Dr. I. Fernini                                               Special Topics

ASTROPHYSICS (Phys 493)

Fall  2003

Homework 2

Due: October 18

Problem 1:

(a)     At noon, Mercury’s surface temperature is roughly 700 K; at midnight, 125 K. Calculate the

peak wavelength at which the surface emits at noon and at midnight.

(b)     Calculate the energy output per square meter of the surface at midnight and noon.

Problem 2:

The Earth, Venus, and Mars all have carbon dioxide in their atmospheres. Find the ratio of the root mean square speed to escape velocity for each.

Make a statement about the retention of carbon dioxide for these planets.

Problem 3:

In discussing the Coriolis effect, we mentioned that a body, which undergoes acceleration a, will travel a distance s=1/2 at2 in a time t.

Show that the speed of the body is proportional to time and that the body’s acceleration is indeed a.

Problem 4:

(a)     What is the energy of one photon of wavelength l = 300 nm? Express your answer both in

joules and in electron volts.

(b)     An atom in the second excited state ( n = 3 ) of hydrogen is just barely ionized when a photon

strikes the atom. What is the wavelength of the photon if all its energy is transferred to the

atom?

Problem 5:

(a)     How much more energy is emitted by a star at 50,000 K than one at 20,000 K.

(b)     What is the predominant color of each star in part (a) ? Use the Wien displacement law and

Problem 6:

At what wavelengths will the following spectral line be observed:

(a)                             the H-alpha line ( n1 = 2 ; n2 = 3) emitted by a star moving toward us at 500

km/s

(b)                             the Ca II line (undisplaced wavelength of 397.0 nm) emitted by a galaxy

receding at 20,000 km/s