At birth, our sun was a protostar, which was nothing more than a giant cloud of matter. It began to collapse in on itself due to gravity, and it began to swirl. Due to conservation of angular momentum, it flattened into a disk, and the center contracted faster, forming the dense stellar core. Denser areas in the disk began to swirl and collapse on themselves, forming planets around the fledgling star. The center of the protostar began to glow and heat up, and that counteracted the gravitational free-fall with pressure. The protostar stayed this way for about 10,000,000 years before it became hot enough for the first nuclear reactions to ignite. Then the sun was truly born.
With the birth of nuclear fusion, the sun entered the hydrogen burning phase of its life. Our sun is currently in this stage. In this phase, the star fuses hydrogen into helium in its core, converting .7% of the mass into energy according to E=mc^2. Our sun has a radius of about 6.95x10^8 m and a temperature of about 15,00,000 K at its center (6050 K at the surface). The star slowly slows its rotation, gets larger, and increases luminosity (brightness) as time goes on.
When the sun is about 10 billion years old, it will begin to use up all its hydrogen fuel, and the core will begin to contract again. This will cause an increase in temperature, which will allow hydrogen fusion to begin in a shell surrounding the core. The star will expand in size further until, in about 1.5 billion years, it will be about three times as big as it is now. As a red giant, it will keep expanding up to 100 times its present size and 500 times its current luminosity. It will remain in the red giant phase for only about 250 million years, with its core contracting and heating up to about 100 million K. At that point, the helium ash from earlier hydrogen fusion will begin to fuse into carbon. This will release a huge amount of energy and raise the core temperature up to 300 million K. This is known as the helium flash, and it is so forceful that up to one third of the sun's mass may be ejected into space. The core will then cool off to about 100 million K again and begin the steady burning of helium.
Once the sun is about 15 billion years old, it will exhaust its supply of helium and shrink and cool into a glowing carbon cinder known as a white dwarf star. It will be extremely dense, packing about 2x10^9 kg/m^3. It will be only 1% of its present size (about the size of the Earth) and 0.1% as luminous. Over a period of several billion more years, the carbon nuclei which make up the dwarf will cool even more until it dies as a cool, dark black dwarf star.
Stars with greater masses will suffer a different fate. They burn their hydrogen and helium supplies faster, and when the core collapses, the temperature is raised high enough for fusion of heavier elements. All of the elements up to iron (the 26th) can be formed in this manner. Thermonuclear fusion cannot, however, continue beyond iron. The iron formed clogs up the fusion, and as the star can no longer generate sufficient energy to be stable, a third collapse eventually follows. The crushing force of gravity forces protons and electrons together into neutrons, and the star finally stabilizes as a neutron star, the final phase of its life. A neutron star is only about 16 km in diameter and has a density on the order of 1,000,000,000,000,000 g/cm^3.
In even larger stars, the collapse of the iron core occurs so quickly that the huge forces blow the star apart in a supernova, the most spectacular event known. For several days, a star exploding in a supernova puts out more energy than an entire galaxy. The pressure and temperature are so great that all of the elements up to uranium and plutonium are created in the blast and flung into space. This incredible pressure also compacts the core into a mass of neutrons, creating a neutron star. This result of the supernova spins rapidly on its axis, typically between 20 and 50 revolutions per second. The star's magnetic field has been focused and strengthened by the collapse, and charged particles spiraling in toward the poles emit radio waves in a kind of beacon. When the star rotates, the beacon flashes Earth once per revolution. Because of the pulsing radio beacon, this type of star is known as a pulsar.
Stars with masses above the Chandrasekhar limit (about 1.5 times the mass of the sun) cannot combat the gigantic, crushing force of gravity generated by their masses. Such a star would theoretically collapse to a single point and become a black hole. A black hole is one of the strangest events that occurs (theoretically) in our universe. Because it consists of a single point, it has infinite density. At such a point, known as a singularity, the laws of physics would break down. The singularity is surrounded by the event horizon, which is the distance away from the singularity inside of which nothing can escape. Scientists speculate on what may happen at the singularity; some believe that it is the end of time, others that it may open up a wormhole, or passage to another part of the universe, still others that it may provide a way to travel through time. In any case, black holes are not well understood, and have not been definitely observed. They exist only in theory, yet they definitely are predicted by Einstein's general theory of relativity. Black holes are strange and fascinating, and they raise fundamental questions about our universe. For many years and many discussions to come, they will tempt the intellect and excite the imagination.