Physics 101 - Astronomy

Study guide and notes for Chapters 11-16 (in the OpenStax textbook)

To review for the exam, I suggest that you go to the OpenStax book and look at the Key Terms, Summary, and Review Questions. Instead of re-reading the chapter (if you read it already) you could try writing down facts that you remember from the chapter. Then check your answers and go back to any areas that you were weak on. The relevant sections of the OpenStax book are 11.1-3, 12.1-5, 13.1-4, 14.1-4, 15.1-4, 16.1-3.

Here are some samples of the kinds of facts you should know. Note that this is an incomplete list; look at the book for facts about planets, etc.

Dwarf Planets and Small Solar System Bodies

Triton (the large moon of Neptune), Pluto, Charon, and the Kuiper Belt objects are all similar, and could be put in the same category.

Rings around the Jovian planets form when moons drop to lower orbits and pass inside the Roche limit, where they break up due to tidal effects.

Asteroids are mostly in the asteroid belt, between Mars and Jupiter, but some are Earth-crossing asteroids (Near-Earth Objects NEO). They are mostly rocky and/or contain iron.

Comets have a nucleus, ion tail, and dust tail. Comets have large amounts of frozen water in them. The long-period comets may come from the Oort cloud, located over 1000 AU from the Sun. The short-period comets may come from the Kuiper belt.

The Sun

You may want to organize your understanding of the Sun by thinking of the structure of the Earth. The Earth has an inner core, an outer core, a mantle where there is convection, a crust (and oceans on top of part of that), and an atmosphere with various layers. So the Sun also has a layered structure.

The core of the Sun is hot due to nuclear fusion, and dense due to all the material above the core pressing down on it because of the gravity of the sun. The temperature of the core is about 15 million K, somewhat hotter than the minimum needed to start nuclear fusion (which is about 10 million K). You can think of nuclear fusion as a process (with several steps) that results in the fusion (combining together) of four protons (from hydrogen) into the nucleus of the helium atom, which contains two protons and two neutrons, all bound together very tightly. In the process, two electrons are destroyed, and two neutrinos are produced, which immediately travel away from the core and pass right through the rest of the Sun. Also, gamma rays, which are like powerful X-rays, are produced, and they also travel away from the core.

Outside the core is the radiation zone, which is transparent, so the gamma rays and other radiation from the core passes right through it.

Outside the radiation zone is the convection zone, which absorbs the gamma rays, and carries the heat generated by the core up to the next layer by the process of convection (movement of hot gas around in circles, similar to boiling water in a pot). See the figures in your book to visualize these.

Outside the convection zone is the photosphere, which is the part of the Sun that we see. The gas in the "atmosphere" of the sun is fairly transparent, so the photosphere radiates lots of light (infrared, visible, and ultraviolet) into space, so the photosphere is actually the coolest part of the Sun. Still, it is at about 5800 K, much hotter than any planets! The granulation of the photosphere is due to convection cells, about the size of Texas (1000 km in diameter, although they vary in size). The luminosity of the sun is due to the radiation coming from the photosphere.

The atmosphere of the Sun has three main layers, the chromosphere, the transition zone, and the corona. The chromosphere is somewhat hotter than the photosphere, and would have a pinkish or reddish color if we could see it separately, and the photosphere didn't blind us. The transition zone is a region where it gets hotter as you get farther above the surface of the photosphere, until you reach the corona, which is very hot, about 1 million K, but very thin and diffuse, so the light from the corona is very weak and we can't see it unless there is a total solar eclipse. Holes in the corona let out some of the gas that makes up the solar wind.

The active Sun occurs on a regular cycle of about 11 years, also called the sunspot cycle. The sunspots are usually formed in pairs, and behave like the poles of magnets. The magnetic field of the Sun comes out through one sunspot of the pair and goes into the other. Prominences are loops of hot gas that tend to travel along the magnetic field lines above the pair of sunspots. Flares are giant explosions that occur over active regions on the photosphere. In class I showed some pictures of coronal mass ejections which blow huge amounts of gas away from the Sun so fast that it doesn't fall back. Sunspots also are useful to watch because they show us how fast the sun rotates, and we find that the equator rotates faster than the poles.