Black Holes - Black and Red: the colors of gravity...Black holes are amongst the universe's family of anomalies that we've just recently begun to understand in the recent decades. The term black hole was laid to claim by John Wheeler in 1969, yet the theory dates back over 200 years. In 1783, John Michell wrote a paper declaring that a star that was massive enough would have a gravitational influence so strong that light would not be able to escape its surface. He also believed there were a number stars like this in the universe, but because light could not escape their gravity they would just be black voids in space. Michell also conjectured that even though we could not see the stars' light we could feel their gravitational influence. It took 200 years before Michells theories could be put to the test, but it came.
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 starThe 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 her late twenties, still full of strength and vigor.
In order to understand black holes, one must understand the life process of a star. Stars form when a large quantity of interstellar gas - mainly hydrogen atoms - begins to contract due to self-gravity. The colliding atoms begin heating up as they collide at greater rates and at high velocities. Eventually, the collapse gets so hot that the atoms no longer repel off of each other, but fuse together into helium atoms. This is called thermonuclear fusion. Eventually, the heat produced from these collisions counters the contraction of gravity and a star is formed. Stephen Hawking's analogy works great: "It is a bit like a balloon - there is a balance between the pressure of the air inside, which is trying to make the balloon expand, and the tension in the rubber, which is trying to make the balloon smaller." Inevitably, the star will run out of nuclear fuel and will no longer be able to melee with gravity. Thus, gravity wins the war and the star is doomed to collapse; but it isn't necessarily doomed to a collapse so severe it creates a black hole.
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 vaporize and all life will be destroyed. After another 5.5 billion years the sun will burn up all of its hydrogen fuel located in the core, and 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 100 times bigger than its present size. By this point it has swallowed Mercury, Venus and very probably the Earth in the process of expansion. At that moment we call the sun a Red Giant.
The sun's exhaust gas, helium - generated through nuclear fusion - will serve as the sun's new fuel, when it has devoured all of its hydrogen. The standard hydrogen core can reach temperatures as hot as 100 million degrees, while a helium core can reach up to 600 million degrees. The temperatures increase and the fuel runs out quicker. The transition from a G2 star (our sun) to a Red Giant is roughly 160 million years. On the cosmic scale that's quite fast. The lifespan of a Red Giant is only 1 billion years, compared to our sun's 10 billion years.
Once all the sun's helium is consumed it will then eject enormous amounts of matter into space. After it ejects its surface layers, the sun will then cool down and contract to be an object with a very high density, but only a few thousand miles in radius. We call this object a White Dwarf. A teaspoon of white dwarf material would weigh five-and-a-half tons or more on Earth. 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, called electron degeneracy pressure, is the reason why stars do not explode very soon after birth. Effectively the sun is now around its dying years.
Shrinking StarWhite 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 the Chandra X-ray Observatory. 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 bottom 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, the light's wavelength stretches so that their color will shift toward the red spectrum. Until that moment (in 1924) it had been very difficult to detect red-shifted light in low-mass stars such as our sun. You're probably now saying, "Light particles and light waves! Which is it!"? 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 a star's 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 stellar explosion which astronomers 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 Massive 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 massive 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, which once burned brightly in the heavens.
A star that exceeds 1.4 solar masses, and is limited to 3 solar masses, after its supernova will collapse further than a white dwarf into a very dense star called a neutron star.
A neutron star is nothing more than an incredibly dense core made of just neutrons. Its mass is packed in a volume roughly 10^14 times smaller than our sun and has a mass density around 10^14 times higher than the sun; it is so dense that a teaspoon would weigh 100 million tons. A neutron star less than 3 solar masses will not contract any further, because the neutrons will resist the inward push of gravity, just like the white dwarf's electrons do.
This is now called neutron degeneracy pressure. When the neutron star's mass far exceeds 3 solar masses (no-one exactly knows the precise critical point) there is a good chance that the process of inward gravity exceeds that of the neutrons' resistance. The core of the neutron star collapses further and then there's no more stopping the ongoing process, the star infinitely collapsing; a black hole is formed.
Black Hole: the making of: What exactly IS a black hole? A black hole is a region in space-time that has a gravitational field so strong that the escape velocity is faster than the speed of light.
This means nothing can escape its clutches, not even light. When the core of a massive neutron star collapses, the inward gravity prevailing over the neutron degeneracy pressure, the 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.
The tale of the black hole has the following chapters: -A singularity
-The Schwarzschild radius
-The event horizon
-The apparent horizon
The SingularityThe 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, where space-time has an infinite curvature. The laws of physics break down at the singularity; it is really a point where space and time as we know them cease to exist. Astrophysicists say the big bang started as a singularity.
The Schwarzschild radiusThe German astrophysicist Karl 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). Thus, the radius between the singularity and the event horizon is called the Schwarzschild radius.
The event horizonThe event horizon is what some would call "the point of no return." Beyond this unseen border the escape velocity for the black hole is greater than that of the speed of light, meaning light would have to travel faster than its constant velocity of 300,000km/h in order to escape. The event horizon is a static state at some point. The event horizon coincides at some point in time with the apparent horizon.
The apparent horizonThe 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 HorizonEven 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.
How can we see a black hole?
If gas from a nearby star is "sucked" into the black hole, the gas will begin orbiting the event horizon, accelerating to velocities near the speed of light and heating up to many millions of degrees. We then will be able to detect the radiation. Another way is through Hawking radiation, where virtual particle and anti-particle pairs are created outside of the event horizon. The two will immediately collide and obliterate themselves, releasing gamma radiation. However, there are times when one of the pair is pulled in beyond the even horizon. The particle pulled into the black hole will then have a negative mass-energy and the one released will have a positive mass-energy, thus being detected as radiation.