellar black holes in the Milky Way. \n\n\nFiguratively, the term black hole is used to refer to a total disappearance: “They never saw the man again — he might as well have fallen into a black hole.”