BLACK HOLE.
Most stars end up as white dwarfs or neutron stars, black holes are the last evolutionary stage in the life times of enormous stars that had been at least 10 or 15 times as massive as our own sun. When giant stars reach the final stages of their lives they often detonate in cataclysms known as supernovae. Such an explosion scatters most of a star into the void of space but leaves behind a large "cold" remnant on which fusion no longer takes place.
A black hole is a region of space time exhibiting such strong gravitational effects that nothing not even particles and electro magnetic radiation such as light can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform space time to form a black hole. The boundary of the region from which no escape is possible is called the event horizon.
Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover,quantum field theory in curved space time predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass.
This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe. Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.Another kind of black hole is called"stellar." Its mass can be up to 20 times more than the mass of the sun.
There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.The largest black holes are called "super-massive." These black holes have masses that are more than 1 million suns together.
Scientists have found proof that every large galaxy contains a super-massive black hole at its center. The super-massive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.
Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nuclei synthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature.
In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight. The collapse may be stopped by the degeneracy pressure of the star's constituents, allowing the condensation of matter into an exotic denser state. The result is one of the various types of compact star. The type of compact star formed depends on the mass of the remnant of the original star left after the outer layers have been blown away.
Such explosions and pulsations lead to planetary nebula. This mass can be substantially less than the original star. Remnants exceeding 5 M☉are produced by stars that were over 20 M☉before the collapse. If the mass of the remnant exceeds about 3–4 M☉the Tolman–Oppenheimer–Volk off limit. either because the original star was very heavy or because the remnant collected additional mass through accretion of matter, even the degeneracy pressure of neutron sis insufficient to stop the collapse.
No known mechanism except possibly quark degeneracy pressure, see quark star is powerful enough to stop the implosion and the object will inevitably collapse to form a black hole. The gravitational collapse of heavy stars isassumed to be responsible for the formation of stellar mass black holes. Star formation in the early universe may have resulted in very massive stars, which upon their collapse would have produced black holes of up to 103 M.
These black holes could be the seeds of the super-massive black holes found in the centers of most galaxies. It has further been suggested that super-massive black holes with typical masses of ~105 M☉could have formed from the direct collapse of gas clouds in the young universe. Some candidates for such objects have been found in observations of the young universe.
While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process.
Even though the collapse takes a finite amount of time from the reference frame of in falling matter, a distant observer would see the in falling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.
Gravitational collapse is not the only process that could create black holes. In principle, black holes could be formed in high-energy collisions that achieve sufficient density. As of 2002, no such events have been detected, either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments. This suggests that there must be a lower limit for the mass of black holes.
Theoretically, this boundary is expected to lie around the Planck mass (mP=√ħc/G≈1.2×1019 GeV/c2≈2.2×10−8 kg), where quantum effects are expected to invalidate the predictions of general relativity. This would put the creation of black holes firmly out of reach of any high-energy process occurring on or near the Earth. However, certain developments in quantum gravity suggest that the Planck mass could be much lower: some brane world scenarios for example put the boundary as low as1 TeV/c2.
This would make it conceivable for micro black holes to be created in the high-energy collisions that occur when cosmic rays hit the Earth's atmosphere, or possibly in theLarge Hadron Collider at CERN. These theories are very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists. Even if micro black holes could be formed, it is expected that they would evaporate in about 10−25seconds, posing no threat to the Earth.
Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its surroundings and omni present cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown. A similar process has been suggested for the formation of intermediate-mass black holes found in globular clusters.
Another possibility for black hole growth, is for a black hole to merge with other objects such as stars or even other black holes. Although not necessary for growth, this is thought to have been important, especially for the early development of supermassive black holes, which could have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes.
Hawking radiation In 1974, Hawking predicted that black holes are not entirely black but emit small amounts of thermal radiation; this effect has become known as Hawking radiation. By applying quantum field theory to a static black hole background, he determined that a black hole should emit particles that display a perfect black body spectrum.
Since Hawking's publication, many others have verified the result through various approaches. If Hawking's theory of black hole radiation is correct, then black holes are expected to shrink and evaporate over time as they lose mass by the emission of photons andother particles. The temperature of this thermal spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which, for a Schwarzs child black hole, is inversely proportional to the mass.
Hence, large black holes emit less radiation than small black holes. A stellar black hole of 1 M☉has a Hawking temperature of about 100 nano kelvins. This is far less than the 2.7 K temperature of the cosmic microwave background radiation. Stellar-mass or larger black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and thus will grow instead of shrink.
To have a Hawking temperature larger than 2.7 K, a black hole would need a mass less than the Moon. Such a black hole would have a diameter of less than a tenth of a millimeter. If a black hole is very small, the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant.
A black hole with the mass of a car would have a diameter of about 10−24m and take a nano second to evaporate, during which time it would briefly have a luminosity of more than 200 times that of the Sun. Lower-mass black holes are expected to evaporate even faster; for example, a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. For such a small black hole,quantum gravitation effects are expected to play an important role and could hypothetically make such a small black hole stable, although current developments in quantum gravity do not indicate so.
The Hawking radiation for an astro physical black hole is predicted to be very weak and would thus be exceedingly difficult to detect from Earth. A possible exception, however, is the burst of gamma rays emitted in the last stage of the evaporation of primordial black holes. Searches for such flashes have proven unsuccessful and provide stringent limits on the possibility of existence of low mass primordial black hole
On 14 September 2015 the LIGO gravitational wave observatory made the first-ever successful observation of gravitational waves. The signal was consistent with theoretical predictions for the gravitational waves produced by the merger of two black holes: one with about 36solar masses, and the other around 29 solar masses. This observation provides the most concrete evidence for the existence of black holes to date. For instance, the gravitational wave signal suggests that the separation of the two objects prior to the merger was just 350 km.
The objects must therefore have been extremely compact, leaving black holes as the most plausible interpretation. More importantly, the signal observed by LIGO also included the start of the post-merger ring down, the signal produced as the newly formed compact object settles down to a stationary state. Arguably, the ring down is the most direct way of observing a black hole. From the LIGO signal it is possible to extract the frequency and damping time of the dominant mode of the ring down. From these it is possible to infer the mass and angular momentum of the final object, which match independent predictions from numerical simulations of the merger.
The frequency and decay time of the dominant mode are determined by the geometry of the photon sphere. Hence, observation of this mode confirms the presence of a photon sphere, however it cannot exclude possible exotic alternatives to black holes that are compact enough to have a photon sphere. The observation also provides the first observational evidence for the existence of stellar-mass black hole binaries. Further more, it is the first observational evidence of stellar-mass black holes weighing 25 solar masses or more.
Astronomers use the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral intermission and very strong radio emission. Theoretical and observational studies have shown that the activity in these active galactic nuclei (AGN) may be explained by the presence of super-massive black holes, which can be millions of times more massive than stellar ones.
The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets perpendicular to the accretion disk. Detection of unusually bright X-Ray flare from Sagittarius A*, a black hole in the center of the Milky Way galaxy on 5 January 2015.
Although super-massive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central super-massive black hole candidates. Some of the most notable galaxies with super-massive black hole candidates include the Andromeda Galaxy,M32,M87,NGC 3115,NGC 3377,NGC 4258,NGC 4889,NGC 1277,OJ 287,APM 08279+5255 and the Sombrero Galaxy.
It is now widely accepted that the center of nearly every galaxy, not just active ones, contains a supermassive black hole. The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself.
WHAT HAPPENS WHEN A SMALL SIZE A BLACK HOLE PRESENT IN EARTH.
First of all, not all of the Earth would simply be sucked into the black hole. When the matter near the black hole begins to fall into the black hole, it will be compressed to a very high density that will cause it to be heated to very high temperatures. These high temperatures will cause gamma rays, X-rays, and other radiation to heat up the other matter falling in to the black hole.
The net effect will be that there will be a strong outward pressure on the outer layers of the Earth that will first slow down their fall and will eventually ionize and push the outer layers away from the black hole. So some inner portion of the core will fall into the black hole, but the outer layers, including the crust and all of us, would be vaporized to a high temperature plasma and blown into space.
This would be a gigantic explosion—a significant fraction of the rest of the mass of the Earth matter that actually fell into the black hole will be converted into energy. For astrophysical black holes, up to 40 percent of the rest mass of the accreted material can be emitted in radiation. This radiation will be absorbed by the outer layers of the Earth and will vaporize them. Examples of this kind of dramatic matter to energy conversion are quasars.
Quasars are the most luminous objects in the universe, and they are powered by matter falling onto a supermassive black hole. So there will be plenty of energy available to blow off the other layers of the Earth and they will escape! For example, when the black hole is first placed at the center of the Earth, the first thing we would all notice is that gravity increased by a factor of two on the surface of the Earth.
However, the escape velocity of an object only increases as the square root of the mass, so the current 11 km/s escape velocity on the surface of the Earth will only increase to about 16.8 km/s. A very significant fraction of the mass of the Earth will become a vaporized hot plasma and will be going faster than that when it passes the radius of what used to be the surface of the Earth.
Secondly, the Earth is rotating, so by conservation of angular momentum, when a significant amount of mass has started to fall into the black hole, the mass will also begin rotating at a higher and higher rate. This angular momentum will tend to slow down the fall into the black hole and will eventually result in something like an accretion disc around the black hole. This will also limitthe fraction of the Earth that will fall into the black hole and will greatly increase the time it takes for the black hole to consume whatever fraction of the mass of the Earth it will consume.
The reason for the delay is that the accretion disc has to use friction to transfer angular momentum from the innermost portion of the disc to the outer edge of the disc, where it will cause material to be ejected from the vicinityof the disc carrying away angular momentum. The lower angular momentum near the center will allow that innermost material to fall into the black hole. In fact, even though the Earth only rotates once per day, the angular momentum of the Earth is huge.
There are limits to how much angular momentum a black hole can have roughly the maximum angular momentum is where the "surface" of the black hole if it had a surface it would approach the speed of light. Trying to make a small black hole with all of the Earth's angular momentum would mean that the surface would have to travel at about 10 9times the speed of light. So most of the mass of the Earth would have to be used to carry away almost all of the original angular momentum of the Earth in order to keep the black hole below its angular momentum limit.
But what if there is no explosion and no angular momentum to stop the surface from falling in to the black hole? How long would it take for the Earth to "fall" into the black hole? Well, imagine that somehow, magically, all the mass of the Earth just became a black hole at the center of the Earth and that you were standing on the North Pole with no angular momentum in a space suit (since you are now in a vacuum).
How long would it take until you are spaghettified as you fall into the black hole? We can get an approximate answer by using Newtonian gravitation instead of general relativity, which is what is really needed for motion into or near a black hole. According to Newtonian gravity, it would take approximately 15 minutes to fall into the black hole.
For a black hole with twice the mass, it would take 10 minutes to fall into the hole. So the more accurate general relativity answer may be slightly different, but the time for the surface to fall in will be something close to 10 to 15 minutes. This would be the time as measured by you as you fall into the hole. For someone on the moon watching you fall, the gravitational time dilation will make it look like you are falling slower and slower when you get very close to the black hole, so it would look like it would take forever to hit the black hole horizon.
However, for you, falling in, it will be all over in approximately 10 to 15 minutes or so from your point of view. Similarly, if there were no explosion and no angular momentum that would retard or prevent the swallowing up of the Earth, then it would take about 10 to 15 minutes for the whole Earth to fall into the new black hole at the center of the Earth.
THANK YOU KEEP VISITING FOR MORE INFORMATION.
0 comments: