The radiant energy is produced by fusion reactions, mainly the conversion of hydrogen into helium. As a result of these nuclear reactions the star emits electromagnetic radiation, especially light, and other forms of electromagnetic radiation. The temperatures and luminosities of stars are determined by their masses. The most massive stars are about 100 times heavier than the Sun. The largest stars are red supergiants - they can be 1000 times bigger than the Sun.
Large stars are often very luminous and hot, and so appear blue.
The hottest stars are blue supergiants. Their surface temperature is five times hotter than the Sun's. Medium-size stars, like the Sun, are yellow, while small stars are a dull red. The oldest stars are 15 billion years old. The universe contains billions of galaxies , and each galaxy contains billions of stars, which are frequently bunched together in star clusters of as many as 100,000.