### Blackholes

Blackholes Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a black hole . Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein’s general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, by thinking about gravity under fairly simple circumstances. Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air.

Assuming you don’t throw it too hard, it will rise for a while, but eventually the acceleration due to the planet’s gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet’s gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet’s gravity is called the escape velocity. As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high.

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A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet’s center: the closer you are, the higher the escape velocity . The Earth’s escape velocity is 11.2 kilometers per second (about 25,000 M.P.H.), while the Moon’s is only 2.4 kilometers per second (about 5300 M.P.H.).We cannot see it, but radiation is emitted by any matter that gets swallowed by black hole in the form of X-rays. Matter usually orbits a black hole before being swallowed. The matter spins very fast and with other matter forms an accretion disk of rapidly spinning matter.

This accretion disk heats up through friction to such high temperatures that it emits X-rays. And also there is some X-ray sources which have all the properties described above. Unfortunately it is impossible to distinguish between a black hole and a neutron star unless we can prove that the mass of the unseen component is too great for a neutron star. Strong evidence was found by Royal Greenwich Observatory astronomers that one of these sources called Cyg X-1 (which means the first X-ray source discovered in the constellation of Cygnus) does indeed contain a black hole. It is possible there for a star to be swallowed by the black hole.

The pull of gravity on such a star will be so strong as to break it up into its component atoms, and throw them out at high speed in all directions. Astronomers have found a half-dozen or so binary star systems (two stars orbiting each other) where one of the stars is invisible, yet must be there since it pulls with enough gravitational force on the other visible star to make that star orbit around their common center of gravity and the mass of the invisible star is considerably greater than 3 to 5 solar masses. Therefore these invisible stars are thought to be good candidate black holes. There is also evidence that super-massive black holes (about 1 billion solar masses) exist at the centers of many galaxies and quasars. In this latter case other explanations of the output of energy by quasars are not as good as the explanation using a super-massive black hole. A black hole is formed when a star of more than 5 solar masses runs out of energy fuel, and the outer layers of gas is thrown out in a supernova explosion.

The core of the star collapses to a super dense neutron star or a Black Hole where even the atomic nuclei are squeezed together. The energy density goes to infinity. For a Black Hole, the radius becomes smaller than the Schwarzschild radius, which defines the horizon of the Black Hole: The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud.

This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant. So, a black hole is an object, which is so compact that the escape velocity from its surface is greater than the speed of light. The following table lists escape velocities and Schwarzchild radii for some objects: The black hole masses ranging from 4 to 15 Suns (1 solar mass = 1 Msun = 2 x 1033 grams.) And are believed to be formed during supernova explosions. The after-effects are observed in some X-ray binaries known as black hole candidates. The velocity depends on the mass of the planet.

The scientists believe if our Sun dies, the sun may turn into a black hole. Black holes were theorized about as early as 1783, when John Michell mistakenly combined Newtonian gravitation with the corpuscular theory of light . The concept of an escape velocity, Vesc, was well known, and even though the speed of light wasn’t, Michell’s idea worked the same. He showed that Vesc was proportional to mass/circumference and reasoned that, for a compact enough star, Vesc might well exceed the speed of light. His mistakes were twofold: he subscribed to the corpuscular theory of light, and he assumed that Newton’s law of universal gravitation could apply to such a situation.

These mistakes happened to cancel each other out, but when the wave theory of light gained favor, the astronomers abandoned these dark stars. In the beginning of the 20th century, Einstein proposed his theory of general relativity. The formula worked out by Michell and rederived, this time without mistakes in the derivation, by Karl Schwarzschild, gives the Schwarzschild radius for any massive body (that is, a body containing mass): RS= 2GM/c2. Vesc for any body smaller than this radius would exceed that of light, and since general relativity forbids this; any matter within RS would be crushed into the center. Thus RS can effectively be thought of as the boundary of a black hole, called an event horizon because all events within RS are causally disconnected from the rest of the universe.

There arent many physical features of a black hole. In an aphorism coined by John Wheeler , black holes have no hair, hair meaning surface features from which details of it’s formation might be obtained. There are no perturbations in its event horizon, no magnetic fields. The hole is perfectly spherical and in fact has only three attributes: it’s mass, it’s spin (angular momentum), and it’s electric charge. Of these properties, it is only the mass that concerns astronomers.

As a cloud of gas contracts, the interior heats up until the core is so hot and dense that nuclear reactions can occur. This nucleosynthesis of hydrogen into heavier elements generates a tremendous pressure, according to the ideal gas law P=NkT, and this pressure holds the star up against further gravitational collapse. This state of equilibrium, during which a star is said to be on the main sequence, lasts until the hydrogen in the core is used up, about 10 billion years for a star like the sun, whereupon gravity will resume shrinking the star. Exactly what occurs next depends on the complicated interactions between different layers of the star, but generally, the star will explode in a supernova. If there is any remnant of this explosion, its further evolution depends almost exclusively on it’s mass. A remnant below ~1.4 M (@) will collapse until it can be supported by electron degeneracy pressure and form a white dwarf.

A remnant between ~1.4 and ~3 M(@) is halted by neutron degeneracy pressure and forms a neutron star. Degeneracy pressure is an effect that results from quantum mechanical interactions when the density of subatomic particles increases. As it depends only on this density, it is non-thermal and will remain no matter how much the star cools down. Still for remnants above ~3 M(@), not even degeneracy pressure can counter the force of gravity, and a black hole is born. This was the general base that general relativity gave to astronomers, but just because something is allowed to happen doesn’t mean that it does. Most astronomers resisted such absurd realities. Astronomers are very conservative by nature, and some of the most respected and influential astronomers of the day rejected this idea so soundly that it wasn’t until the 60’s that any actual searches began.

At first, the only instruments available were the old familiar optical telescopes. Optical telescopes are just what they sound like, telescopes sensitive to the visible portion of the electromagnetic spectrum . This spectrum can reveal much information regarding the source of the light. The color indicates the temperature of a star. By combining the type of star, identified by observing lots of other stars with similar characteristics, and our models of stellar processes with a measurement of the star’s luminosity, it is possible to calculate the distance to the star. We can even determine the chemical composition of the star by observing any emission or absorption lines in the spectra. Furthermore, these lines are very distinctive, and if they appear in the correct relation to each other but have been Doppler-shifted towards the red or blue ends of the spectrum, a measurement of the star’s speed relative to the earth can be obtained.

The only distinguishing feature of a black hole is its gravity, however, and searching for a black hole with an optical telescope is next to impossible. A black hole does not give off any light. It’s too small to observe by blocking out stars behind it. It could act as a gravitational lens, but to do so it would have to be directly in line with the Earth and some bright object, and even then there would be no way to distinguish between a black hole or a very dim star. Still, there was on promising method proposed by Russian astronomers Zel’dovich and Guseinov in 1964.

If the black hole was in a binary system with another, normal star, the light curve of the system would give it away. Binary systems comprise about half of all known stars, so it is not unlikely that a black hole might be found next to a normal star. In a spectroscopic binary system, the stars rotate about their center of mass and the light will be Doppler shifted. The light curve of a star is a graph of the intensity or Doppler-shift of light from the star versus time. Here the light curve of the visible companion can yield much information. The period of rotation about the center of mass can be determined by inspection of the Doppler-shifted light curve itself, and the mass of the visible star is given by the type of star and how luminous it is.

All that is then needed is a reasonable estimation of the …

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