The life history of a Star
The birth of a star:
Stars form out of clouds of dust and gas. The particles in the clouds gather together under their own gravity. The clouds merge and become more and more concentrated to form a Protostar.
As a Protostar becomes denser, it gets hotter. If it becomes hot enough, the nuclei of Hydrogen atoms and other light elements fuse together. Energy is released in the process, so the core gets hotter and brighter and stars to shine. A star is born!
Stars like the Sun radiate energy because of Hydrogen fusion in the core. This is the main stage in the life of a Star. It can continue for billions of years until the star runs out of hydrogen nuclei to fuse together.
- Energy released in the core keeps the core hot so the process of fusion continues. Radiation flows out steadily from the core in all the directions
- The force of gravity that makes a star contract is balanced by the outward pressure of radiation from its core. These forces stay in balance until most of the Hydrogen nuclei in the core have been fused together.
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The end of a Star:
When a Star runs out of Hydrogen nuclei to fuse together, it will swell out.
- As it swells, it cools down and turns red. It becomes a Red Giant. At this stage, Helium and other light elements in its core fuse to form heavier elements.
- When there are no more light elements in its core, fusion stops. No more radiation is released and the Star collapses on itself. As it collapses, it heats up and turns from Red to Yellow to White. It becomes a White Dwarf, a hot, dense White Star much smaller in diameter than it was. Stars like our Sun then fade out and go cold.
Bigger stars end their life much more dramatically.Their collapse continues past the White Dwarf stage, and then suddenly reverses in a cataclysmic explosion known as a Supernova.
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After a Supernova?
The explosion compresses the core of the star into a Neutron Star, an extremely dense object composed only of neutrons. If a neutron star is massive enough, it becomes a Black Hole. Its gravitational field would then be so strong that nothing could escape from it, not even light or any other form of Electromagnetic Radiation.