Stars are large gaseous bodies that generate energy through nuclear fusion processes at their cores -- where temperatures and pressures are high enough for hydrogen nuclei to collide and fuse into helium nuclei, converting matter to energy in the process. Stars are born out of clouds of hydrogen, that collapse under gravity to form dense knots of gas. This collapse continues until enough pressure builds up to heat the gas and trigger nuclear fusion. The energy released by this "fusion-engine" halts the collapse, and the star is in equilibrium.
A star's brightness, temperature, color and lifetime are all determined by its initial mass. Our Sun is a typical middle-aged star halfway through its ten billion-year life. Stars can be 100 times more massive than our Sun, or less that 1/10 its mass. A Hubble Space Telescope search for dim stars suggests that most stars in the galaxy are about 1/5 the mass of our Sun.
Following a fiery birth, stars lead tranquil lives as inhabitants of the galaxy. Late in a star's life, fireworks can begin anew as changes in the core heat the stars further, eject its outer layers, and cause it to pulsate. All stars eventually burn out. Most collapse to white dwarf stars -- dim planet-sized objects that are extraordinarily dense because they retain most of their initial mass. Extremely massive stars undergo catastrophic core collapse and explode as supernovae --the most energetic events in the universe. Black holes and neutron stars -- ultra dense stellar remnants with intense gravitational fields -- can be created in supernova blasts.
At least half of the stars in the galaxy have companion stars. These binary star systems can undergo complicated evolutionary changes as one star ages more rapidly than the companion and dies out. If the two stars are close enough together, gas will flow between them and this can trigger nova outbursts. Supernovae and novae are key forces in a grand cycle of stellar rebirth and renewal. Heavier elements cooked up in the fusion furnaces of stars are ejected back into space, serving as raw material for building new generations of stars and planets.
