Black and Red: the colors of gravity...
In this article I’ll try to make black holes a bit easier to understand. Why? Because – up to recently – black holes were a mystery to me too. Lazserus helped me on my way to search more information. And now I feel confident enough to write something about the subject. I used many internet sources on this subject. As this is only an article to spread information, I hope the authors won’t mind I used their info, pictures and diagrams.

The death of Re
Re was the Egyptians’ sun god. Re was believed to travel the sky in a barge, starting at sunrise as the god Re-Harachte and sailing the sky towards the highest point as the god Khepri, then sailing back to the horizon as the god Re-Atum, the Creator who gives light and warmth and growth.
At night Re would fight the Big Snake Apep, who was trying to prevent Re from rising again: the battle between light and dark, right and wrong.
Of course Re won the battle each and every day, to shine his rays onto the fertile lands surrounding the river Nile, bringing food and prosperity to the realm. It’s not surprising that the most important god of Egypt was the sun, source of all wealth. The Pharaohs didn’t take on the name and depiction of Re for no reason. The sun was the embodiment of life AND eternal life. But how eternal is the life of the sun really?

A relatively small star
Well that’s not so hard: the expected life span of OUR sun is about 14 billion years. The sun is about one-third through that time, and can be compared to a human being in his late twenties, still full of strength and vigour.

How will the sun die eventually? During the next billion years or so, the sun will become brighter by 10%. This will heat up our planet as a result of a severe greenhouse effect. All of the oceans on earth will vaporise and all life will be destroyed. After another 5.5 billion years the sun will burn up all of its hydrogen fuel, most of it located in the core. Then it will start using up the hydrogen from the layers surrounding the core. This will cause the sun to swell like a big balloon. 2.5 billion years later the sun will have become about 160 times bigger than its present size. It has taken Mercury, Venus and very probably the Earth for breakfast. At that moment we call the sun a Red Giant.

The sun’s exhaust gas, helium – generated through nuclear fission – will serve as the sun’s new fuel, when it has devoured all of its hydrogen. Helium apparently tastes better than hydrogen and the sun races through these reserves in only a mere 100 million years.

As an after-dinner surprise the sun will then eject enormous amounts of matter into space, cool down and contract to be an object with a very high density, about the size of our earth. We call this object a White Dwarf, and this has nothing to do with a certain Gimli. A teaspoonful of white dwarf material would weigh five-and-a-half tons or more in the Earth's gravity! Yet a white dwarf can contract no further; its electrons resist further compression by exerting an outward pressure that counteracts gravity. This balance between gravity and outward pressure is the reason why stars do not explode very soon after birth. Effectively the sun is now around its dying years.

White dwarfs are very common objects in the universe. Most of them are very dim and invisible to our eye and telescopes. A very famous one is Sirius B. Astronomer W.Bessel was the first to suspect that Sirius had an invisible companion when he observed that the path of the star wobbled. In the 1920's it was determined that Sirius B, the companion of Sirius, was a "white dwarf" star. The pull of its gravity caused Sirius's wavy movement.

Here is an X-ray image of the Sirius star system located 8.6 light years from Earth. This image shows two sources and a spike-like pattern due to the support structure for the transmission grating. The bright source is Sirius B, a white dwarf star that has a surface temperature of about 25,000 degrees Celsius which produces very low energy X-rays. The dim source at the position of Sirius A – a normal star more than twice as massive as the Sun – may be due to ultraviolet radiation from Sirius A leaking through the filter on the detector. The picture was taken with Chandra, an X-ray obervatory. Since its launch on July 23, 1999, the Chandra X-ray Observatory has been NASA's flagship mission for X-ray astronomy, taking its place in the fleet of "Great Observatories."

The picture to the right shows the same star system, now through a “normal” visible light telescope, to show exactly how small Sirius B is, compared to Sirius A, which is about 1.6 times the size of our own sun, but 22 (!) times the luminosity of our sun. Sirius B has a luminosity of 1/400 of our sun, making it very dim.

Next to these facts it was also discovered that Sirius B had another important trick up its sleeve: it was the first star of which the light showed a gravitational red-shift, making a nice piece of evidence to support Einstein’s theory of relativity.

Einstein had predicted that photons (light particles) that meet a strong gravitational pull, will lose energy. Thus their colour will shift toward the red. Until that moment (in 1924) it had been very difficult to detect red-shifted light in low-mass stars such as our sun. We will discuss this effect of light shifting toward the red again when the black hole is being explained.

A big star dies
Contrary to what you might think, a larger star burns out more quickly than a small star like our sun. The moment all of their fuel is consumed, the big star will shed most of its mass into space – much like our own sun will do, but then with an incredible force, a bang the scientists call a Supernova. There are more spectacular explosions, called Hypernovae, but scientists are still in doubt as to their cause. What happens before the bang of a supernova?

We are stardust
Big stars burn up hydrogen, which is converted to helium. They do that at tremendous rates: a star, 25 times the mass of our sun will live its life a thousand (!!) times faster. It will also burn a 100,000 times brighter. Because a big star has more mass, gravity will build up pressure and temperature around the core, which will help to fuse the fuel into elements of increasing atomic weight. There are many of these processes going on in a star, and depending on the distance from the core, we will see different layers. At the stars surface we would see hydrogen being fused to helium, somewhat deeper there would be a layer where helium was fused into carbon and oxygen, carbon would be fused into neon and magnesium and so on. At the stars deepest point, where it is really hot ( 8 billion degrees Kelvin), iron is created by fusing silicon. The creation of this iron core takes place in about a week.

Once the iron core is formed it is no longer possible to produce more energy just by compressing it to start a new fusion reaction. Gravity is indifferent to this and will go on compressing the core, raising temperatures to about 10 billion degrees Kelvin. At this temperature the photons split the iron nuclei into protons and neutrons. They don’t do that quietly: in a tenth of a second a 12,000 km iron core collapses into a neutron star of about 20 km in diameter. The outer layers of the star are suddenly without support, and they now collapse and bounce on the dense, incompressible neutron core, resulting in the instant release of a huge amount of gravitational potential energy. Boom!!

As you see: during its lifetime and especially toward the end the sun is the creator of all elements we find on earth and in ourselves. Truly we are stardust, the remains of a dead star, that once burned brightly in the heavens.

Neutron Star
If the material that remains after a supernova is more then 1.4 solar masses but less than 2, it will contract to form a neutron star. A neutron star is nothing more than an incredibly dense core made of just neutrons. These stars are so dense, a teaspoon would weigh a 100 million tons. A neutron star like that will not contract any further, because – however crushed – the neutrons will resist the inward pull of gravity, just like the white dwarfs electrons do.
When the neutron star’s mass far exceeds 2 solar masses (no-one exactly knows the precise critical point) there is a good chance that the process of inward gravity wins over the neutrons resistance. The core of the neutron star collapses further and then there’s no more stopping the ongoing process: a black hole is formed.

Black Hole: the making of…

What exactly IS a black hole? To put it bluntly, it’s the absolute nothingness, and it is NOT a hole. When the core of a big neutron star collapses – the inward gravity wins – this process will go on and on, until we reach a point in which all matter of the star if being compressed into a point of infinite density. Why is it called a black hole? Here we come back to our colour red.
The enormous gravitational field of the hole of course affects the photons inside. They are red-shifted to the extreme and can’t get out! In other words: the escape velocity of the hole, caused by the enormous gravity is equal to the light speed. Photons can not travel at greater speeds than the speed of light, so they can not escape the hole.

The tale of the black hole has the following chapters:
• A singularity
• The Schwarzschild radius
• The event horizon
• The apparent horizon

We’ll have a little discussion about all four of these.

The Singularity
The singularity lies at the heart of the black hole. This is where all matter has been crushed to an infinitely small point of infinite density, with an infinite gravitational pull. Space-time has an infinite curvature. All laws of physics cease to apply; it is really a point where space and time as we know them cease to exist.

The Schwarzschild radius
The German astrophysicist Kurt Schwarzschild used the equations in Einstein’s theory of relativity to determine the radius for a given mass at which matter would collapse into a singularity. An example: A black hole with a mass of about 10 of our suns, will have a radius of only 30 (!!) kilometers (19 miles). Picture the black hole as a sphere, then the surface of the hole is known as…

The event horizon
Beyond this horizon the inward gravitational pull is so great that absolutely nothing from the hole can escape to the space outside. The event horizon is a static state at some point. The event horizon coincides at some point in time with…

The apparent horizon
The collapsing dying star will show an “apparent” event horizon forming all of a sudden. This horizon moves out like a balloon expanding until it coincides with the event horizon of the black hole (see diagram). This horizon – during its existence – will separate trapped light rays from the light rays that can still move away. Some of these rays can be trapped later when more matter or energy falls into the hole, increasing the gravity inside.

Apparent versus Event Horizon
Even before the star meets its final doom, the event horizon forms at the centre, balloons out and breaks through the star's surface at the very moment it shrinks through the critical circumference. At this point in time, the apparent and event horizons merge as one: the horizon. The distinction between apparent horizon and event horizon may seem subtle, even obscure. Nevertheless the difference becomes important in computer simulations of how black holes form and evolve.
Beyond the event horizon, nothing, not even light, can escape. So the event horizon acts as a kind of "surface" or "skin" beyond which we can venture but cannot see.

Schwarzschild and Kerr

What are Gravitational Waves?
Matter has gravity. The motion of matter will generate disturbances in the fabric of space-time, like a pebble, thrown into a pond. These “gravitational waves” are small and weaken further when travelling away from their source. A wave reaching earth will stretch and shrink distances on a very small scale, a change like something the size of an atom in the distance between Earth and the Sun. These waves are very hard to detect, as you can imagine.

Gravitational waves and the black holes
What happens to a black hole after it forms? Does it vibrate? Radiate? Lose mass? Grow? Shrink?

Partial solutions of the Einstein equations point to two possible outcomes:
• A non-rotating, spherical black hole, first postulated by Schwarzschild.
• A rotating non-symmetrical black hole, predicted in 1964 by the New Zealand mathematician Roy Kerr.

These two types of black holes have become known as Schwarzschild and Kerr black holes.
Both are "static": they don’t change in time, except when disturbed in some way. When a "real" black hole forms from the collapse of a very massive star, or when two black holes collide this could result in the generation of gravitational waves. By emitting gravitational waves, non-stationary black holes lose energy, eventually become stationary and cease to radiate in this manner. In other words, they "decay" into stationary black holes, namely holes that are perfectly spherical or whose rotation is perfectly uniform. According to Einstein's theory, such objects cannot emit gravitational waves.

How can we see a black hole?
We can’t. All light is trapped inside the hole, so we can not see it. What we can see is the effect the hole has on matter it encounters. If gas from a nearby star is sucked into the hole, the gravity will heat up the gas to many millions of degrees. We will be able to detect the radiation.
The real evidence of black holes will be presented when scientists build a gravitational wave detector that is sensitive enough to detect the waves emitted by a hole.

Journey into a Black Hole
A trip into a black hole has been the subject of much speculation. Let’s take an unmanned probe and send it into a black hole. The spectators watching the probe would see it approach the event horizon, but it would seem - strangely enough - never to reach it. The probe would seem to halt, then turn orange and then red and then fade from view. Now if you were inside the probe, you would notice a total inability to steer into any direction but toward the centre of the hole. At some moment you would cross the event horizon although you’d never know when. Gravity would turn you into spaghetti instantly. Too bad, because you wouldn’t be able to test the theory about time behaving strangely in a black hole, allowing you to travel through time.

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