Stars – Life and Death

Stars begin as clouds of dust and gas so far off in space that we would need 100,000 years to reach them. Gravity gathers gas and dust together and the clouds grow denser squeezed together by gravity. Eventually these clouds turn into spheres of compressed gas with internal temperatures of an incredible few million degrees Celsius. At this point nuclear fusion occurs. During this stage hydrogen fuses together to form helium and large amounts of energy is given out as electromagnetic radiation. A star is born.

A star like our sun has enough hydrogen to fuel it for billions of years. Once a star exhausts its core hydrogen it becomes larger, redder and more luminous. The star is now a red giant.
After a medium star like our sun exhausts its core hydrogen there is no longer any force to support the core against the crushing force of gravity so the core begins to contract. Hydrogen burning continues in a shell outside the core and the outer layers puff up engulfing nearby planets. This larger star is called a red giant. Meanwhile the core continues shrinking until it reaches a density high enough to fuse helium into carbon. When the helium in the core too is exhausted the star would evolve into a red supergiant. Eventually a red supergiant would lose all its mass in the outer envelope and leave behind only a hot core of carbon. The Carbon core would cool gradually and become a white dwarf, a dense, dim remnant of a once bright star.

When a much bigger star, about ten times as massive as the sun, exhausts its helium in the core, the nuclear burning cycle still continues. The carbon core contracts further and reaches temperatures high enough to burn carbon into oxygen, neon, silicon, sulphur and finally to iron. Iron, being the most stable of nuclear matter, cannot be fused to make another element. Without any heat to counteract gravity, the iron core collapses until it reaches nuclear densities. This high density core resists further contraction and in falling matter bounces off the core producing a supernova explosion. These supernova explosions eject carbon, oxygen, silicon and other elements into interstellar space. Without supernova, the fiery deaths of massive stars, there would be no carbon, oxygen and other elements that make life possible. The fate of the core depends upon the mass of the original star. If the mass of the original star was around ten times the mass of the sun, the core will become a neutron star. If the original star was much bigger then the resultant core would be so dense and its gravity so high that nothing, not even light can escape it. Such invisible remnants of huge stars are called black holes.

Thilina Rajapakshe
Grade 11-E
Dharmaraja College Kandy

Date: 6/30/2009