Chapter 19

Why do nuclei of elements other than hydrogen require higher temperatures to undergo fusion than does hydrogen?

Suppose the temperature of a normal gas doubles while its density remains the same. What happens to the pressure of the gas?

Suppose two single stars form at the same time. They have the same masses and chemical compositions. What can be said about the evolution of the two stars from that point forward?

In cluster \(1,\) the main sequence extends from spec tral class \(\mathrm{O}\) to spectral class \(\mathrm{K}\). In cluster 2 , there are no main sequence stars cooler than spectral class G. Which cluster is older and how do we know?

What do all main sequence stars have in common?

How do the temperatures, masses, radii, and luminosities of the most massive main sequence stars compare with those of the least massive main sequence stars?

Why doesn't the core of a \(1 \mathrm{M}_{\odot}\) star expand when helium fusion begins to raise the temperature of the core?

Describe the relationship between pressure and gravity throughout a pulsation cycle of a Cepheid variable.

Why are AGB stars faint in the visible part of the spectrum but very luminous in the infrared?

What is the origin of the gas that can be seen as a planetary nebula around a hot, post-AGB star?

Why must there be a hot star at the center of a planetary nebula?