Stars. Computer artwork showing a combination of stars of different colours, sizes and brightness. The colour of a star will depend on several of its properties, including its chemical composition, but in general a high-mass star is hot and blue, while a low-mass star is cool and red. Medium- mass stars, like the Sun, will be yellow. A star's appearance also changes during its lifetime, as it can expand and form a red giant star, and then contract to form a white dwarf star. The apparent brightness of a star will depend upon its distance, its luminosity (how much light it emits per unit area) and its size.