Deep inside, hydrogen atoms fuse together to form helium. This nuclear reaction creates so much energy that the temperature at the heart of a star may reach millions of degrees, making the surface glow brightly. It will go on glowing, and sending out light, heat, radio waves and other kinds of radiation, until nearly all the hydrogen is used up. Giants and dwarfs Most stars are much the same size as our Sun, which is 1.4 million km across. But some giant stars, such as Aldebaran, are 20 to 100 times as big.
Supergiants are vast.
Aurigae double-star system 3000 million km across. If this star were a football, the Sun would be a grain of salt. There are also tiny stars. Some dwarf stars are smaller than the Earth. Neutron stars may be only 15km across - yet contain as much matter as the Sun.