Chapter Questions
When the Sun expands to be a red giant, its radius may be $100 R_{\odot} .$ What will the average density of the Sun be at that time?
When the Sun becomes a red giant, its core region (containing about $20 \%$ of its mass ) may contract to $0.01 R_{\odot} .$ What will the density of the core be at that time?
By how many times its current radius would the Sun have to expand before its outer atmosphere reached the Earth?
If the Sun becomes a red giant that has a radius 100 times larger than its current radius and a luminosity 1000 times its current luminosity, what will its surface temperature be? (Use the Stefan-Boltzmann equation from Unit 57 and solve for $T$.)
We observe a red giant with a luminosity 127 times the Sun's and a wavelength of maximum radiation of $675 \mathrm{nm}$. What is its radius? (Use Wien's law, Unit 55, to find its temperature.)
Estimate the radius of the $15-M_{\odot}$ star shown in the H-R diagrams in this unit from when it is initially a blue supergiant to when it becomes a red supergiant. (You will need to estimate luminosities and temperatures from the H-R diagram and then use the StefanBoltzmann equation from Unit 57 to solve for radius.