Final Frontier...

What is a black hole?

A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent light or anything else from escaping.The existence of black holes was first proposed in the 18th century, based on the known laws of gravity. The more massive an object, or the smaller its size, the larger the gravitational force felt on its surface. John Michell and Pierre-Simon Laplace both independently argued that if an object were either extremely massive or extremely small, it might not be possible at all to escape its gravity. Even light could be forever captured.The name "black hole" was introduced by John Archibald Wheeler in 1967. It stuck, and has even become a common term for any type of mysterious bottomless pit. Physicists and mathematicians have found that space and time near black holes have many unusual properties. Because of this, black holes have become a favorite topic for science fiction writers. However, black holes are not fiction. They form whenever massive but otherwise normal stars die. We cannot see black holes, but we can detect material falling into black holes and being attracted by black holes. In this way, astronomers have identified and measured the mass of many black holes in the Universe through careful observations of the sky. We now know that our Universe is quite literally filled with billions of black holes.There are different types of black holes depending upontheir size. the sizes are measured in thousands of solar massesSupermassive black holes: Contain hundreds of thousands to billions of solar masses are believed to exist in the center of most galaxies, including our own Milky Way. They are thought to be responsible for active galactic nuclei, and presumably form either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. The largest known supermassive black hole is located in OJ 287 weighing in at 18 billion solar masses.Intermediate-mass black holes: They have been proposed as a possible power source for the ultra-luminous X ray sources. There is no known mechanism for them to form directly, so they most probably form via collisions of lower mass black holes, either in the dense stellar cores of globular clusters or galaxies. Such creation events should produce intense bursts of gravitational waves, which may be observed in the near- to mid-term. The boundary limit between super- and intermediate-mass black holes is a matter of convention. Their lower mass limit, the maximum mass for direct formation of a single black hole from collapse of a massive star, is poorly known at present. Stellar-mass black holesThey have masses ranging from a lower limit of about 1.5–3.0 solar masses (the Tolman-Oppenheimer-Volkoff limit for the maximum mass of neutron stars) up to perhaps 15–20 solar masses, and are created by the collapse of individual stars, or by the coalescence (inevitable, due to gravitational radiation) of binary neutron stars. Stars may form with initial masses up to ≈100 solar masses, or possibly even higher, but these shed most of their outer massive layers during earlier phases of their evolution, either blown away in stellar winds during the red giant, AGB, and Wolf-Rayet stages, or expelled in supernova explosions for stars that turn into neutron stars or black holes. Being known mostly by theoretical models for late-stage stellar evolution, the upper limit for the mass of stellar-mass black holes is somewhat uncertain at present. The cores of still lighter stars form white dwarfs. Micro black holes (also mini black holes) They have masses much less than that of a star. At these sizes, the effects of quantum mechanics are expected to come into play. There is no known mechanism for them to form via normal processes of stellar evolution, but certain inflationary scenarios predicted their production during the early stages of the evolution of the universe. According to some theories of quantum gravity they may also be produced in the highly energetic reaction produced by cosmic rays hitting the atmosphere or even in particle accelerators such as the Large Hadron Collider. The theory of Hawking radiation predicts that such black holes will evaporate in bright flashes of gamma radiation. NASA's Fermi Gamma-ray Space Telescope satellite (formerly GLAST), launched in 2008, will search for such flashes as one of its scientific objectives.

Final Frontier...

Black Holes?

A black hole is an object with a gravitational field so powerful that a region of space becomes cut off from the rest of the universe – no matter or radiation, including visible light, that has entered the region can ever escape। The lack of escaping electromagnetic radiation renders the inside of black holes (beyond the event horizon) invisible, hence the name। However, black holes can be detectable if they interact with matter, e।g. by sucking in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of light, X-rays and Gamma rays in the process while still outside of the event horizon. Black holes are also thought to emit a weak form of thermal energy called Hawking radiation.

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