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Stars are created when dust and gas are pulled together by gravity into a disk until they collapse enough in the center that the atoms of helium are smashed together and nuclear fusion ignites the gas and gives the young star life adn light. Stars can range in sizes from a small star like our sun to stars tens of times the Sun's mass. Larger stars burn through their fuel faster than smaller stars and die sooner, though they create more new elements in their lifetime. All of the elements in your body and the objects around you are created in the core of a star.
Stars stay "alive" by producing energy through fusion that combats the gravity constantly pulling the matter of the star inward. When stars can no longer fuse atoms to produce new elements and energy, the star begins to die and collapse in on itself. Large stars have the gravity to keep fusing elements past where a small star would be able to, but once a star gets to iron it can go no further. Iron is the true killer of stars.
When smaller stars like the Sun die, they slowely expand over millions of years, which is called a Red Giant. The outer layers of the star blow away, leavnig a star remnant called a White Dwarf. The white dwarf has the same mass and gravity of the original star but it is much smaller. When the Sun dies it will expand all the way to Mars and the Earth will be destroyed in the process, though life would have been dead on its surface for thousands of years becasue of the greater heat the Sun will give off as a Red Giant.
When stars that are less than thirty solar masses (the mass of our sun) but much larger than our sun die, they create what is called a Supernova. The gravity of the star crushes inward faster and harder than that of a smaller star and compresses the core of the star into a ball a fraction of the star's original size. Then it explodes in a blinding flash of light that can be seen throughout the universe. In the explosion, all the elements heavier then Iron are created and spread throughout the galaxy. The remnants left by supernovi are called Neutron Stars. They have an extremely strong gravitational field which is more than that of the original star.
Hypernovas are similar to Supernovas, but they are created when stars more than thirty solar massese die and are much more rare. When their core collapses under the pressure of gravity it does not stop at the point where a pulsar is created. The star's core is crushed in to a singularity and the following explotion is much more energetic than that of a Supernova. Both Hypernovas and Supernovas give off gamma rays and superheated jets of energy which would devistate life on earth in seconds.