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Gravity and Stellar Evolution

star_structure.jpgStars are born when gravity causes a comparatively dense interstellar gas and dust cloud to collapse.  The centre of the collapsing protostar becomes hotter and denser, until it is hot enough to ignite the nuclear fusion reactions that convert hydrogen to helium.  These reactions release energy that heats the gas at the centre of the star, producing a pressure which resists the force of gravity.  As long as the fuel lasts, the star adjusts its internal structure so that there is an exact balance between the force of gravity and the force generated by the pressure of the hot gas. 

Outside the protostar, the remains of the cloud forms into a rotating disc.  By collisions and the action of gravity the dust and gas slowly forms into ever-larger clumps.  Eventually planets, moons, asteroids and comets are created.

The ultimate fate of a star depends on its mass.  A star about he same mass as the Sun will first become a red giant before ejecting its outer atmosphere, enriched with elements it has made, into the space between the stars.  There the matter will be ready to form a new generation of stars, while the Sun becomes a white dwarf and slowly fades away.

supernova1987a.jpgA star more than about eight times the mass of the Sun will finally destroy itself in a supernova explosion, which can be as bright as the light of an entire galaxy of stars.  The force of gravity causes the central regions of the star to collapse, releasing vast amounts of gravitational potential energy.  During the first few minutes of the supernova, heavy elements are created by nuclear fusion and dispersed into space in the explosion/

In a supernova explosion, the central regions of the star collapse to form either a black hole or a rapidly rotating neutron star.  Often neutron stars emit intense beams of radio energy.  If the orientation of the neutron star is right then as the radio beam sweeps past the Earth, typically many times a second, the remnant is observed as a radio pulsar.
Supernova 1987A exploded in the Large Magellanic Cloud on 24 February 1987.  Shown before (left) and after (right).