Most stars are main sequence stars that fuse hydrogen to form helium in their cores - including our sun.

The Sun is very much a main sequence star, in the G-type category (G2V to be precise). It’s been fusing hydrogen for 4.6 billion years, but it still has almost three times as much as it has helium.

While main sequence stars trace a diagonal line on the H-R diagram, red giant stars create a horizontal line in varying shades of red. This is called the red giant branch.

The diagram contains a group of stars that form a diagonal line below the main sequence, which means they are dimmer than main-sequence stars of the same temperature and color.

The early HR diagrams only had data for about 300 stars, but even then it was clear most stars lay along a linear path known as the main sequence. Since most stars are within the main sequence, it ...