# ASTR1000 University of Colorado Thermal Radiation and Astronomy Questions

**(1) Thermal Radiation**

**A telescope is observing another solar system, watching planets orbiting the star Ork. **

**The planet Mork has an average surface temperature of about 350K. **

**The planet Orson has an average surface temperature of about 50K.**

**(2 pts.) (a)** What is the wavelength of maximum thermal emission from each of these two planets? (Hint: use Wien’s law)

**(2 pts.) (b)**In what region(s) of the electromagnetic spectrum are these emissions?

**(2 pts.) (c)**Which planet emits more energy per square meter of surface area per second? By a factor of how much? (Hint: use the Stefan-Boltzmann law)

**(3 pts.) (d)**Mork has a radius that is about 4 times Orson’s radius. Mathematically compare the total amount of energy emitted by each planet per second over the whole planet. Mork emits how many times more energy per second? (Remember: the surface area of a sphere is 4 pi R2)

**(2 pts.) (e)** Planet Mindy orbits Ork at an average distance of 3.76 AU while, on average, Planet Fred 15.04 AU. from Ork. Mathematically compare the intensity (flux) of the light that reaches the surface of the two planets. How many times stronger is the light on Mindy in comparison to the light on Fred?

**(2) Spectra**

**(3 pts.) (2 a)**The plot above shows an example of a blackbody spectrum, the spectrum given off by a opaque object. What is the wavelength in meters where this spectrum is brightest?

**(3 pts.) (2 b)** Use Wein’s Law to estimate the temperature of the object emitting the spectrum above.

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**(3 pts.) (2 c)**The blackbody spectrum above would be produced by an object at a different temperature. What is the wavelength where it is brightest?

**(3 pts.) (2 d)** Estimate the temperature of the object emitting the spectrum above.

**(3) Doppler Shift ****(2 pts.)** We know that hydrogen on the Sun emits hydrogen line emission at 656 nanometers. When we look at a spectrum of light that has been reflected by a planet we see that the hydrogen line appears at 658 nanometers. What does this tell us about the planet?

**(4) Telescopes**

**(1 pts.) (a)**What are the 3 main functions of a telescope?

**(2 pts.) (b)** Astronomers like to brag about the size of their telescopes. What dimension in particular are we interested in when we say “bigger”? Give 2 reasons why a bigger telescope is better.

**(2 pts.) (c)** “Size is not everything.” By some important criteria, the 94″ diameter Hubble Space Telescope can make better observations than the 230″ diameter Keck Telescope on Mauna Kea, Hawaii. What is the most important reason why the Hubble is able to make better observations than the Keck?

**(5) The History of Astronomy** Describe the main constributions of the following individuals to the science of astronomy . Be sure to write your responses in complete sentences.

** (1 pt.) (a) **Aristarchus of Samos

** (1 pt.) (b) **Eratosthenes of Cyrene

** (1 pt.) (c) **Hipparchus of Nicaea

** (1 pt.) (d) **Claudius Ptolemy

** (1 pt.) (e) **Nicholaus Copernicus

** (1 pt.) (f)**Tycho Brahe

** (1 pt.) (g) **Johanness Kepler

** (1 pt.) (h) **Galileo Galilei

** (1 pt.) (i) **Isaac Newton

** (1 pt.) (j) **William Herschel