Name: ________________________


                                        Student Number: ______________

4. (3 pts) If an astronomer would like to estimate the total time from birth to death of a particular star, what is the one property of the star she most needs to know?

5. (5 points) Spectra of Spica and Procyon.
Look at locations of Spica and Procyon in the HR diagram, figure 15.9. Describe how the strength of the calcium and visible hydrogen lines in their spectra differ.

Spica, a hot star, has stronger hydrogen lines and weaker calcium lines than the star Procyon.

This is because the more energetic photons in the atmosphere of Spica have ripped off the outer electron shell of the elements which have less strongly-bound electrons, like calcium. So calcium absorption lines do exist for Spica, but they're seen in the bluer end of the spectrum, not the visible part.
Since hydrogen's one electron is physically close to its nucleus, it takes the energetic photons of Spica to interact with it.

6. (3 points) In class we have discussed four tests of astrophysical models of stars. Three of the tests (helioseismology, mass-luminosity relation, and HR diagram) confirmed the model, but one (neutrinos from the Sun) did not. Why aren't the models rejected?

(a) The neutrino observations are wrong.
(b) The solar model should be rejected. Astrophysicists are stubborn.
(c) The Sun may be unique and not the same as other stars.
(d) The neutrino experiment depends on both the solar model and the theory of neutrinos. Either one might be wrong.
(e) Since three out of four tests agree with the predictions of the models, astrophysicists ignore the one test that is in disagreement with the models' predictions.

7. (5 points) 1. The structure of a star is determined by two equilibrium or balance conditions. These are the balance between: PRESSURE and GRAVITY, and the balance between ENERGY GENERATION and ENERGY LOSS. Apply these balance conditions to describe what would happen to a star if its insulation increased, by circling the correct choice in the paragraph below.

When the insulation increases the luminosity of the star - rate at which energy is lost - will (decrease) (remain unchanged) (increase). This will make the temperature in the core (decrease) (remain unchanged) (increase). As a result the rate of nuclear reactions will (decrease) (remain unchanged) (increase) and pressure in the core will (decrease) (remain unchanged) (increase). Then the pressure will be (less than) (the same as) (greater than) the gravity. So the star will (contract) (not change) (expand). As a result the insulating ability of the star will (decrease) (remain unchanged) (increase). This will make the luminosity (decrease) (remain unchanged) (increase). This (contraction) (unchanged state) (expansion) will also (decrease) (leave unchanged) (increase) the pressure in the the core. As a result, both the balance conditions will again be satisfied.