Black+Holes


 * Black Holes **﻿By Max & Adam

Black holes are clusters of atoms consisting of dead stars which are incredibly dense. Black holes are divided into two main parts, the event horizon and the black hole's core.

Black holes absorb all light, so when they pass in front of a galaxy such as this one... it will absorb and bend the light, distorting your view. Jets of hot gas are occasionally found streaming out of the area surrounding a black hole. These gas jets flow perpendicular to the accretion disk and can extend to millions of light years in length. All matter that comes within a certain distance of the black hole is trapped forever. That distance is represented by the event horizon, an imaginary sphere that surrounds a black hole. The black holes core is mostly made of super-condensed hydrogen and other residue from a supernova that implodes upon itself, creating a small amount of black hole matter that can range from a in size basket ball to earth or even bigger. Radiation generated inside a star's core creates outward pressure that keeps the star from collapsing under its own gravity. When a massive star runs out of fuel, the sudden drop in outward pressure causes the star to collapse. This triggers a supernova that leaves behind a black hole or neutron star. 

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a “singularity ". Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the “event horizon ". But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the Earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape. The Schwarzschild radius can be calculated using the equation for escape speed: vesc = (2GM/R)1/2 For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be R = 2GM/c2 If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the centre of our Solar System. If We Can't See Them, How Do We Know They are There? Since stellar black holes are small (only a few to a few tens of kilometres in size), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see. For instance, the photograph above shows the optical companion star to the (invisible) black hole candidate Cygnus X-1. However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission. Binary X-ray sources are also places to find strong black hole candidates. A companion star is a perfect source of in falling material for a black hole. A binary system also allows the calculation of the black hole candidate's mass. Once the mass is found, it can be determined if the candidate is a neutron star or a black hole, since neutron stars always have masses of about 1.5 times the mass of the Sun. Another sign of the presence of a black hole is its random variation of emitted X-rays. The in falling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity. Additionally, if the X-ray source is in a binary system, and we see it from certain angles, the X-rays will be periodically cut off as the source is eclipsed by the companion star. When looking for black hole candidates, all these things are taken into account. Many X-ray satellites have scanned the skies for X-ray sources that might be black hole candidates. Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates. It is a highly variable and irregular source, with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3,000 kilometres. This is one fourth of Earth's diameter! So the region emitting the X-rays around Cyg X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses, or else it would not exert enough gravitational pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar masses are too large to be a white dwarf or neutron star, it must be a black hole. However, there are arguments against Cyg X-1 being a black hole. HDE 226868 might be under massive for its spectral type, which would make Cyg X-1 less massive than previously calculated. In addition, uncertainties in the distance to the binary system would also influence mass calculations. All of these uncertainties can make a case for Cyg X-1 having only 3 solar masses, thus allowing for the possibility that it is a neutron star. Nonetheless, there are now about 20 binaries (as of early 2009) for which the evidence for a black hole is much stronger than in Cyg X-1. The first of these, an X-ray transient called A0620-00, was discovered in 1975, and the mass of the compact object was determined in the mid-1980's to be greater than 3.5 solar masses. This very clearly excludes a neutron star, which has a mass near 1.5 solar masses, even allowing for all known theoretical uncertainties. The best case for a black hole is probably V404 Cygni, whose compact star is at least 10 solar masses. With improved instrumentation, the pace of discovery has accelerated, and the list of dynamically confirmed black hole binaries is growing rapidly. = What about All the Wormhole Stuff? = Unfortunately, wormholes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to faraway places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat. The throat, or passageway, is called an Einstein-Rosen bridge. It has never been proven that wormholes exist, and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create.

The anomalous black holes are concentrated areas of mass so immense, that the mammoth force of gravity denies anything within a certain area around it from passing. This area is called the event horizon of a black hole. We have given black holes their name because light inside the event horizon can never be seen by mankind, or any outside observer. We believe that black holes in space are created by the collapse of a red super giant star. As these stars reach the end of their lives, an imbalance of inward and outward pressure forces the star to collapse. Information on black holes is limited, though numerous schools of theory exist. We know black holes exist not because we can see them, but because of the impact they have on the space around them. Scientists like Karl Schwarzschild, Jayant Narlikar and Stephen Hawking have built upon ideas from Einstein and others to offer theories on black holes. And yet, they remain an enigma. Because extensive, proven black holes information is scarce, they remain a constant area of intrigue and curiosity. What happens if my friend watches me fall into a black hole? Your Friend sees things quite differently from you. As you get closer and closer to the horizon, they see you move more and more slowly. In fact, no matter how long they waits, they will never quite see you reach the horizon. In fact, more or less the same thing can be said about the material that formed the black hole in the first place. Suppose that the black hole formed from a collapsing star. As the material that is to form the black hole collapses, Your Friend sees it get smaller and smaller, approaching but never quite reaching its Schwarzschild radius. This is why black holes were originally called frozen stars: because they seem to 'freeze' at a size just slightly bigger than the Schwarzschild radius. Why does they see things this way? The best way to think about it is that it's really just an optical illusion. It doesn't really take an infinite amount of time for the black hole to form, and it doesn't really take an infinite amount of time for you to cross the horizon. (If you don't believe me, just try jumping in! You'll be across the horizon in eight minutes, and crushed to death mere seconds later.) As you get closer and closer to the horizon, the light that you're emitting takes longer and longer to climb back out to reach Your Friend. In fact, the radiation you emit right as you cross the horizon will hover right them at the horizon forever and never reach them. You've long since passed through the horizon, but the light signal telling them that won't reach them for an infinitely long time. There is another way to look at this whole business. In a sense, time really does pass more slowly near the horizon than it does far away. Suppose you take your spaceship and ride down to a point just outside the horizon, and then just hover them for a while (burning enormous amounts of fuel to keep yourself from falling in). Then you fly back out and rejoin Your Friend. You will find that they has aged much more than you during the whole process; time passed more slowly for you than it did for them. So which of these two explanations (the optical-illusion one or the time-slowing-down one) is really right? The answer depends on what system of coordinates you use to describe the black hole. According to the usual system of coordinates, called "Schwarzschild coordinates," you cross the horizon when the time coordinate t is infinity. So in these coordinates it really does take you infinite time to cross the horizon. But the reason for that is that Schwarzschild coordinates provide a highly distorted view of what's going on near the horizon. In fact, right at the horizon the coordinates are infinitely distorted (or, to use the standard terminology, "singular"). If you choose to use coordinates that are not singular near the horizon, then you find that the time when you cross the horizon is indeed finite, but the time when Your Friend sees you cross the horizon is infinite. It took the radiation an infinite amount of time to reach them. In fact, though, you're allowed to use either coordinate system, and so both explanations are valid. They're just different ways of saying the same thing. In practice, you will actually become invisible to Your Friend before too much time has passed. For one thing, light is "red shifted" to longer wavelengths as it rises away from the black hole. So if you are emitting visible light at some particular wavelength, Your Friend will see light at some longer wavelength. The wavelengths get longer and longer as you get closer and closer to the horizon. Eventually, it won't be visible light at all: it will be infrared radiation, then radio waves. At some point the wavelengths will be so long that they'll be unable to observe them. Furthermore, remember that light is emitted in individual packets called photons. Suppose you are emitting photons as you fall past the horizon. At some point, you will emit your last photon before you cross the horizon. That photon will reach Your Friend at some finite time -- typically less than an hour for that million-solar-mass black hole -- and after that they'll never be able to see you again. (After all, none of the photons you emit *after* you cross the horizon will ever get to them.)