A star is a ball of hot, glowing gases that are reacting with each other in a similar way to the reaction that takes place in a nuclear reactor. It is this reaction, which is called nuclear fusion, which produces the light emitted by a star. Stars cluster together to form galaxies with each galaxy containing many different types of stars. Stars are light sources. The nearest one to Earth is the Sun and provides the Earth with light and heat energy. Most stars are made up of the two gases hydrogen and helium, together with small amounts of other elements. These gases become compressed at the centre of the star where it becomes so hot and dense that nuclear fusion occurs. This process results in the star releasing energy from its core to the surface from where it is released as light and heat. As stars reach this point, they are said to have joined the Main Sequence and are classified according to their mass.
- High mass stars are the largest, hottest and brightest Main Sequence stars and are blue, blue/white or white in colour. They have masses between three and sixty times greater than that of the Sun and use their hydrogen fuel very quickly so are short lived. Their temperatures range from about 10,000oC to 20,000o
- Low mass stars are the smallest, coolest and dimmest Main Sequence stars and are orange, red or brown in colour. They have masses between half and one twelfth that of the Sun and use their hydrogen fuel very slowly, so having much longer lives. Their temperatures range from about 3000oC to about 4700o
- Intermediate mass stars have life cycles between the low and high mass stars. They are yellow in colour and the Sun is a good example of an intermediate mass star. They have temperatures of about 6000o
Stars are often viewed as part of a constellation or group of stars. Many years ago, astronomers divided stars into groups and, to help them remember them, they drew imaginary pictures around the group (e.g. the Great Bear). This was the start of the system of constellations that we have today. In fact, the stars are largely unrelated and only appear to make these groups when seen from Earth. In reality, they may be many light years away from each other and in another plane (they are not all on the same flat plane as we see them).
Astronomers have also classified stars according to their brightness or magnitude. This is only an apparent magnitude as seen from Earth and does not describe the stars’ real brightness. The scale is numerical with 1 being the brightest. Stars classified 1 to 6 are those visible with the naked eye from the Earth, of which there are about 6,000.