Let's examine the sizes of stars, and see how they relate to the H-R diagram.

• Stars such as Ori which are cool in temperature yet huge in size are found in the upper right of the HR diagram and are given the name Red Giants. They can grow in size up to 1,000 times the radius of the Sun, ballooning outward in size as they run out of fuel and migrate away from the Main Sequence.

• We can play the same trick with the low-luminosity, hot stars at the left side of the H-R Diagram, to determine their sizes. Despite the fact that they have lots of energy radiated per unit surface area, they have a small amount of total energy being radiated. Therefore they must have small surface areas, and so we call them White Dwarfs .

• You can play this game with all the stars in the sky, and find an enormous range of sizes from around 1/100 R to nearly 1,000 R .

• Most stars fall upon the Main Sequence (the blue curve drawn below). They form a steady progression, starting with cool, small, faint stars (lower right-hand corner) and increasing in size, temperature, and brightness to hot, large, bright stars (upper left-hand corner). The red giant and white dwarf populations are isolated from the Main Sequence. How else might they differ from those stars? Are they the same age?

  Red giants have high luminosities, but their large surface areas make them cool. In contrast, white dwarfs have low luminosities but their small surface areas mean that the energy per unit area is high, so they are quite hot. [NMSU, N. Vogt]