The left axis arranges stars according to their luminosity which is a measure of the brightness of a star independent of its distance.

If you observed the beam from a small flashlight held just a few feet away it might appear brighter than a powerful searchlight which was many miles distant. So it is with stars. A dim star which is relatively close might appear as bright or brighter than a very bright star which was much farther away.

When we consider the luminosity of stars we ask how bright they would appear if they were all observed at the same distance as our Sun. A star that would be just as bright as the Sun has a luminosity of 1. A star like Pollux would be almost 100 times as bright so its luminosity is just under 100.

Luminosity depends on temperature and size. Large, hot stars are the most luminous. Small, cool stars are the least luminous.

 ESRT Page 15 - Characteristics of Stars

Stars are also arranged by size with the smallest stars, dwarfs, near the bottom and the largest, giants and supergiants near the top.

When stars are arranged by temperature, size, and luminosity as they are here it becomes apparent that they are not randomly distributed. Most stars are located in a narrow band that runs from the top left to the bottom right. These are known as main sequence stars. Our Sun is a very average main sequence star.

As stars age they may go off the main sequence and become giants or dwarfs. Some may even explode, becoming supernovas.

 

Along the bottom axis stars are arranged according to temperature with hot stars on the left and the cooler star on the right. It is the temperature of a star that determines its color. The hottest stars are blue and the coolest stars shown on this chart are red. Between these extremes are a range of colors with our Sun, a yellow star near the center. Other yellow stars such as Alpha Centauri and Polaris are yellow because, like the Sun, they have a surface temperature of about 6000K. 

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