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Thursday 27th April:

Without the extension known as inflation, the standard Big Bang model fails to explain:
  A: Hubble's Law
  B: The existence of the cosmic microwave background
  C: The uniformity of the Universe on very large scales (correct)
  D: The abundances of the light elements

Tuesday 25th April:

Which of these elements do we think was made mostly in the Big Bang rather than in the cores of stars?
  A: helium (correct)
  B: carbon
  C: oxygen
  D: iron
  E: uranium

Matter can be created in the very early Universe because:
  A: Of the very rapid expansion that we call inflation
  B: The Universe at early times was dominated by radiation
  C: It was hot enough that energetic photons could transform into matter (correct)
  D: Dark energy was the dominant force

Tuesday 18th April:

Two galaxy clusters are studied. Cluster A has typical galaxy velocities of 600 km/s, cluster B 1000 km/s. Which is most likely?
  A: Cluster A has more galaxies than cluster B
  B: Cluster A is more massive than cluster B
  C: Cluster B galaxies are more likely to be spirals
  D: Gas between galaxies in cluster A will have a lower temperature than gas in cluster B (correct)
Lower galaxy velocities and lower gas temperatures are both consequences of lower cluster masses

Evidence that the expansion of the Universe may now be accelerating (a phenomenon thought to be caused by `dark energy') comes from:
  A: Observations of the cosmic microwave background
  B: Observations of distant supernovae (correct)
  C: Observations of the rotation of galaxies
  D: Observations of galaxy clusters

Dark matter is best described as:
  A: A mysterious substance that appears to be causing the expansion of the Universe to accelerate
  B: Very low mass stars that emit little light
  C: Black holes within the halo of the Milky Way
  D: Unseen mass whose gravity governs the observed motion of stars and gas clouds within galaxies (correct)
Answer A describes dark energy

Thursday 13th April:

What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk (recall that the rotation curve for the Milky Way is roughly flat)?
  A: Its mass is concentrated at the center
  B: It rotates like the Solar System
  C: Its especially rich in dark matter (correct)
  D: It's just like the Milky Way

Tuesday 11th April:

If the attractive force of gravity between the galaxies is causing the expansion of the Universe to slow down over time, the true age of the Universe would be:
  A: less than 13.6 billion years (correct)
  B: 13.6 billion years
  C: greater than 13.6 billion years
Some explanation... 13.6 billion years is the number we get by assuming that age = 1 / Hubble's constant. This effectively assumes that the expansion rate has remained constant over time. If, instead, the expansion was faster in the past (because gravity is slowing it down), then the Universe would have reached its present size in less time than suggested by this estimate. So A is the right answer. However (!) in the real Universe there's evidence for dark energy, which is now speeding up the expansion. So although gravity has slowed down the expansion, the combined effects of gravity and dark energy cancel in the age calculation so that the real age of the Universe is currently thought to be 13.6 billion years.

We see a car 30 miles away (we have good vision!) that is receding at 60 miles per hour. How long ago did it depart?
  A: 30 minutes ago (correct)
  B: 60 minutes ago
  C: 2 hours ago

What would be the surface brightness (i.e. the brightness per unit area) of the Sun if it were twice as far away?
  A: one quarter as bright
  B: one half as bright
  C: the same brightness (correct)
  D: twice as bright
  E: four times as bright

Hubble's Law cannot be used to estimate the distance to very nearby galaxies because:
  A: we cannot measure the velocities of nearby galaxies accurately enough
  B: we do not know the Hubble constant accurately
  C: the velocities of nearby galaxies are dominated by rotation
  D: nearby galaxies are gravitationally bound to the Milky Way and so do not obey Hubble's Law (correct)

Thursday 6th April:

You observe two supernovae (of the type that are good standard candles). Supernova 2006a is 4 times brighter (in apparent brightness) than supernova 2003f. You conclude:
  A: 2006a is 4 times closer than 2003f
  B: 2006a is at half the distance of 2003f (correct)
  C: 2006a is at twice the distance of 2003f
  D: 2006a is at 4 times the distance of 2003f
  E: 2006a is at 16 times the distance of 2003f

Hubble's Law tells us that:
  A: more distant galaxies are moving away faster than nearby ones (correct)
  B: more distant galaxies are moving away slower than nearby ones
  C: light from distant glaxies is blueshifted more than light from nearby galaxies
  D: galaxies fall into two main classes, spirals and ellipticals
  E: galaxies fall into two main classes, spirals and irregulars

Tuesday 14th March:

Gamma-ray bursts are known to be extremely powerful explosive events because:
  A: they originate from neutron stars within the Milky Way
  B: they signal the birth of black holes
  C: gamma rays are the most energetic photons
  D: their short duration implies very high energies
  E: they are known to be both very bright and very distant (correct)

We can distinguish between neutron stars and black holes in binary systems such as Cygnus X-1 because:
  A: X-ray emission from neutron stars is more luminous than from black holes
  B: the compact star is too massive to be a neutron star in some cases (correct)
  C: We have proof of the existence of an event horizon, which only black holes have
  D: radio pulses indicate when we have a neutron star

Thursday 9th March:

What would be the mass (roughly) of a thimble of neutron star stuff (recall the density of a neutron star is about 10^{17} kg per cubic meter, and one cm is 0.01 meter)?
  A: 10^{9} kg
  B: 10^{11} kg (correct)
  C: 10^{13} kg
  D: 10^{15} kg
  E: 10^{17} kg
Calculation is: one cubic cm (roughly the volume of a thimble) is one million-eth of a cubic meter (i.e. there are a million - 100 x 100 x 100 - cubic cm in a cubic m). A cubic meter of neutron star matter would have a mass of 10^{17} kg, so a cubic cm would be a million times less which is 10^{11} kg.

Would you expect the red giants we see in the sky with the naked eye to be (on average):
  A: closer to us
  B: further away from us (correct)
  C: at the same distance the average star of the same mass while it is on the main sequence.

Tuesday 7th March:

Today's clicker questions were graphical - points to remember are:
  In an eccentric orbit, the speed is fastest at the point of closest approach
  In any orbit, the stars orbit around their center of mass (which is closest to the more massive object of the pair)

Thursday 2nd March:

The Galactic Center is difficult to study in visible light because:
  A: of the distance
  B: stars along the line of sight obscure our view
  C: of obscuration by interstellar dust (correct)
  D: stars near the Galactic Center do not emit visible light
  E: it's only visible from the Southern hemisphere

A pulsar is:
  A: a star subject to pulsational instability
  B: a compact object known as a white dwarf
  C: a type of black hole
  D: a neutron star that emits radio pulses (correct)
  E: a binary containing two black holes

Tuesday 28th February:

What is the most fundamental reason why you should be suspicious of our Newtonian reasoning (in which we derived the escape velocity) when applied to black holes?
  A: no object can be small and dense enough to meet the criteria for forming a black hole
  B: photons of different colors have different energies
  C: light is always measured to travel at c, so photons don't slow down like material objects (correct)

Which of the following properties can be measured for a black hole?
  A: its mass (correct)
  B: its chemical composition
  C: its age

Thursday 23rd February:

Astronauts in the Space Shuttle feel weightless because:
  A: Of the vacuum in space
  B: There's no gravity in space
  C: The Space Shuttle is free falling (correct)

If you stand on weighing scales as an elevator starts to accelerate downward, your weight:
  A: Decreases (correct)
  B: Increases
  C: Stays the same
If in doubt, try this yourself!

Which of the following is not a prediction of General Relativity?
  A: Time dilation in a strong gravitational field
  B: Light bending
  C: Length contraction (correct)
  D: Black holes
  E: The idea that mass curves spacetime

Tuesday 21st February:

Collisions of cosmic rays with atoms in the upper atmosphere produce unstable particles with a lifetime of only one microsecond. From the point of view of the particles (i.e. in their inertial frame), which relativistic effect allows them to reach the ground?
  A: time dilation, so they live longer
  B: length contraction - the atmosphere appears thinner (correct)

Thursday 9th February:

Every star similar to the Sun eventually becomes a giant. So why does an H-R diagram show many more stars on the main sequence than giants?
  A: red giants are faint, so hard to see
  B: red giants are cooler than other stars
  C: the red giant phase is short - have to be lucky to catch a star as a giant (correct)
  D: red giants are more luminous than main sequence stars of the same mass
  E: our galaxy isn't old eneough yet for Solar mass stars to have become giants

If we were somehow able to quadruple (x4) our distance from the star Vega, how much dimmer would it appear?
  A: one half as bright as originally
  B: one fourth as bright
  C: one eighth as bright
  D: one sixteenth as bright (correct)
  E: one sixty-fourth as bright

What types of star are most useful for the observational determination of stellar masses?
  A: white dwarfs
  B: red giants
  C: spectroscopic binaries
  D: pulsating stars
  E: eclipsing binaries (correct)

The most important factor determining the luminosity of a star is its:
  A: chemical composition
  B: mass (correct)
  C: rotation rate

Tuesday 7th February:

Light that has a higher frequency has:
  A: a shorter wavelength, higher energy photons (correct)
  B: a shorter wavelength, lower energy photons
  C: a longer wavelength, higher energy photons
  D: a longer wavelenegth, lower energy photons
  E: a higher speed

How many different energy photons can an atom emit considering only transitions between the ground state and three excited states?
  A: 1
  B: 3
  C: 4
  D: 6 (correct)
  E: 10
This was a counting argument - important point to remember is that each transition corresponds to a different wavelength spectral line, and that the pattern of those lines is a unique signature of the chemical element.

Thursday 2nd February:

Light and other forms of electromagnetic radiation are understood to be:
  A: waves which travel at speed c in a vacuum
  B: particles called photons
  C: have both wave and particle properties (correct)

A spectrum of a star does not tell us (directly anyway) about the star's
  A: age (correct)
  B: surface temperature
  C: surface or atmospheric composition
  D: velocity relative to us
It's important to understand how light can tell us about B, C and D - this will be part of lecture #7. Note that a spectrum can be analyzed to give clues to a star's age - but it's difficult and indirect.

Tuesday 31st January:

Imagine an initially stable star made out of a material whose pressure does not depend upon temperature. What would happen if nuclear reactions got started in such a star?
  A: steady nuclear burning, as in the Sun
  B: collapse of the star under gravity
  C: slow contraction producing heat
  D: slow expansion
  E: the star would explode (correct)
This is not as crazy as it sounds - what we call `degenerate material' has exactly this property and explosions called white dwarf supernovae are thought to result from the initiation of nuclear fusion in such stars. The important point is the one we mentioned in class - it is the ability of the Sun's core to expand and cool if the nuclear reaction rate increases that keeps the Sun in a stable state.

The most dangerous (to us) radiation is that which:
  A: can penetrate a distance of 1 micron through water
  B : can penetrate a distance of 10cm through water (correct)
  C: can penetrate the whole Earth (neutrinos)

Thursday 26th January:

Why does the Sun shine?
  A: because it's burning
  B: nuclear fission
  C: because it's contracting
  D: nuclear fusion (correct)
  E: radioactive decay

The rate of nuclear fusion in the core of the Sun:
  A: varies on an approximately 11 year cycle
  B: changes very slowly over billions of years (correct)
  C: varies strongly from day to day

Why do nuclear reactions (involving the nuclei of atoms) involve much larger energies than chemical reactions (involving only the electrons)?
  A: chemical reactions are slower than nuclear reactions
  B: the strong force binds nuclei together more tightly than electric forces bind electrons to the nucleus (correct)
  C: protons and neutrons are much more massive than electrons
  D: electrons have a negative charge

Tuesday 24th January:

Which of the following is not a form of energy?
  A: Kinetic
  B: Temperature (correct)
  C: Gravitational potential
  D: Mass
This question gave some trouble: the point to remember is that temperature is a measure of the average speed of atoms - the amount of thermal energy depends on the number of atoms as well as their temperature

Question 2 asked which gas was hotter - one with high density but slow particle velocities, or one with low density but high particle velocities. The latter possibility is correct.

Collision between nuclei become more violent when:
  A: the density is increased
  B: the density is decreased
  C: the temperature is increased (correct)
  D: the temperature is decreased

Thursday 19th January:

The Milky Way galaxy is made up of:
  A: The Sun, the planets, asteroids and comets
  B: About 100 billion stars, including the Sun (correct)
  C: The Local Group
Note: distinction between a galaxy and the Solar System

Most other galaxies are observed to be:
  A: Moving away from us (correct)
  B: Moving toward us
  C: Neither moving away nor toward us, on average

Who has to worry most about getting a high dose of radiation from Solar storms:
  A: A passnger on a transatlantic flight
  B: A passenger on a transpolar flight (correct)
  C: Sitting at home in Boulder
  D: A deep sea diver

Instructor: Phil Armitage (JILA A909, 303-492-7836, email