Blue stars are the brightest, and red stars are the darkest. Aside from temperature and brightness, a star's color typically—with the same qualification—indicates its size: the hottest and most energetic blue stars are usually larger, whereas red ones are smaller. The term "star" applies to objects that are massive enough to fuse hydrogen into helium in their cores. The sun is a star, as is it sister star, Mercury. Stars are also called luminaries because they emit light, and that light comes in many colors, depending on the composition of the star. Blue stars are more common than red ones, which are more common than yellow ones. In fact, almost all stars are either blue or red, with only a few being yellow. Stars are classified by how much energy they give off, with main sequence stars like our own sun being the most abundant type. Main sequence stars are those that have exhausted the hydrogen fuel in their cores and are now burning helium instead. As they age, they gradually move away from the main sequence toward higher temperatures and lower gravities (in other words, they become less dense). This happens because as stars get older they shrink due to loss of energy through radiation and gravity, so they need to consume material to add mass and restore their original size. A star's evolution depends largely on how much mass it has acquired - the more, the faster it will burn through its nuclear fuels and die.