Space

- the solar system formed from gas and dust clouds that gradually became more and more concentrated because of gravitational attraction

- a protostar is a concentration of gas and dust that becomes hot enough to cause nuclear fusion

- energy is released inside a star because of hydrogen nuclei fusing together to form helium nuclei

- the sun is stable because gravitational forces acting inwards balance the forces of nuclear fusion energy in the core acting outwards

- stars become unstable when they have no more hydrogen nuclei that they can fuse together

- stars with a similar mass as the Sun:

    protostar > main sequence star > red giant > white dwarf > black dwarf

- stars which are much larger than the Sun:

    protostar > main sequence star > red super giant > supernova > neutron star (or black hole if there is enough mass)

- the Sun will eventually become a black dwarf

- a supernova is the explosion of a red supergiant after it collapses

- the force of gravity between: 

    a planet and the Sun keeps the planet moving along its orbit 

    a satellite and the Earth keeps the satellite moving along its orbit

- the force of gravity on an orbiting body in a circular orbit is towards the centre of the circle

- as a body in a circular orbit moves around the orbit:

    the magnitude of its velocity (speed) does not change

    direction of its velocity continually changes and is always at right angles to the direction of the force

    it experiences an acceleration towards the centre of the circle

- to stay in orbit at a particular distance, a small body must move at a particular speed around a larger body

- the red-shift of a distant galaxy is the shift to longer wavelengths (and lower frequencies) of the light from the galaxy because it is moving away from you

- the faster a distant galaxy is moving away from you, the greater its red-shift is

- all the distant galaxies show a red-shift, the further away a distant is from you, the greater its red-shift is

- the distant galaxies are all moving away from you because the universe is expanding

- the universe started with the Big Bang, which was a massive explosion from a very small point

- the universe has been expanding ever since the Big Bang

- cosmic microwave background radiation (CMBR) is electromagnetic radiation that was created just after the Big Bang

- the red shifts of the distant galaxies provide evidence that the universe is expanding, CMBR can be explained only by the Big Bang