The H-R Diagram: Practice Problem Solutions

1. Surface temperature (or color or spectral class) and luminosity (true brightness).

2. It is a historical convention: the temperature scale is drawn running from hot on the left to cool on the right.

3. The main sequence, which holds about 90% of stars.

4. A red giant or supergiant. To be so bright while cool it must have a huge surface area, so it is very large.

5. A white dwarf, which is hot but tiny.

6. They have more fuel, but they burn it far faster (luminosity grows about as mass to the 3.5 power), so their fuel runs out sooner.

7. About 3,000 times as bright, and it lives about 1/300 as long (roughly 30 million years).