## ASTR 503 Homework 5 : Sun and Stars

ALWAYS SHOW YOUR WORK; Keep track of units!

1. When the Sun dies, it will swell into a "red giant", with a diameter around the size of the orbit of Mars. What will be the surface area of the red giant shell of the Sun then? _________ km2.

2. Assuming that the Sun's surface temperature goes down by a factor of 2.5 from its present temperature of 6000 K, what will that surface temperature be? _______ K

3. What will the wavelength of the peak of the solar spectrum at that time? _________ nm. Use Wien's law λ = (0.3 / T) (cm/K).

4. What will the amount of energy radiated per m2 of the red giant surface? _____________W/m2 . Use Stephan-Boltzman law J = σ T4
where σ = Stephan-Boltzman constant = 5.67 10-8 W/(m2 K4)

5. What will be the total amount of energy radiated from the Sun then? ______________ W
How does that compare to the Sun's total energy output now? (3.84 x 1026 W)

6. (Back to today's Sun): If a Coronal Mass Ejection has an average speed of 1000 km/s on its way to Earth from the Sun, how long will it take to arrive?

Last updated 9/9/2014