Physics 2083 - Complete Study Guide

Updated through Wednesday, May 5. Current study questions can be found here.

Explain how we can use parallax to find the distances to stars. How does the distance depend on the parallax angle? If star A has a parallax angle 10 times smaller than star B, then how does the distance to star A compare with the distance to star B?

Why can we only use the parallax technique to find the distances to stars fairly close (about 100 parsecs) to the Sun?

How were parallax observations used to "disprove" the heliocentric model of the solar system by contemporaries of Kepler and Copernicus? Why were these observations invalid?

Given the equation of the inverse square law, be able to answer questions such as "Star A has twice the absolute luminosity of star B and is twice as far away as star B. How does the apparent luminosity of star A compare with that of star B?"

What are the two main functions of the telescope that we have discussed in class? How and why does increased aperture diameter affect these two things?

Explain the concept of frequency and how it relates to wavelength. Know how the wavelength, speed and frequency of light are related, and be able to identify from two wave patterns which has the higher/lower wavelength/frequency.

Explain how energy and wavelength are related for light. An object with more energy to emit will tend to emit light that has higher energy. Use this fact to explain why hotter objects tend to be blue.

Explain qualitatively and graphically why the wavelength of maximum emission is inversely proportional to the temperature of an object emitting continuous radiation.

Given that light is more easily scattered at short wavelengths, use this fact to explain why the sky is blue and why the Sun appears red at sunset. On a spectrum graph, show how the distribution of light from the Sun changes between noon and sunset.

How does scattering affect the apparent surface temperature of the star that is derived from the color of the star? Explain how your temperature estimate will be wrong (too high or too low?) if you fail to take scattering effects into account.

Be sure you understand how electrons change energy levels within an atom. Given a simple energy level diagram, be able to identify which electron transition corresponds to the longest/shortest wavelength photon emission/absorption.

Explain how absorption/emission line spectra are generated with the help of a simple diagram.

How can we use line spectra to determine the composition of a cloud of gas?

Explain how we can calculate the total luminosity (amount of energy given off per second) of the Sun just by knowing how much energy per second falls on a square meter of the Earth's surface.

Explain how did scientists discount the possibility of chemical or gravitational forces as the source of the Sun's energy.

What is the source of energy in the proton-proton chain? Why does nuclear fusion require high temperatures and densities?

Explain how (and why) spectral line strengths are related to the abundance of the particular atom that is creating the spectral line.

Use a simple diagram to show how the bulk motion of an object results in spectral line shifts due to the Doppler effect. Be sure you understand the difference between radial motion (which causes shifts) and transverse motion (which does not cause shifts).

Explain how and why internal motions in a cloud of gas are related to the line widths. Understand why larger motions lead to larger line widths.

Explain why temperature and density of an object are all considered to be sources of internal motion, leading to broader lines.

Explain how rotational motion of a cloud of gas might lead to line broadening. Would a rotating star or cloud exhibit rotational line broadening if viewed pole-on? Explain. (TQ #2)

What is ionization? How are the rules of light absorption by electrons different for simple absorption compared to the process of ionization? Explain.

Is it possible for light to pass through a cloud of Hydrogen without suffering the effects of absorption? Explain the circumstances under which this might occur. (TQ #1)

Explain why the inner envelope of the Sun transports energy radiatively from the core outward toward the surface (instead of transporting the energy mechanically).

Use the temperature structure of the outer layers of the Sun to explain why the chromosphere is thought to be the source of most of the absorption lines in the Sun's spectrum.

What is the physical mechanism that we think causes the gas in the corona to be so hot?

Explain how observations of emission line widths from coronal gases could confirm the theory of the temperature structure of the corona stated in question 26.

Explain how observations of different ionization species at various altitudes above the photosphere could confirm the temperature structure of the corona stated in question 26.

Explain how observations of limb darkening confirms the theoretical model of the photosphere that states temperatures in the photosphere increase the deeper one observes below the outer edge of the photosphere.

The solar corona has temperatures at least as high as the core of the Sun, so why doesn't nuclear fusion occur in the corona? (TQ #3)

Briefly explain the "solar neutrino problem". Why can we use neutrinos to observe the Sun's core instead of just using light, and why are neutrinos so hard to observe?

Given the conflict of observation with theory, why do Astronomers still believe that the proton-proton chain is still the fundamental source of the Sun's energy?

Given the equation for the absolute luminosity of a star, describe how we can use the radius (size) of a star along with the star's surface temperature to determine the distance to that star.

Explain why stars with the same atmospheric composition but different surface temperatures will have spectra showing different patterns of absorption lines.

Suppose a star has no lines of a particular element, like Calcium. How can we tell whether this is due to a lack of Calcium or a stellar temperature that is too high or too low for Calcium lines?

How and why do spectral line widths correlate with stellar sizes? (TQ #4)

Be able to answer questions such as, "Star X is twice as large as the Sun and has a surface temperature of 3000 K (the Sun's surface temperature is 6000 K). How does the absolute luminosity of star X compare to that of the Sun?" Be able to answer these kinds of questions quantitatively and graphically (plotting the position of X on a typical H-R Diagram relative to the Sun).

Use the Copernican Principle to help explain why a collection of the stars nearest to the Sun is more representative than a collection of stars with the largest apparent brightness.

Describe how Astronomers can use either spectral line observations or light curves to deduce that a given star on the sky is actually some kind of eclipsing binary system.

Understand what a light curve is and what causes the dips in the curve for an eclipsing binary system. Be able to identify the period of a binary system from a light curve.

Given the equation of orbital velocity, how could you deduce the mass of the Sun by knowing only Earth's orbital distance from the Sun and Earth's orbital period (just describe how it is done; you do not need to actually repeat the calculation).

Be able to manipulate the two equations of orbits in order to answer questions such as "The orbital distance for binary system A is the same as the orbital distance for binary system B, but the companion star in binary system A has only half the period of system B. Which system's companion star has a higher orbital velocity? Which system's central star has a higher mass?"

Explain how we find the masses for central stars in edge-on binary systems. What two quantities are easy to measure directly, and how do we use that information to find the central star's mass?

Explain how we find the masses for central stars in face-on binary systems. What two quantities are easy to measure directly? Why is it that we can only easily find the orbital distance for such a system if that system is relatively close to us?

By determining the mass of several hundred stars in binary systems, Astronomers have found that there is a correlation between mass and absolute luminosity. Explain why theorists believe this is true.

Explain why a star's mass is inversely proportional to its lifetime, both quantitatively (with equations) and qualitatively (with words).

Given the relationship between mass and absolute luminosity, be able to answer questions such as, "Star X is twice as massive as our Sun. How does the luminosity of star X compare with our Sun? How does the lifetime of star X compare with our Sun?"

Be able to discuss the concept of ISM extinction of starlight. If, due to ISM extinction, a star appears dimmer than it would otherwise, how does that affect our estimate of the distance?

Just by looking at star counts on the sky (e.g. page 207 in the text), explain how we can tell where an interstellar cloud is found.

Be able to discuss the concept of ISM reddening of starlight. If, due to ISM reddening, a star appears redder than it would otherwise, how does that affect our estimate of the distance?

How and why do ISM absorption lines differ from stellar absorption lines? Given a spectrum with both kinds of lines, be able to identify which set belongs to the ISM and which belongs to the star (as well as relative Doppler shifts due to bulk motion of either or both).

Hydrogen's 21-cm radiation is considered to be a "forbidden line" because we can't reproduce it in laboratory conditions. Why is this emission line only seen in interstellar Hydrogen?

We probably wouldn't be able to detect the presence of most of the cold Hydrogen gas in our galaxy if it weren't for the fine splitting of the first energy level that corresponds to a wavelength of 21 cm. Explain why.

Explain why it is easiest to see the dust component of the ISM by observing at infrared wavelengths.

Explain the concept of pressure equilibrium in the ISM. Understand what the heliosphere is, and explain how this boundary represents an example of pressure equilibrium.

Explain what happens to the temperature and density of an interstellar cloud as it collapses due to its own self-gravity. Explain what happens to the internal (outward-pushing) pressure of the cloud during a collapse.

Why is there a minimum mass for stars?

What is hydrostatic equilibrium? Use the HSE cycle to describe what happens to a star after its gravity is somehow increased (by adding mass) or after its core temperature is somehow increased.

Describe how the core of a star changes during the course of its main sequence lifetime (a few simple diagrams would help). Use the concept of differentiation to explain why the inert Helium "ash" collects at the center of the star's core.

What happens to the overall size of a star near the end of its main sequence lifetime? Use HSE to help explain your answer.

After the main sequence lifetime of a star ends, what happens to the star's size, core temperature and core density? Again, use HSE to help explain your answer.

Why is the minimum stellar mass for Helium core burning larger than the minimum stellar mass for Hydrogen core burning? As part of your answer, explain three reasons why Helium fusion requires higher core temperatures and higher core densities than Hydrogen fusion.

Why do stellar colors change to redder shades when they expand into giants near the end of the main sequence?

After Helium fusion begins, Hydrogen in a shell around the core ignites to fuse into Helium. This Hydrogen was part of the envelope during the main sequence. Why is it now a part of the core?

Explain the pulsations of a variable star in the context of the Hydrostatic Equilibrium cycle. When a star is at its smallest, explain what is happening to the outward-pushing pressure and the inward-pushing gravity. Likewise, when a star is at its largest.

Explain how to calibrate the Cepheid Period-Luminosity relationship. Why is it best to calibrate using a sample of nearby Cepheids? Because we're limited to using nearby Cepheids, is our sample likely to be representative? Explain

Explain how to the use the P-L relationship to find the distance to a galaxy in which a Cepheid is located. Given a couple of Cepheid light curves, be able to tell which has the longer period and therefore, the higher absolute luminosity.

Why do stars have a maximum possible mass? (TQ #1)

Explain how to find the distance to a planetary nebula with the following pieces of information: angular size, time since the original explosion, and the velocity of the expanding gaseous shell. (TQ #2)

Explain what a planetary nebula is. Why do stars with less than about four solar masses undergo this experience?

What is different about iron fusion as opposed to the fusion of lighter elements? Explain how this difference leads to the collapse of the star's core.

Every atom in your body with an atomic number higher than that of Helium comes from the cycle of stellar evolution, and every atom with an atomic number higher than that of iron comes from a supernova explosion. Explain why we can make these statements with confidence.

Use conservation of angular momentum (mass*size*rotation speed) to explain why neutron stars spin so quickly.

Explain how the gravity of black holes is similar to and different from the gravity of ordinary matter. For example, if the Sun were replaced by a one solar mass black hole, would the Earth's orbit change? Also, is it possible for the gravitational force of a one solar mass black hole to be greater than the Sun? Explain each answer. (TQ #4)

Explain how to use binary star system techniques to find the mass of a black hole around which a companion star is orbiting.

When a black hole is part of a binary system, the region around a black hole contains an accretion disk of material spiraling into the black hole. Explain how this accretion disk radiates energy.

Explain how we can "prove" the existence of black holes indirectly.

Why does a representative sample of stars contain mostly stars that are burning Hydrogen in their cores (in other words, mostly stars that lie along the main sequence strip of the H-R Diagram)?

Why does a representative sample of stars contain more stars with absolute luminosities less than the Sun compared to stars with absolute luminosities greater than the Sun?

Explain the Malmquist bias. Why are stars with low absolute luminosities typically excluded from samples that are limited by apparent luminosity?

How and why has the metallicity of the ISM changed over time?

Explain the relationship between age and metallicity for stars. (TQ #3)

Describe how the H-R Diagram for stars in a globular cluster evolves over time. Given a couple of different cluster diagrams, be able to identify and explain which cluster is older and which has a higher metallicity.

Discuss how astronomers use the main sequence turnoff point of a globular cluster to estimate the age of the cluster.

Discuss how astronomers find the distance to a globular cluster based on the properties of a star at the main sequence turnoff point.

Explain how the distribution of globular clusters on the sky is seen as evidence that our Sun is not at the center of our galaxy. How are the distances to globular clusters used to estimate the size of our galaxy?

Why are star-forming regions usually blue? Why are regions devoid of gas and dust (e.g. the bulge and the halo of our galaxy) typically red?

If you see a blue star, can you say conclusively that it is young (that it formed recently)? Explain. If you see a red star, can you say conclusively that it is old? Explain.

Use the rotation of the galaxy and the concept of centrifugal force to explain why the galaxy collapsed into a disk shape.

Use the fact that there are no zero-metallicity stars in the disk to argue that there must have been a generation of star formation either before or during the collapse of the galaxy into a disk.

Explain why the gas and dust originally in the halo of the galaxy is now a part of the disk while stars originally in the halo of the galaxy are still there.

Describe how astronomers discovered that there is a super-massive black hole at the center of our galaxy (and most if not all other galaxies).

Why are the spiral arms so bright and blue compared to the rest of the material in the disk?

What is a Keplerian rotation curve? How does the rotation curve of our galaxy differ from a Keplerian curve, and why is this considered to be evidence for the existence of dark matter?

Given the equation of orbital velocity, explain how we can determine the amount of dark matter in the galaxy. How is the "M" in the equation of orbital velocity different from the visible mass of the galaxy, obtained by counting up all the stars, gas and dust?

Why are WIMPs (Weakly Interacting Massive Particles) so difficult to study? Even though the mass of a WIMP may be extremely small (even compared to the mass of an atom), they still may constitute a large part of the dark matter. Explain why.

Explain how MACHOs use gravitational lensing to make starlight appear brighter for a short period of time. A simple diagram would help.

How and why does the light curve of a MACHO-lensed star differ from the light curve of a Cepheid variable or a star that periodically brightens or dims due to instability?

The colors of MACHO-lensed stars typically do not change during a brightening while other variable stars change colors when they change brightnesses. Explain the difference.

Summarize the scientific results of the two photographs of page 343 of the text, in which astronomers predicted and counted the number of very low mass stars in a representative region. In other words, explain how much these stars contribute to the dark matter in the Milky Way, and how we know this.

Black holes are also thought to be a candidate for the dark matter. Why can we use x-ray observations to detect black holes? If black holes don't have accompanying accretion disks, why can't we detect them easily via lensing like MACHOs?

Explain the concept of lookback time. Why is lookback time only significant for objects that are very far away (more than a few hundred million light years)?

The average size of elliptical galaxies has been found to be inversely proportional to their distances from Earth (when we look at very large, cosmological-scale distances). If this is so, explain how the average size of elliptical galaxies has changed as the Universe has gotten older.

The ratio of spirals to elliptical galaxies has been found to be inversely proportional to the distance from Earth. Based on this, explain how the number of elliptical galaxies in the Universe has changed as the Universe has gotten older.

The observation that elliptical galaxies tend to be located near the centers of galactic clusters is also seen as evidence that ellipticals are formed via galaxy mergers. Explain why the observation leads to this conclusion.

How does the "merger hypothesis" for the origin of elliptical galaxies explain why elliptical galaxies have very small amounts of gas and dust today? Why does a small amount of gas and dust tend to imply that an elliptical galaxy will have an overall red color?

If we measure the radial velocity of a galaxy that is part of a galaxy cluster, explain how this velocity compares to the true velocity of the galaxy.

Why do we assume that the average velocity of galaxies in a cluster must be less than or equal to the escape velocity of the cluster?

Given the equation of escape velocity and knowledge about the angular size and distance to the galaxy cluster, explain how we can use the average galaxy velocity to deduce the existence of dark matter in a galaxy cluster.

How would you expect the metallicity of the ISM of galaxies (seen by way of their absorption lines in quasar spectra) to correlate with redshift? Explain. (TQ #2)

Cepheid variables are very reliable distance indicators. Unfortunately, we can only use them to find distances to galaxies that are very close to our own. Why are we unable to utilize the Cepheid P-L relation for galaxies very far away?

Explain how the standard ruler method of distance determination works, using a simple diagram to help. How does the angular size of an object correlate with its distance from Earth?

Why is the standard ruler method not a reliable method of distance determination? Is there any way to improve this reliability?

Explain how the standard candle method of distance determination works. Assuming all galaxies have the same absolute luminosity, how would the apparent luminosity of galaxies correlate with distance from Earth?

Why is the standard candle method (using galaxies as standards) not a reliable distance indicator? Is there any way to improve this reliability?

Why are the brightest globular clusters in a given galaxy a better "candle" for use with the standard candle method? Why are supernovae peak brightnesses even better, besides the fact that they are often as bright as entire galaxies?

Given a graph that shows the relationship between absolute luminosity and rotation velocity for a galaxy, explain how to use the Tully-Fisher relationship to determine the distance to a galaxy.

Why can the TF method only be used on relatively nearby galaxies?

Given a Hubble diagram that shows the relationship between distance and redshift, explain how to use the redshift of a galaxy's absorption lines to determine the distance to that galaxy.

How would the Hubble diagram look in a Universe that weren't expanding away from us? (TQ #1)

Why do some galaxies close to use have radial velocity components that are in the direction toward Earth?

How do we know that quasars are so far away? Explain.

How do we know that quasars are much brighter than most galaxies? Explain.

How do we know that quasars are much smaller than galaxies? Explain the relationship between size and time variability.

Be able to graph a simple Hubble's Law analogy, like the car race described in class. Be able to calculate the "age" of any car race based on your graph.

How does the slope of the Hubble relation change as the race time (or the age of the Universe, by analogy) grow longer? Explain with the help of a simple graph.

Why does the Hubble relation imply that the Universe has a finite age (in other words, why does it imply some kind of initial "big bang")?

We observe that virtually every galaxy in the sky seems to be moving away from our location in the Milky Way galaxy. Explain why this is not a violation of the Copernican Principle.

Define the critical density of the Universe. Explain how and why the relationship between the observed density of the Universe and the critical density of the Universe will dictate the ultimate fate of the Universe (i.e. whether it will collapse or expand forever).

Why is the Anthropic Principle compelling philosophically (what makes us think it may be true?) Use observed vs critical density as an example to argue this (i.e what would happen if the observed density were very different from the critical density?)

Explain two different versions of the Anthropic Principle in your own words and justify your belief in one or the other. (TQ #4)

Explain how the observation that the night sky is dark leads us to believe that the Universe is finite in some way (start by assuming that the Universe is infinite in space and time and explain how it would differ from what we see today).

Explain how the observation that the night sky is dark also leads us to the conclusion that the Universe is expanding.

If the Universe is slowing down due to gravity, how would that change the appearance of the Hubble relation? Explain.

How does the Hubble relation change if the Universe is accelerating? What does this imply about the age of the Universe compared to the case where gravity slows the Universe down? Explain.

Explain how the idea of an accelerating Universe helps solve the "age discrepancy", which is the idea that, under the old theory (that the Universe was slowing down), the ages of the oldest globular clusters were apparently older than the age of the Universe.

What is the Microwave Background Radiation? Explain how it leads us to believe that the early Universe was smaller, hot and dense.

Why does the Microwave Background look bluer (hotter) in one direction and cooler (redder) in the opposite direction? (TQ #3)

Explain why observations of a lumpy structure of galaxies in the Universe as a whole led theorists to predict the presence of similar "lumps" in the Microwave Background Radiation (which were subsequently observed by later, more sensitive observations).

Conditions in the early Universe were ripe for nucleosynthesis to occur prior to a time when the Universe was about 3 minutes old. Why wasn't nucleosynthesis an important process in the Universe prior to a time of about 1 second? Why was it limited to only occuring between times of about 1 second and 3 minutes?

How does the Big Bang theory predict the current composition of the Universe? In other words, how does it explain the fact that the Universe is mostly Hydrogen, instead of something like Carbon?

Given problems with the Big Bang theory like the age discrepancy, why don't scientists abandon the theory as incompatible with observations, like the scientific method says we should?

Explain the importance of the observation that there seems to be an upper limit to the ages of all things in the Universe. What can we conclude from this?

What is the horizon problem? Explain why the observation of this problem requires a mechanism like inflation.

Explain why the number of intelligent, communicative civilizations in our galaxy (the "N" of the Drake equation) depends on L, the average lifetime of an intelligent civilization.

Why do we think that searching for and communicating via radio signals is a more effective way to find out about the possibility of extraterrestrial life as opposed to actually visiting other stellar systems (you don't need to go through all the math, just summarize the reason).

Explain the concept of bandwidth in broadcasting. How does the bandwidth of a signal correlate with the amount of energy it takes to broadcast the signal? How does it correlate with the number of possible channels one would have to search to find the signal?

Explain the tradeoff an observer must make between sensitivity and sky coverage when trying to detect signals. If an observer takes the time to observe with very high sensitivity in a given direction, how does that affect the amount of time it will take to cover the whole sky?