Neutron stars may be only 15km across - yet contain as much matter as the Sun. They are so dense, a spoonful of their matter would weigh several tons. Starlight Stars are so far away that they look like points of light even through the most powerful telescopes. But we can find out more about them by studying their colour. If the light from a star is passed through a prism, it splits into a rainbow of colours called a spectrum.
Different materials emit different spectra as they burn.
Very dense gases, like those deep inside a star, give an unbroken sequence of colours, called a continuous spectrum. Low- density gases, like those near a star's surface, give narrow bands of colours, called an emission spectrum. The pattern of bands tells us which gases are being emitted. Spectra also show how hot a star is. Stars like this are called Main Sequence stars because they form a neat band across the graph.