Tuesday, 9 February 2010

Black Holes

Another article on Black Holes. 

A star is dying 
It is a giant star, ten times as massive as our Sun, a thousand times as bright. For millions of years, hydrogen has “burned” in nuclear reactions in the star’s core, powering the star. But now the supply of hydrogen is running out, and as it does, the star burns ever brighter. The outer layers expand until the star is a vast red giant, as large as our entire solar system. 

Deep within the core, hydrogen first reacts to produce energy, then helium, and then carbon. Even more complicated elements are then produced in nuclear processes scientists still do not completely understand. The core temperature keeps rising; the nuclear reactions proceed ever more quickly, until the element iron is formed deep within. And then a crisis point is reached. 

When hydrogen is “burned” in a nuclear reaction, it releases excess energy; it is this energy that makes the stars shine. Helium and carbon release energy in nuclear reactions as well. But iron is different. When iron is involved in a nuclear reaction, it absorbs nuclear energy like a sponge absorbing excess water. 

Imagine, if you will, the core of this giant star. The pressure is millions of times Dearth’s atmospheric pressure, so intense that even atoms are broken down into their component parts. The temperature is tens of billions of degrees Fahrenheit, far hotter than a normal star. To maintain this temperature, to keep the outer layers of the star from crashing inward, the nuclear reactions have been proceeding faster and faster. 

But when iron is formed, it does not produce more energy for the star. Instead, it absorbs energy. Production of energy within the star’s core stops. There is no longer any pressure to withstand the pull of gravity, there is no longer any radiation pressure to hold up the weight of the star’s outer layers, and the inevitable occurs – collapse. 

Within a remarkable short period of time – some astronomers say hours, some minutes, some even seconds – the star falls inward upon itself. As the outer layers rush inward, enormous heat is created by compression, and much of the star is blown off into space in a supernova explosion. 

But the inner core of the star remains. The explosion has an opposite effect on it. Instead of exploding outward, it is imploded inward. Incredibly, it is compressed until the very atoms are forced together into neutral subatomic particles. As the implosion continues, those particles are compressed even further, the core of the star growing much smaller, its gravitational pull becoming more concentrated, until nothing can halt the collapse. And then the matter of the star disappears from our universe entirely, literally compressed out of existence, leaving only the gravitational pull behind. 

What results is called a black hole. It can be neither seen nor sense, apart from the effects of its immense gravity. It is probably the strangest object we know of in all the universe. 

Not every star will, in its death throes, form a black hole. A smaller star, such as the Sun, will have a much quieter death; it will collapse to form a white dwarf, a dense star about 10,000 miles in diameter. Matter within a white dwarf is very tightly packed; a cubic centimeter of white dwarf material would weigh a ton. A star one and half times as massive as the sun would collapse much the same way, but its gravity would be too intense for a white dwarf to be formed. Instead, it would collapse yet further, forming an neutron star a dozen miles across. The material of a neutron start is the densest in the universe. 

But if a star is still more massive, more than 3.2 times the mass of the Sun, it encounters a third and strangest fate. Collapse of such a star will not halt at the white dwarf or neutron star stages. The star’s mass is just too great, its gravity too intense to allow that to happen. Instead, collapse continues – and there is nothing to stop it. Eventually the star’s gravity grows so intense that not even light can escape its clutches – and here the laws that govern the universe seem to break down. 

The black hole collapses, apparently, until its matter is compressed out of existence. Only the gravitational field is left to show where the star had been, just as the Cheshire cat in Alice in Wonderland left only his grin behind as he disappeared. 

The Event Horizon 
The boundary of the black hole – if a black hole can be said to have boundaries – is the “event horizon.” This is the effective cut-off line between the black hole and the rest of the universe. Anything that goes through the event horizon into the center of a black hole can’t come back out. Once an astronaut passes the even horizon in his spaceship, he can’t return to the universe he knew. For him there is only one path – right down into the back hole. 

And what would he find there? It is difficult to say. In the center of a black hole, matter may have mass and yet not take up any volume. As an astronaut fell into a black hole, time for him would seem to run normally. But for outside observers watching him, his ship would seem to fall ever more slowly, until finally it would stop, poised on the edge of the event horizon. In a black hole, the laws of physics, and common sense, don’t seem to work. 

But if black holes exist, how are we going to find them? Radiating no light, a black hole is by definition invisible. First of all, we can observe the effect of a black hole on other objects around it. If the original star were part of a binary system – if it had a companion star – then after it collapses to form a black hole, the other member of the binary will still remain, radiating light and thus visible. Because of the black hole’s gravity, the visible star will seem to be orbiting some unseen center; its path through space will be affected by the invisible black hole. Astronomers on Earth, observing the change in the path of the other star, and not seeing a visible start that could be producing such an effect, will decide that the star in question must have a dark companion. By analyzing the motion of the visible star, astronomers can determine the mass of the companion. If that mass is less than 3.2 times the mass of the Sun, then the unseen companion is very likely a white dwarf or neutron star. If the mass of the companion, five or more times the mass of probably a black hole has been found. 

What is more, even though black holes are invisible, what falls into them is not. The gravity of the black hole is likely to drag in a great deal of material – dust, gas, even other stars, if the black hole is large enough. When matters falls into the black hole, it is accelerated to extremely high speeds, and as this happens x-ray radiation is released. Any large-scale x-ray source in the sky is possibly material being pulled into a black hole. 

As a matter of fact, astronomers may have found a black hole, using both the above methods of observation. In the constellation of Cygnus is a certain star that is a binary star. It is not visible to the naked eye, and has no name, only a number: HD 226868. The visible member of the binary is a large, hot blue star with about thirty times the mass of the Sun. It seems to be orbiting another invisible object once every 5.6 days. And this other, unseen object has a mass of five to eight times that of the Sun. 

Additionally, this binary star corresponds to an extremely powerful source of x-ray radiation. An invisible companion, five or more times the mass of the Sun, could very well be a black hole. The x-ray radiation which would be produced as material from the larger star is sucked into the black hole. 
Astronomers have done what might seem, at first glance, to be impossible – locating a black hole in the vast back night of interstellar space. 

And how many more might there be? 
Black holes are formed from very large stars, and such stars are not really common. On the other hand, such stars are short-lived – many of them have died already – and the galaxy is very large. Most estimates put the number of black holes in our galaxy at about one billion. The odds are, then, that a black hole is located within twenty light years of us – or five times the distance of the nearest star. 

Of course black holes can be much larger than a single star. Indeed, black holes can only grow in size, for while anything can fall into a black hole, nothing ever comes out. Some astronomers suggest a giant black hole may exist in the center of our galaxy. In the galactic core, the stars are very concentrated, separated by only a tenth or a hundredth of the distance that lies between stars in our region of space. Collisions between stars in such a crowded area of space are common, and once a black hole formed, every star it collided with would be absorbed. As the black hole grew in size, a “chain reaction” would occur, more and more stars being pulled in, until eventually an immense black hole would be formed. Indeed, some astronomers say this super black hole may have the mass of 100 million stars – 1/1000 the mass of our entire galaxy! As stars approached such a mammoth black hole, its immense gravity would tear them apart: if they approached quickly enough, they would be swallowed whole. 

We can take solace in the fact that if such an immense black hole exists at the center of the galaxy, it is very far away – 30,000 light years, or 18 quadrillion miles! It should take a long time in getting to our neighborhood! 

It has even been suggested that black holes may serve as a route to another universe; that by entering a black hole, one would be carried billions of light years away, to surface in another galaxy. Whether black holes really act as such a universal subway system is pure speculation. It is hard to believe that one could survive the immense pressures at the center of a black hole to emerge from the other side. But, as we have said before, in the center of a black hole both the laws of physics and common sense apparently break down. 

In any case, black holes are useful in that they have reshaped our ideas of what the universe is like, and how it works. The back hole is a mind-stretching and exciting concept: who can imagine a place where matter is squeezed out of existence? The existence of back holes, their dramatic formation, and their lives as cosmic vacuum cleaners are witness to the wonder and complexity of our universe. Black holes, quite simply, lie outside the realm of our experience, and in that fact lies their fascination. 
The English scientist J. B. S. Haldane said it best: The universe is not only stranger than we imagine; it is stranger than we can imagine. 

- By Dave Stover 

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