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Before understanding how stars dies, we first have to understand what they are and how do they live.
I always though they [the stars] were balls of gas burning billions of miles
away.
Pumba the Warthog - From The Lion King
Pumba is absolutely right. The stars are giant balls of gas “burning” billions of miles away. Our Sun is also a star although it’s only about 93 million miles away from us and we’ll be using it as an example in this article.
Stars are made up mostly of the light gas hydrogen. The hydrogen in a star does not burn in the conventional way most things burn on Earth but by a “ fusion reaction” in which two hydrogen atoms, each consisting of a single proton and single electron, are fused under enormous pressure to become a single helium atom. The final mass of the helium atom is LESS than that of the two hydrogen atoms. Some of their original mass is converted directly into energy according to Einstein’s famous equation E=mc2. According to this equation a small amount of mass is converted into an enormous amount of energy. For instance, if it was possible to convert a pound of potatoes (or anytoodg else for the matter) directly into energy, it would create enough energy to feed the entire human population for a day. Less than 1% of the original hydrogen is converted into energy but that’s enough to produce all the heat and light created by the stars including our own Sun. The fusion reaction within the Sun also produces neutrinos which are a strange sort of particle without any mass so they can pass through almost any solid object. In fact there are billions of neutrinos passing through your body at this very moment without having any effect on you what so ever.
The fusion reaction is what takes place in a hydrogen bomb and releases much more light and heat than from an atomic bomb which is based on splitting large atoms. So if somewhere in the center of the Sun (and other stars) there is a nuclear furnace generating huge amounts energy, why doesn’t the whole thing just blow apart like one of those hydrogen bombs dropped on isolated Pacific Islands in the 1950s and 1960s? Because while the fusion reactions taking place at the center of a star creates an OUTWARD pressure, the gravity of the massive stars creates an INWARD pressure. In a stable star such as our Sun, the outward pressure from the nuclear furnace and the inward pressure created by gravity balance each other and keep the star at a constant size. The Sun which is a fairly typical star is not as dense as the Earth but is much, much larger and weighs about 4.4 followed by 30 zeros pounds which is the equivalent of 340,000 Earths. Such a large amount of matter creates an enormous gravitational pull which keeps the Sun from blowing apart.
The Sun, like other stars, lives because it generates massive amount of heat and light by converting hydrogen to helium in a nuclear furnace a confined by gravity. But what happens when it runs of hydrogen?
In part 1, a star was described as sort of a nuclear furnace which is held together by gravity and powered by hydrogen being fused into helium and producing heat and light. This can go on for billions of years but eventually the hydrogen will run out. But before it does, there will be a contracting helium core in the center of the star which is only twice the volume of the Earth. The remaining hydrogen still gets converted into helium in a spherical shell that grows larger and larger. During this phase, which can last a billion years, the star both expands and cools. It becomes a red giant, maybe 50 times its original size. During this time, the helium has contracted to a very high density and heat (over 100 million degrees Centigrade) and another type of nuclear reaction starts taking place. The helium atoms (that were originally created from hydrogen) are now fused into atoms such as beryllium and carbon. The star of Antares in Scorpio is an example of a red giant.
Eventually all the star’s fuel is exhausted and it starts to contract. Remember it was the pressure created by nuclear fusion that prevented gravity from shrinking the star. Once the fusion has stopped, gravity takes over and a star the size of our Sun will shrink to something like the size of the Earth. This is called a white dwarf and although it’s too small to be seen with the naked eye there is a white dwarf called Sirius B which is a companion star to Sirius in Canis Major.
The shrinking of a white dwarf releases some energy (as light and heat) but eventually it reaches its minimal size. During the shrinking it changes color from blue to white to yellow and to red. Finally when it has shrunk to its final size and no longer can either generate energy through nuclear fusion or even release energy by shrinking, the star becomes a black dwarf. A black dwarf has mass and it can still generate a gravitational pull but no longer produces any light or heat. This is the fate of our Sun, but the death of larger stars is quite different.
Yesterday we described how our Sun and similarly sized stars will eventually end up as a Black Dwarf about the size of the Earth. But a star that has 1.4 times the mass of the Sun has a very different death. The end life of a star whose mass is greater than 1.4 times the Sun’s mass is so different that we call this dividing line the “Chandrasekhar’s limit” after the brilliant Indian born, American physicist.
If a star is above the Chandrasekhar’s limit at the end of its life, outwardly it will appear like a red giant (see part 2) but inside its core will be shrinking and triggering nuclear fusion reactions. Helium is fused into carbon and oxygen, oxygen into neon, sodium, magnesium. Magnesium, silicon, sulfur, phosphorus, cobalt, nickel and eventually iron will also be created. Finally the expanding iron core colliding with the collapsing gases within the star create a tremendous explosion called a “Supernova”. Most of the original material forming the Earth and the planets and our very bodies was created within a supernova. As Carl Sagan said, “we are stardust”.
A supernova explosion is so powerful that for a few days it outshines the light from all the other stars of a galaxy combined. Because of this, it has been possible to observe supernovas in other galaxies from time to time. A supernova is a relatively rare event and the last one in our own Milky Way galaxy took place during the 17th century. It was bright enough to be seen during the day.
Eventually gravitation pulls the remains of a super nova explosion together and it begins to contract. To understand what happens next we have to know a bit about the structure of an atom. A simplified way to think of an atom is very small core of relatively heavy neutrons and protons (which make up most of the atom’s mass) surrounded by an electron “cloud”. The term “cloud” is used because although the number of electrons for each atom is known, there is no way to precisely determine their location at any one time. The core is 100,000 times smaller than the entire atom. Under ordinary conditions electrons keep a certain distance away from each other and the nucleus. This is why a white dwarf will remain about the size of the Earth with a radius of a few thousand miles. But the inside of a collapsing super nova is far from ordinary. The gravitation created by the huge collapsing object forces the electrons and nuclear together into a sort of a giant nucleus. Under ordinary circumstances, most of an atom is empty space, but when a supernova collapses the empty space is “squeezed” out by gravitational forces and instead of a dwarf, a neutron star is created. A neutron star is only about 10 miles across and “weighs” billions of tons per cubic inch.
What about stars that are two or even three times more massive than the Sun? We don’t know exactly what is the upper limit of mass for a neutron star, but at some point a star is so massive that even after throwing off material during the supernova explosion, it creates a gravitation force that is literally irresistible. In a neutron star, the enormous gravitation squeezes the neutrons, protons and electrons into a tiny space, but the electrical forces keeps them from getting any closer to each other. But if the star is massive enough, the gravitation will overpower even these powerful electrical forces. The star will not stop shrinking when a few miles across, but its entire mass will continue to collapse into a point with no volume called a “singularity” or as it has been come to be called – a black hole. The laws of nature break down in a black hole although there are interesting theories that rival science fiction about black holes being connected to each other through “worm holes” than could be used to jump through time and space. A black hole can only be detected indirectly. If a star is moving as if under the influence of a massive, invisible star, than there is very likely a black hole. A black hole exerts all the gravitational force of a large star, but can not be seen. It’s considered highly likely that a massive black hole is in the middle of the galaxy.
I wouldn’t recommend it. There is a border around the black hole called the event horizon. If the black hole is massive enough (remember that a black hole has mass even though it takes up no space) you wouldn’t notice anything as you passed the black hole’s event horizon, but at the moment your fate would be sealed. Nothing in the universe is energetic enough to escape the gravitational pull of a black hole from within its event horizon. Not even light can escape. If you shined a flashlight away from the singularity, the black hole’s gravitation would not allow its light to escape beyond the event horizon.
An excellent account of the end of stars and black holes can be found in Kip Thorne’s book Black Holes & Time Warps
Black Holes - A Traveler's Guide by Clifford A. Pickover gives the reader a chance to “play” with black holes by using simply mathematics.