Space

• Stars form in a cloud of dust and gas called a nebula.
• The force of gravity pulls the dust and gas together to form a protostar. When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei. This gives out lots of energy.
• Stars enter a long stable period when the outward pressure caused by the nuclear fusion is balanced by the inward pressure caused by gravity. This is when it's called a main sequence star.
• The Sun is a main sequence star
• When stars run out of hydrogen for nuclear fusion, they become red giants (or red super giants if they are large stars). Fusion of heavier elements like helium all the way up to iron are created in the core of the star.
• Smaller stars like the Sun eject their outer layer of dust and gas, leaving behind a small, dense solid core - a white dwarf. Eventually this cools to form a black dwarf.
• Bigger stars glow brightly again as more fusion takes place, forming elements as heavy as iron. Eventually they explode in a supernova, ejecting the elements they formed into space to create more planets and stars.
• After a supernova, a very dense core called a neutron star will be left behind. If the star was massive enough, it will form a black hole.
• Planets have to be large enough to have 'cleared their neighbourhoods' - their gravity has pulled in all surrounding objects apart from their natural satellites.
• If an object is moving in a circle (eg. orbiting) it is constantly changing direction, so it is constantly accelerating. This also means it is constantly changing velocity (but not speed).
• The instantaneous velocity keeps the object travelling in a circle instead of falling into whatever it's orbiting. It acts at a right angle to the acceleration.
• Faster objects will orbit with a smaller radius (closer) than slower objects.
• When we look at light from distant galaxies, we find the wavelength has been increased - shifted towards the red end of the spectrum. This is called red shift.
• Red shift suggests that the source of light is moving away from us. Distant galaxies are receding faster than nearer ones.
• The whole universe is expanding.
• The current theory for the start of the universe is the Big Bang, although new evidence may change this theory.