Wednesday, December 8, 2004
LAST DISCUSSION SESSION!
Please come to the front of the room and take back your papers.
Concept Check Winners – add 3 bonus points to your discussion grade
Section 203: Team ATeam B
Section 204: Team ATeam B
Extra Credit! 5 pts.
You are planning on going online to do the course evaluation, right? (I’m really looking forward to hearing feedback from you!) Remember, those need to be completed by December 10. Well, while you’re online, you might as well complete the extra credit assignment on the astronomy place web site – the "Detecting Dark Matter in Spiral Galaxies" tutorial and exercises. It is due the last day of classes, December 10. Remember that you must join the online class to get credit.
Remember the Q&A review session in CSS 2324 on Mon Dec 13 from 4-5 pm. Your final will be Thursday December 16 from 8:00 am – 10:00 am.
The afternoon will be/was my last official office hour. I will be available on Tuesday December 14 from 10:30-11:30 to answer last minute questions and hand back your homework.
Short answer practice: The Sun and stars produce absorption spectra. Using what you know about the conditions that lead to the three different types of spectra (emission, absorption, and thermal), explain why we see an absorption spectrum for the Sun and other stars.
Non-answer:
We see an absorption spectrum from the Sun and other stars because the light that we see from these stars covers a large amount of the light spectrum and not just certain parts of the spectrum.
Note how this answer fails to explain why we see the absorption line in terms of the conditions within the star. Although it is good that this student knows what an absorption spectrum is, he/she failed to address the question asked.
I generously gave this answer one point out of four, but I would have been fully justified in giving no credit at all. Make sure you are answering the question asked and answering it fully! Don’t waste time writing down material irrelevant to the question asked – you won’t get any extra points for it, and if you say something wrong you could get a little bit off.
Partial credit answer:
We would see an absorption spectrum for the Sun and other stars because between the Sun and us would be clouds of gas absorbing some of the wavelengths of light. These cloud/dust particles interfere with our visibility of the direct source of light we want to see.
Sounds good, right? Well, it’s not a bad start. Unfortunately, some of the answer is only partially correct, and there is information that’s altogether missing. The “clouds of gas” are the stellar atmosphere – it is important to note that the gas absorbing the photons is part of the star. Also, this answer does not mention that the gas is cool and thin, which is also important. Finally, the initial state of the light is not discussed at all. Remember, the light is generated in the hot, high pressure core; at this point, it is a continuous spectrum of thermal radiation. Make sure that you fully answer the question!
My answer:
The hot, high pressure gas within the stellar core produces a continuous (thermal) spectrum of thermal radiation. All stars have relatively cool, low densityatmospheres; the atoms in the atmosphere absorb photonsof certain wavelengths that are coming out of the core. This wavelength corresponds to the difference between two electron energy levels. The electrons move up to a higher energy level when atoms absorb photons, and then the electrons typically fall back down to their original levels, emitting photons of the original absorbed wavelength in a random direction. Since some of the absorbed photons that were initially traveling in our direction are not any longer, we cannot see a continuous spectrum. Rather, we see an absorption spectrum.
Course Review
Questions I would focus on are in bold. I give no guarantees, though! Questions that students emailed me are in italics. Remember, he probably won’t ask questions identical to those in previous exams, although he could modify some old questions.
Most important material covered recently:
What are the parts of a spiral galaxy? Be able to draw and label a picture of a spiral galaxy.
See figure 14.1.
What other kinds of galaxies are there? Can you draw a simple sketch of these?
What are the steps of the star-gas-star cycle? What happens during each step? What are the consequences of this cycle – do older stars have more or less metal in them? How does the star-gas-star cycle work? What are the different effects of the low mass star and high mass star within the star-gas-star cycle?
What two types of nebula did we discuss? What do they look like, i.e. what colors are they? Where can you find these nebula? What causes them to form?
What are active galactic nuclei? What are quasars? Where would you look to find them? When did these exist? How are they powered? Are radio galaxies and quasars the same? Why is it that quasars were only active during the beginning of time and have since become dormant? Do ALL galaxies have a black hole with quasars or that used to have quasars?
An active galactic nucleus is an unusually luminous galactic center. Quasars are very bright active galactic nuclei. Remember that at large distances we are seeing into the past. Since active galactic nuclei are far away, they are young. Therefore, activity is more common among young galaxies than older ones. More active galaxies existed in the past. Current theory holds that active galactic nuclei are powered by supermassive black holes. The gravitational potential energy of matter that is falling toward the supermassive black hole (we say that this matter is being accreted onto the black hole) is converted into kinetic energy, which in turn is converted into thermal energy.
In what ways can we measure distances to stars, galaxies, and other celestial objects? How did Hubble use the Cepheid variable stars to prove that there are galaxies outside of our own? What exactly is a Cepheid Variable? Is it a kind of star or is it something else? And, how are they related to measuring distances, etc.?
When Hubble looked at galaxies distances and velocities, what relationship did he find? What does this imply about the formation and age of the universe? Astronomers claim that the fate of the Universe is to expand forever. However, when asked what is the Universe expanding into, they say that it is not expanding into anything. Can you explain this theory more clearly?
How do galaxies form? What roles do angular momentum (from the protogalactic spin) and density play? How can spiral galaxies become an elliptical galaxy? How is dark matter involved in galaxy formation? What are the conditions in protogalactic clouds? I know collisions were pretty dominant earlier in time, and galaxies were found to be a lot closer together compared to the present galaxies. I was wondering did collisions play a major role in the formation of protogalactic clouds or were they just responsible for the creation of elliptical galaxies? How does dark matter influence the formation of galaxies, and what are the ways that we can detect dark matter? If dark matter in the universe did not exist, what are the major ways that our universe would change?
What is dark matter? What evidence is there that dark matterexists? What types of matter are candidates for dark matter? Which do we favor? Why?I really don’t understand the concept of black matter and how they came to find black matter. Also, the effects it has on the objects around black matter. Separate email: I do not understand the statement that if most of the mass is in the halo, the velocity of the stars closer to the edge of the disk would have to be the same as stars near the bulge. Why would the velocity be the same, wouldn't the velocity of stars in the halo be greater because there is more dark matter? How do you determine the velocity of stars? Is it based on the amount of matter in that region?Could you explain the differences between ordinary and extraordinary dark matter? If astronomers cannot explain or prove that dark matter really exists, then how are they able to determine such things as MACHOS and WIMPS? I just don't understand how they can gather so much information about something that might not even be there in the first place.
Dark matter is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds. We think there is dark matter because the amount of light-emitting mass that we see does not agree with the amount of mass that is needed to explain observed motions. When looking at these motions, we can consider rotation curves of spiral galaxies, absorption line widths in elliptical galaxies, orbits of galaxies in clusters, gas temperatures in clusters, and gravitational lensing of celestial objects. (In the interest of time, I won’t go into these. Feel free to ask me questions, though.) There are two candidates for dark matter: MACHOs and WIMPs. MACHOs are just ordinary or baryonic matter in the form of dim stars or planetlike objects, while WIMPs are weakly interacting massive particles.There does not appear to be enough ordinary matter (MACHOs) to account for all the dark matter. Most of it is probably extraordinary or nonbaryonic matter consisting of undiscovered particles (WIMPs). WIMPs also help to explain why dark matter is in the halo: they don’t interact much, so they aren’t able to radiate away their orbital energy and collapse down into a disk.
What are the possible fates of the universe? Which do we believe will happen? Why?
The universe could recollapse in a “big crunch,” expand forever at a slower and slower rate, expand at a constant rate, or expand at an accelerating rate. These patterns are called recollapsing, critical, coasting, and accelerating, respectively. (Remember figure 16.15?) We know that the universe is currently expanding. There doesn’t seem to be enough matter in the universe to cause a high enough gravitational force to stop the expansion. Oddly enough, observations of white dwarfs show that the expansion of the universe is speeding up. We explain this by saying that dark energy is pushing the galaxies in the universe apart.
What is the big bang theory? What evidence is there for it? The solar system has a "center," protogalactic clouds have a "center" (or nucleus), and galaxies also have a center. Then why don't we believe that the universe has a center?
What eras has the universe been through? What happened in each of these eras (forces dividing, matter changing forms, universe expanding)?
The universe began at a much higher temperature, and as it cooled it allowed certain things to take place.The following lists the eras in order and describes what happened during these eras
Planck era -We don’t really know what was happening, but we think the four forces may have all behaved as one at this time.
GUT era – At the start of thisera, it has become warm enough forgravity to become distinct. The other three forces, still combined, are called grand unified theories. The GUT eraended when it got warm enough for the strong force became a distinct force, perhaps leading to the rapid expansion called inflation.
Electroweak era – The two forces that were still combined are known as the electroweak force. Electromagnetism and the weak force became distinct at the end of the electroweak era.
Particle era – During this era, the amount of matter and antimatter was roughly equal. This era ended when matter particles annihilated all the antimatter particles.
Era of nucleosynthesis – This era began when matter annihilated the remaining antimatter. It ended when fusion of protons and neutrons into helium ceased.
Era of nuclei – During this era, it is cool enough for nuclei to form. This era ended whenhydrogen nuclei captured all the free electrons, forming hydrogen atoms. It was at the end of this era that the background radiation currently known as the cosmic microwave background was released.
Era of atoms – At the end of this era, galaxies began to form.
Era of galaxies – This is the current era.
Also not that, as the universe cooled it expanded. We can see much of the above information illustrated in figure 17.3.
How does inflation explain the things that the Big Bang theory doesn’t answer?
Most important material from exam one:
Seasons – what cause the seasons?
The axis tilt causes seasons. The tilt primarily affects the concentration of the rays of sunlight. During the summer, the rays of light are more concentrated, producing more heat. During the winter, the rays are less concentrated, so it is not as warm.
Eclipses and phases of the moon – What cause eclipses? Lunar phases? What is the difference between a solar and lunar eclipse?
Lunar eclipses are caused by shadows - the Earth’s shadow covers the Moon when the Earth, Sun and Moon line up. This doesn’t happen every month due to the tilt of the Moon’s orbit around the Earth with respect to the Earth’s orbit around the Sun. Lunar phases are caused by the changing position of the Moon with respect to the Sun.
Could you go over the phases of the moon? Could you explain again the moon rise and set times? I never completely understood how drawing it out showed what phase the moon was in.
Geocentric vs Heliocentric models of the solar system – what arguments supported each? Who did we study that supported geocentric? Heliocentric?
Kepler’s and Newton’s laws
Gravity and the inverse square law – How does your weight change if the mass and radius of the planet changes?
Tides – what causes?
Solar nebula theory of solar system formation – asked on first exam, probably won’t show up again as a short answer question
What are the characteristics of the terrestrial planets? The jovian planets? Be able to compare one planet to another, especially comparing other terrestrial planets to Earth.
How are extrasolar planets detected?
How can planet surfaces be reshaped? How does an atmosphere influence what happens to a planet?
Most important material from exam two:
Be able to describe how a comet gets its tail and why there are annual meteor showers.
Know what is “special” about Io, Europa, Ganymede, Callisto, Titan, Triton, Miranda.
H-R diagrams, especially where different types of stars are located and how a given star's position on the diagram will change over time
Relationship between comets and meteor showers
The sun's structure and methods of energy production via fusion
Emission and absorption spectra, thermal emission; specifically, how they are produced and the information they can tell us about a star
Doppler shift, especially how it applies to binary stars and extrasolar
planets
Star formation
Life stages of a low mass star, including the final stages (planetary
nebula and white dwarf)
Can you explain why white dwarfs have a mass limit?
Life stages of a high mass star, including final stages (supernova leaving either neutron star or black hole behind)
What are black holes? How do they work?
Othe Emailed Questions:
Since Io has volcanoes and resurfaces itself often, is the temperature on the planet still much too cold for life? I understand that the heat from the sun is not enough, but since tidal heating causes the core to stay warm, does the surface stay warm or are there just extremes in temperature (from the surface to the molten rock)?
How do large planets migrate closer to a star, and why don't they fall apart once they pass threw the frost line?
If the universe is expanding and it does not have edges, what will happen when it expands into the space of another separate universe?
When there is an explosion in space, can we hear it?