Getting Into Black Holes

Knowing Black Holes:

When giants star dies and a black hole gets formed, all its mass gets squeezed into a single point. At this point, both space and time stops. It's very hard for us to imagine a place where mass has no volume and time does not pass, but that's what it is like at the center of a black hole.

Mathematically, a black hole is an object of zero size and infinite density (but finite mass)-a singularity.

Singularity is the point at the center of a black hole. Within a certain distance of the singularity, the gravitational pull is so strong that nothing- not even light can escape. That distance is called the event horizon. The event horizon is not a physical boundary but the point-of-no-return for anything that crosses it. When people talk about the size of a black hole, they are referring to the size of the event horizon. The more mass the singularity has, the larger the event horizon.

Many people think that nothing can escape the intense gravity of black holes. If that were true, the whole Universe would get sucked up. Only when something (including light) gets within a certain distance from the black hole, will it not be able to escape.
Imagine an object with such an enormous concentration of mass in such a small radius that its escape velocity was greater than the velocity of light. Then, since nothing can go faster than light, nothing can escape the object's gravitational field, that is, only if that thing is in the event horizon. Even a beam of light would be pulled back by gravity and would be unable to escape.

Event horizon is the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than t–he speed of light, so if you fire your rockets hard enough, you can give yourself enough energy to get away. But if you find yourself inside the horizon, then no matter how powerful your rockets are, you can't escape.

But farther away, things do not get sucked in. Stars and planets at a safe distance will circle around the black hole, much like the motion of the planets around the Sun. The gravitational force on stars and planets orbiting a black hole is the same as when the black hole was a star because gravity depends on how much mass there is, the black hole has the same mass as the star, it's just compressed.

You need to get close to black hole (i.e. near the gravitational radius) for its strong gravity to "suck you in" or for General Relativistic effects to be important.

Black holes are truly black. Light rays that get too close bend into, and are trapped by the intense gravity of the black hole. Trapped light rays will never escape. Since black holes do not shine, they are difficult to detect.

Mass and the radius of horizon of black holes:
We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} or 10,000,000,000,000,000,000,000,000,000,000 kilograms. Astronomers also suspect that many galaxies have extremely massive black holes at their centers. These are thought to weigh about a million times as much as the Sun, or 10^{36} kilograms. (You may write that one out if you wish.)

The more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius (which means the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that super-massive black hole has a radius only about four times bigger than the Sun.

Einstein's theory of General Relativity predicted that there would exist massive objects where the gravity is so strong that not even light can escape from it. These objects would in effect be endless, black holes. Modern physicists in general agree that the black hole is centered around a singularity, which is a point with infinite mass. In the singularity all physical laws break down.

If a black hole consists of a singularity, there are many possibilities. Some scientists believe that black holes curve space so strongly that it may be possible to travel from one place to another at a speed faster than light. A singularity may also connect our universe to baby universes, which would mean that our universe is not the only one.
There are lots of other theories about black holes. For this hypography we have chosen some websites which offers insight into these theories, and some explanations on the current knowledge about black holes.

Formation of Black Holes:
Black holes and neutron stars form when stars die. While a star is burning, the heat in the star pushes out and balances the force of gravity. When the star's fuel is spent, and it stops burning, there is no heat left to counteract the force of gravity. Whatever material is left over, collapses in on itself. How much mass the star had when it died determines what it becomes. Stars about the same size as the Sun become white dwarfs, which glow from left over heat. Stars that have about 3 times the mass of the Sun compact into neutron stars. And a star with mass greater than 3 times the Sun's gets crushed into a single point, which we call a black hole.

1. Neutrons:
Neutron stars are very dense and spin very fast and are typically only 10-15 km in radius. Because neutron stars form burnt-out stars, they do not glow. The collapse of the star causes the matter to be converted into mostly neutrons, hence the name neutron star.

2. Pulsars:
Some neutron stars emit radio waves that pulse on and off. These stars are called pulsars. Pulsars don't really turn radio waves on and off--it just appears that way to observers on Earth because the star is spinning. What happens is that the radio waves only escape from the North and South magnetic poles of the neutron star? If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. Far away on Earth, radio astronomers pick up the radio waves only when the beam sweeps across the Earth.

3. Supernovae:
A supernova explosion is usually associated with the formation of black holes and neutron stars. Young stars are hydrogen, and hydrogen is converted to helium due to nuclear reaction with emission of energy. The left over energy is the star's radiation i.e. heat and light. When most of the hydrogen has been converted to helium, a new nuclear reaction begins that converts the helium to carbon, with the left over energy released in the form of radiation. This continues converting the carbon to oxygen to silicon and at the end to iron. Nuclear fusion stops at iron.

The star has layers of different elements. The outer layers of hydrogen, helium, carbon, and silicon are still burning around the iron core, building it up. Eventually, the massive iron core succumbs to gravity and it collapses to form a neutron star. The outer layers of the star fall in and bounce off the neutron core that creates a shock wave that blows the outer layer outward. This is the supernova explosion.

Detection of Black Holes:
Light is not emitted by black holes and neutron stars, because of this we can't just look for them. However, astronomers can find black holes and neutron stars by observing the gravitational effects on other objects nearby.

1. X-rays
Astronomers can discover some black holes and neutron stars because they are sources of X-rays. The intense gravity from a black hole or a neutron star will pull in dust particles from a surrounding cloud of dust or a nearby star. As the particles speed up and heat up, they emit X-rays. So the x-rays don't come directly from the black hole or neutron star, but from its effect on the dust around it. Although x-rays don't penetrate our atmosphere, astronomers use satellites to observe x-ray sources in the sky.

2. Rotating stars
Many stars rotate around each other, much as the planets orbit our Sun. When astronomers see a star circling around something, but they cannot see what that something is, they suspect a black hole or a neutron star.

3. Gravity lenses
Astronomers use a technique called gravity lensing to search for black holes and neutron stars. When a very massive object passes between a star and the earth, the object acts like a lens and focuses light rays from the star on the Earth. This causes the star to brighten.

How can a black hole or a neutron star act like a lens? The answer comes from Albert Einstein, who proved in 1919 that light follows in the path of the bent time and space which is warped due to the gravitational force of a massive object. Einstein predicted that a star positioned behind the sun would be visible during a total eclipse. The Sun bent the light rays coming from the star and made it appear next to the sun.

There are two kinds of systems in which astronomers have found such compact, massive, dark objects: the centers of galaxies (including perhaps our own Milky Way Galaxy), and X-ray-emitting binary systems in our own Galaxy. In many galaxies one should find a central concentration of (dark) mass.