8. Astrophysics

• red shift of galaxies and cosmic microwave background radiation are pieces of evidence for the Big Bang theory
• stars initially form from a cloud of dust and gas called a nebula
• the force of gravity pulls the dust and gas together to form a protostar. the temperature rises as the star gets denser and more particles collide with each other. then the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei. this gives out huge amounts of energy, which keeps the core of the protostar hot. a star is born
• the star enters a long stable period. during this period the outward pressure caused by thermal expansion (the energy produced by nuclear fusion tries to expand the star) balances the force of gravity pulling everything inwards. in this stable period its called a main sequence star and it typically lasts several billion years. the heavier the star, the shorter its time on the main sequence
• eventually the hydrogen in the core begins to run out and the force due to gravity is larger than the pressure of thermal expansion. the star is compressed until it is dense and hot enough that energy (and so pressure) created makes the outer layers of the star expand. the star becomes a red giant (if it is a small star) or a red supergiant (if it is a larger star). it becomes red because the surface cools
• a small to medium sized star like the sun then becomes unstable and ejects its outer layer of dust and gas. this leaves behind a hot, dense solid core. a white dwarf
• big stars however start to glow brightly again as they undergo more fusion to make heavier elements. they expand and contract several times, as the balance shifts between gravity and thermal expansion. eventually they explode in a supernova
• the exploding supernova throws the outer layers of dust and gas into space leaving a very dense core called a neutron star. if the star is massive enough, it will collapse and become a black whole - a super dense point in space that not even light can escape
• For stars much bigger than the sun they initially form from a gas cloud called a nebula. The force of gravity pulls the dust and gas together to form a protostar. The star enters a long stable period. In this stable period it's called a main sequence star and it typically lasts several billion years. The star will eventually explode in a supernova. The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star. If the star is massive enough it will collapse and become a black hole.