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. The stars visible to the unaided eye are all in our own galaxy, the milky way. The visible stars are divided into six classes according to their apparent magnitude.
Stars differ widely in mass, size, temperature, age and luminosity.
Variable stars fluctuate in luminosity. Red giants, the largest stars, are hundreds of times greater in size than the sun. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius. The central region, or core, has a temperature of millions of degrees. At this temperature nuclear energy is released by the fusion of hydrogen to form helium.