Revise GCSE Physics

Question:What are planets, stars, comets, asteroids, meteors and meteorites, galaxies and black-holes.

A galaxy is a large group of stars held together by gravity.

When we look up in the night sky, many of the objects that we call stars are actually galaxies. They are so far away we cannot see the individual stars; in the same way that we are not able to resolve the individual dots that make up the picture on the TV screen from the other side of a room.

A star is a large ball of gases that are so hot they glow. stars are large spheres of very hot gases held together by gravity that produce their own light due to heating caused by the process of thermonuclear fusion at their cores.

Planets are large objects that orbit stars. (** Planets are too small to have heated up enough to start nuclear fusion at their cores, but are big enough to have become rounded by their own gravitational forces and to have cleared all the planetesimals in their vicinity by attracting them to collide with the growing planet in the early times of the Solar system.**)

Asteroids are rocky bodies that orbit stars. Asteroids are less than 500 km in diameter and therefore too small to have become rounded by their own gravity (like planets), so they have all sorts of shapes, and are often covered in impact craters too.

Comets are large balls of dust and ice that orbit stars. Comets often display a tail when they are close to their star. The tail is composed of gases produced by sublimation/melting of solid substances when the comet is warmed by the Sun.

Black-holes are objects in space whose gravity is so strong that not even light can escape, thereby making it impossible to actually see the black-hole directly. Black-holes are formed when supermassive stars explode at the end of the lifecycles. There is believed to be a black-hole at the centre of the Milky Way galaxy.

Satellites are objects that orbit planets.

The Moon is the 'natural' satellite of the Earth and the other satellites are 'artifical' satellites. 'Moon' is the proper name for the Earth's natural Satellite, so when we talk about the Moons of the other planets, what we really mean is the natural satellites of the other planets, which all have proper names of their own. For instance, Mars has two natural satellites called 'Phobos' and 'Deimos' ('Fear' and 'Dread' in Greek).

A meteorite is a very small asteroid that has fallen to the Earth's Surface from outer space without completely burning up in the atmosphere. A meteor is the blazing trail such an object makes through the atmosphere due to being superheated by frictional forces as it falls through the atmosphere at very high speeds.

The term meteor is often used interchangeably for asteroid but this is incorrect. The small Asteroids that become the meteorite and display the meteor effect are sometimes called meteoroids - the term meteoroid is reserved for asteroids smaller than 10m diameter which are no threat to life to Earth.