Gravity is responsible for many of the processes studied in space physics. It holds the solar system together and drives the life and death of stars. The expansion of the Universe is an exception. Gravity works against the expansion rather than causing it.
The solar system contains the Sun, a star at its centre, eight planets and several dwarf planets orbiting it, and natural satellites (moons) orbiting the planets. Asteroids and comets orbit the Sun too.
The Sun formed from a cloud of dust and gas, a nebula, pulled together by gravitational attraction. Our solar system is one small part of the Milky Way galaxy.
A star begins as a nebula pulled together by gravity into a protostar. When it is hot and dense enough, hydrogen nuclei fuse to form helium, releasing energy. During this main sequence stage the star is stable: fusion keeps the core hot, and the outward pressure that results balances the inward pull of gravity.
When the hydrogen runs low the star swells into a red giant. A star about the size of the Sun then sheds its outer layers and leaves a white dwarf. A much more massive star becomes a red super giant, explodes as a supernova, and leaves behind a neutron star or, if massive enough, a black hole.
The GCSE model is that fusion in stars builds the elements up to iron, and that heavier elements form in the extreme conditions of a supernova, whose explosion scatters them through space, which is where the material of the Earth came from. Modern astrophysics is more complicated: hydrogen and helium are mostly primordial, and much of the material heavier than iron is now thought to form by neutron capture in evolved stars and in neutron-star mergers as well.
Gravity provides the force that keeps a planet in orbit round the Sun and a moon or artificial satellite in orbit round a planet.
An object in a circular orbit moves at constant speed but its velocity is constantly changing, because its direction changes. That change requires a resultant force towards the centre. This is the centripetal force, and gravity supplies it. For a stable orbit the speed and the radius are linked: a smaller orbit requires a greater speed. Astronauts feel weightless not because gravity is absent but because they are in free fall around the Earth along with their spacecraft.
Light from distant galaxies is shifted towards the red end of the spectrum: the observed wavelength is longer than it should be. This is red-shift. It is evidence that the Universe is expanding, so the distance between us and those galaxies is increasing.
The more distant the galaxy, the greater its red-shift and so the faster it is receding. That is evidence that the whole Universe is expanding, and it supports the Big Bang theory, that the Universe developed from an extremely hot, dense early state. It was not an explosion into space that already existed. Space itself has expanded since then. Cosmic microwave background radiation, found throughout space, is further evidence for a hot, dense beginning.
Understanding of the Universe has changed as observation has improved: from the naked eye, to optical telescopes, to telescopes detecting radio waves, infrared, ultraviolet, X-rays and gamma rays.
The atmosphere absorbs much of that radiation and blurs images, so some telescopes are placed in orbit above it. That gives sharper images and access to wavelengths that never reach the ground.
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