Chapter 19

Why does the chemical composition of a main sequence star change most rapidly at its center?

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?

In cluster \(1,\) the main sequence extends from spectral 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?

Describe the changes in the Sun that will occur as a result of the Sun's main sequence evolution. What effect will these changes have on the Earth?

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 planefary nebula around a hot, post-AGB star?

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