The sun provides a handy benchmark for describing other stars. The mass of this solar system's sun gives us a unit for measuring other stars' masses. Similarly, the sun's luminosity and surface temperature define the center of the Hertzsprung-Russell Diagram (H-R Diagram). Plotting a star on this chart reliably predicts other qualities of the star, such as mass and age. The main sequence phase of a star's life cycle is that time during which hydrogen fusion takes place in its core. But in terms of the H-R Diagram, "main sequence" also refers to a roughly diagonal, slightly S-curved line stretching between the upper-left and lower-right corners on which main sequence stars chart. They maintain a predictable relationship between luminosity and temperature: the brighter, the hotter. Both of these traits increase with a star's mass; a star indicated closer to the upper-left corner will be "heavier" than our sun, while lower-right main sequence stars will be "lighter."
Red Giants
Should astronomers plot a newly discovered star in the upper-right corner of the H-R Diagram, being both bright yet cool, they'll immediately know what phase of its life cycle the star is enduring. A red giant's core, hot enough to fuse helium and even heavier elements, has pushed its shell layers so far out that they can cool into the red spectrum. They owe their great luminosity not to their temperature, but to their size: bigger stars radiate more light energy.
