1) The Sunıs corona is filled with a diffuse, extremely hot gas. We discussed in this past week how we are able to determine the temperature and composition of that gas.

a) (8 pts) Name and explain one line of evidence that tells us how hot the coronal gas is. If you just answer "we know the gas is hot because of X", thatıs going to get little credit. You need to go on and say "...and X means temperatures are high because of the following reasoning..."

b) (6 pts) Until we truly understood the composition of the corona, many astronomers thought it consisted of something called "coronium". What exactly is coronium (what kinds of elements and how are they different from "normal" element spectra)?

2) We can deduce the temperatures of stars by looking at their colors. This is due to the laws governing continuous radiation.

a) (8 pts) Name and explain the two changes that happen to a star's spectrum as it gets hotter. Describe how each helps the star to more efficiently give off energy.

b) (6 pts) Explain where the term "blackbody" comes from as it pertains to continuous radiation.

3) Because of our knowledge of how atoms and electrons behave, we are able to use spectral lines to deduce the composition of a gas.

a) (8 pts) Explain how and why we can use spectral lines to identify an element in a gas.

b) (6 pts) We know about the inner workings of the nucleus and about the general nature of the atom thanks to experiments by Ernest Rutherford. Explain how Rutherford discovered that atoms are mostly empty space with a very small, dense nucleus.

4) The "solar neutrino problem" tested Astronomers' theory that the Sun is powered by nuclear fusion.

a) (8 pts) Explain the "solar neutrino problem", and then explain one reason why Astronomers didn't abandon the mechanism of nuclear fusion to explain the Sun's energy.

b) (6 pts) Though not perfect, the scientific method did help us better understand what was happening with fusion and neutrinos. On the other hand, when irrefutable beliefs (which cannot be tested or proven wrong) are involved, it makes for a very shaky system of knowledge. What is the biggest weakness with relying on irrefutable beliefs to explain things?

5) The Sun's outer layers include the photosphere and the chromosphere. Both have distinguishing characteristics that teach us about the Sun's properties.

a) (9 pts) Explain what causes limb darkening in the photosphere of the Sun. A simple diagram may help.

b) (5 pts) One feature seen in the spectrum of the Sun (caused by the chromosphere) is Fraunhofer lines. Briefly state what are Fraunhofer lines.

6) Nuclear fusion only occurs in the core of the Sun, because that's the only region within the Sun where the temperature and density of the gas is sufficiently high.

a) (8 pts) Explain why high temperature and why high density is needed in order for nuclear fusion to take place (it may help to briefly describe what exactly is happening between nuclei undergoing fusion).

b) (6 pts) Here on Earth, we sometimes use nuclear processes in medicine. Explain briefly how Positron Emission Tomography works.

7) For stars that are extremely far away, determining their radial distance from the Earth can be very important and very challenging.

a) (8 pts) Briefly explain how the parallax method works to find the distances to nearby stars. Also, state in a single sentence why the parallax method is not useful for stars that are extremely far away.

b) (6 pts) For a star with spectral characteristics identical to our Sun but too far away for parallax measurements, explain how we can use the inverse square law to find the distance to that star.

8) Some basic principles of Physics have really helped us advance our technological abilities in all walks of life. Lasers are particularly useful for lots of applications.

a) (7 pts) Explain how we use lasers in adaptive optics to reduce the blurring effect of the Earth's atmosphere on starlight.

b) (7 pts) Explain how lidar works to measure speeds.