## Astronomy 120: Fall 2008

### Name ___________________________ Section _______

1. In 1910, Halley's Comet passed extremely close to the Earth - so close, in fact, that the Earth passed through the tail of the comet in May 1910. As the date arrived, many people worried that life on Earth would be extinguished by all of the poisonous gasses that would flow onto the Earth.

Their fears were unfounded (my grandmother remembered this, by the way). Why? Explain why such worries were not based in fact given what we now know about comets and their tails.

2. Why does nuclear fusion require high temperatures and pressures?

3. Describe, in a few sentences, what would happen in the Sun if nuclear fusion reactions suddenly turned off. Would the sun continue to shine? If not, what would happen to it? If so, how long would it last? How long would it take for us to find out?

Choose one of the following two questions (4 or 5) to answer

4. Let's consider the huge output of the Sun in terms of the energy needs of the United States:
1. The total annual US energy consumption is about 2x1020 Joules of energy. A Joule is the amount of energy produced by 1 Watt in 1 second. With this information, and remembering that there are 3x107 seconds in a year, what is the power (in Watts) needed to run our country?

2. Solar collectors can harvest solar energy with an overall efficiency of about 5% (averaging in cloudy weather and nightime), so they can on average generate about 70 Watts/m2. What area (in m2) would we need to cover to satisfy the US power needs with solar power?

3. The US has a surface area of about 20 million km2, or 2x1013m2. What fraction of the area of the US needs to be covered with solar collectors to satisfy all of our power needs?

4. Where in the US would you want to install solar energy farms? Why there?

5. As we've seen in this class, the Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are quite different from the terrestrial planets (Mercury, Venus, the Earth, and Mars). In an essay of one page or a little less, explain why the Jovian and terrestrial planets differ in the following ways: composition, size, density, distance from the Sun, and number of planets.