The Life Cycle of Stars

Stars  Stars are huge masses of gas with a massive gravitational pull that keeps the planets orbiting around them.  They also give out massive amounts of heat, light and radiation.  To understand how and why stars evolve over a long time, we need to understand how they generate this heat and radiation.  The process which stars use to give out this energy is called nuclear fusion

Stage 1: Nebula Stars form in clouds of dust and gas called nebulae. We think this happens in the spiral arms of galaxies Our Sun formed about 4,600 Million years ago. Gravitational forces cause the large masses of gas to be squashed together.

Stage 2: Protostar Dense regions in the clouds collapse due to gravity As it gets smaller the protostar at its centre gets hotter Initially the gravitational energy of the collapsing star is its source of energy Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.

Stage 3: Main Sequence The protostar grows and eventually reaches equilibrium It becomes white hot and nuclear fusion starts These fuse H to He for a long time. Our Sun is in this stage Since stars have a limited supply of hydrogen in their cores, they have a limited lifetime as main sequence stars.

Stage 4: Red Giant Once the star runs out of fuel it expands and cools There are two directions stars can take depending on their size. Small stars like that of our sun: Stage 4: Red Giant Once the star runs out of fuel it expands and cools It becomes a red giant When this happens to our Sun it will expand and kill any life on Earth

Stage 5: White Dwarf Stage 6: Black Dwarf The red giant uses up all its fuel and collapses under gravity It forms a hot, dense white dwarf Stage 6: Black Dwarf The white dwarf cools to become a black dwarf The whole life cycle for a star like our Sun lasts for approximately 10 billion years

Stage 4: Red Supergiant Stage 5: Supernova Stars with a large mass: Stage 4: Red Supergiant Large stars run out of fuel quicker than smaller stars They expand and cool to form red supergiants Stage 5: Supernova Red supergiants run out of fuel, become unstable and collapse. They blow off their outer layer in an explosion called a supernova Supernova can last for weeks

Two things may happen: If the star was not massive a neutron star will be the result Not massive is about ten times the mass of the Sun! If the star was very massive it will result in a black hole The life cycle of a star 10 – 30 times the mass of the Sun is only 5 million years