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Simple Explanation of Black Hole….

by Shivaranjan on February 17, 2006


I was always fascinated by the concept of black hole right from my school days. Now came across very simple explanation which a layman can also understand.

What is a Black Hole????

Once a giant star dies and a black hole has formed, all its mass is squeezed into a single point. At this point, both space and time stop. 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.

The point at the center of a black hole is called a singularity. 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. 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.

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.

What are Neutron Stars and Pulsars???

Neutron stars are very dense and spin very fast and are typically only 10-15 km in radius. Because neutron stars form from 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.

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.

How are these things formed????

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.

What is a Supernovae????

A supernova explosion is usually associated with the formation of black holes and neutron stars. To understand what explodes and what collapses, we need to talk about what happens during a supernova explosion.

Young stars are hydrogen, and the nuclear reaction converts hydrogen to helium with energy left over. The left over energy is the star’s radiation–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 as radiation. This process continues converting the carbon to oxygen to silicon 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 which creates a shock wave that blows the outer layer outward. This is the supernova explosion.

How do we Know that Black holes are present????

Black holes and neutron stars don’t give off light, so 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.

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.

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.

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.

Source: Mr.Chris Miller

Isn’t this very simple to understand?????

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