General Astronomy/The Death of High Mass Stars
As with location in real estate, a star's mass is everything. In fact, aside from the exact chemical composition or the gravitational effects of companion stars, mass is the factor that determines the fate of a star. Even chemical composition is fairly unimportant at this stage of the universe, because the vast majority of stars are still primarily hydrogen and helium. The initial mass of a star sets the various parameters of a specific star's lifetime far more rigidly than any factor in the life of a biological organism such as a person. Mass determines the temperature, physical size and span of life, and the exact progression from one stage to the next. Two stars with the same initial mass will follow the same path through life and are in effect identical twins in the absence of any disturbing factor such as a companion star.
Unlike smaller stars, the largest stars have even more spectacular ends. The life track of a high mass star is similar to that of a low mass star like the Sun, except shorter and more violent. High mass stars effectively Live hard and die young. Near the end of their short lives, they puff up into a giant red star, although in this case a supergiant, and then they collapse.
A giant star consumes its nuclear energy more quickly than smaller stars, as one after another layer of core develops, where it fuses lighter elements into heavier elements. All of those reactions produce more energy than they consume. No star can go further in fusing elements than the creation of iron without producing a net loss of energy from the star.
The iron core shrinks, and one after another layer of core falls upon it. The iron core becomes extremely hot and unstable. Implosion is imminent once the star's temperature reaches 10 billion Kelvin. The star suddenly explodes releasing an extreme amount of energy in a very brief period of time as a supernova.
You don't want to be near a supernova; if you were in a pleasant zone near a star about to erupt as a supernova, you would be incinerated when the radiation from the supernova reached you. If one of the nearer stars like Alpha Centauri, Sirius, or Altair were to go supernova, the intense radiation would be deadly, but no nearby star is going to go supernova. That will happen to a giant like Betelgeuse, Deneb, or Antares, each of which is far enough away that the radiation is not a hazard.
While these hypothetical observers near a supernova find them unpleasant, astronomers on Earth -- a safe distance away -- find them very useful. There is a certain type of supernova called type Ia that is very useful for measuring distances. The nice thing about type Ia supernova is that they all have the same absolute brightness. Because they all have the same absolute brightness, they are called standard candles. The differences in apparent brightness of one type Ia supernova from another are entirely due to their different distances from Earth. The most popular theory is that a white dwarf accumulates material from a companion star, slowly growing towards the Chandrasekhar limit mass, until it has just barely enough -- about 1.38 solar masses -- to trigger an explosion. It's still a mystery as to what triggers the explosion and what happens during the explosion, but because it occurs at the same mass for all Ia supernova, all Ia supernova produce the same brightness. The differences in apparent brightness of Ia supernova are entirely due to their different distances from Earth. Astronomers measure the apparent brightness of a supernova as seen from Earth, and combine this with the known absolute brightness of all Ia supernova (also taking Doppler shift into account), to find the distance of the supernova from Earth -- and, by implication, the distance to the entire galaxy containing that supernova. The distances measured this way have led to a greater mystery discussed in General Astronomy/Current Unsolved Mysteries.
If the core ends up with a mass more than 1.44 times that our Sun (known as the Chandrasekhar Limit), not even quantum mechanical laws can prevent a massive star from compacting further.
For a remnant core between about 1.44 and 2 times the Sun's mass, the force of gravity pushes negative electrons into the positive protons, producing neutrons that occupy less space than their original particles and the core shrinks to what is essentially one large neutron. This neutron has more mass than our Sun but is about the size of a city on Earth, perhaps 20 kilometers across. It has become a neutron star. A teaspoon of neutron star material would weight about a billion tons on Earth.
The status of a remnant core between 2 and 3 times the mass of the Sun is not entirely clear, as it could become a neutron star, or perhaps an even smaller quark star which has only been theorized, or a black hole.
If the remnant core has more than 3 times the mass of our Sun, the gravitational field at its surface is not only large, but infinite. Nothing that gets too close to this object can escape. In fact, in theory no force of the Universe is strong enough to pull an object away that has wandered too close to a black hole. This distance away from the black hole is called the Event Horizon, or the surface of the Schwartzchild Sphere. Nothing that falls through the Event Horizon can escape, as escape velocity is greater than the speed of light. Hence not even light can escape, and the object appears completely black, hence the name, black hole.
A typical black hole with a mass three times that of the Sun would be roughly 18 kilometers across. Strictly speaking, this is the diameter of the Schwartzchild Sphere which is the visible surface of the black hole. However, the total mass of a black hole is considered to be concentrated into a single point with zero volume. Thus it is meaningless to speak of how much a tablespoon of black hole material would weigh. Effectively, the gravitational force and density at the black hole singularity are infinite.
We have not observed black holes directly (for obvious reasons), but we have ample evidence of their existence by the detection of radiation from objects falling into black holes, and in the gravitational effects they have on other bodies.