The planets and stars
The Sun is a star that gets its energy from nuclear reactions. It is at the centre of the Solar System, which contains planets, dwarf planets, moons, comets and asteroids in orbit around the Sun. The orbits of the planets are almost circular, but the orbits of comets are ellipses (very squashed circles). Asteroids sometimes hit the Earth and this could have serious consequences.
The Milky Way is the galaxy containing our Solar System and billions of stars. The distances between stars in a galaxy are so huge we measure them in light-years, rather than in kilometres. The universe contains at least one billion galaxies.
The foremost theory of the origin of the Universe is the hot Big Bang theory. It suggests that the Universe began about 13.7 billion years ago in an explosion that caused it to expand (extremely rapidly at first), and to continue expanding. The evidence for the Big Bang theory includes (1) the existence of a microwave background radiation, and (2) the red shift of light from distant galaxies.
Stars do not remain the same but change as they age. We can measure the distance to them using the parallax method, or by studying their relative brightness.
Hubble's Law describes the relationship between the distance a galaxy is from us and how quickly it is moving away. It is evidence that space is expanding.
The Earth itself is about 4.5 thousand million years old, about a third of the age of the Universe.
The conditions in space are hostile to life and spacecraft must be designed to protect their occupants. Space probes can explore other planets without needing astronauts.
Scientists are using different methods to see if there is life on planets other than the Earth. There is, as yet, no evidence of this, but many scientists think it is likely somewhere in the Universe.
The Universe contains extremely dense objects, called black holes, and may consist mostly of dark matter that cannot be seen.
Looking at the Sky
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science