quarks to cosmos final practice problems / midterm exam questions

Let’s say there is a parallel universe to our universe, which has dark energy within it, but the value of the dark energy density is a factor of 100 lower than the dark energy density in our universe. Everything else is the same. Would observers in that universe make the same inference that we have in ours— specifically, would they have detected accelerated expansion of the universe?

A. Yes
B. No
C. Cannot determine from the given information
D. Do not pick this option

You have three universes, A, B, and C, and all universes have only matter and radiation contributing to the total density. All three universes have the same Hubble constant today. The densities of these universes, relative to the critical density, are Ω_A=0.5, Ω_B=0.7, Ω_C=0.9. Which one of these universes is the oldest?

A. A
B. B
C. C
D. all the same age

You take a spectrum of a galaxy, and find that the Sodium line, expected to be at 500 nanometers, is actually at 600 nanometers. What is the recessional velocity of this galaxy relative to the Milky Way?

A. 5.4x10^7 m/s

B. 6.6x10^7 m/s

C. 6.0x10^7 m/s

D. 6.6x10^8 m/s

You are in an elevator with no windows. You feel your own weight and can stand on the floor. You could be in one of two situations— at rest in a gravitational field, or being accelerated by a force like a rocket engine on the elevator. Can you tell the difference between these two situations?

A. You are in a gravitational field

B. You are not in a gravitational field, but rather being accelerated

C. I can make an experiment to determine which situation I am in.

D. You cannot determine which situation you are in.

Two twin astronauts leave Earth at the same time and visit two other planets in our solar system. One on Venus and one on Mars. They arrive back on Earth at the same time. Which twin is older when they arrive? Recall that the mass of Mars = 0.1 M_earth, and mass of Venus = 0.7 M_earth. Recall that the radius of Mars = 0.5 R_earth and radius of Venus = 1.0 R_earth.

A. The one who went to Mars is older

B. The one who went to Venus is older

C. They arrive at the same age

D. You cannot determine from the information

You have two atomic clocks, one on Earth and one on Mars. You synchronize the clocks and start them going. After the clock on Earth reads 1 hour, what does the clock on Mars read? Note that the mass of Mars is 1/10th that of earth, and the radius of Mars is 1/2 that of earth.

A. The same
B. Less than one hour
C. More than one hour
D. Cannot determine with given info

You take a spectrum of a galaxy, and determine its redshift to be z=0.01. How far away is this galaxy, assuming a Hubble constant of 70 km/s/Mpc?

A. 43 pc
B. 43 Mpc
C. 43,000 Mpc
D. 210,000 Mpc

The Andromeda galaxy, our closest galactic neighbor, has a negative recessional velocity, meaning is is moving toward us. This does not agree with Hubble’s law. What causes this? (Choose the best answer)

A. Andromeda is within the dark matter halo of the Milky Way
B. It is a measurement error
C. the gravitational pull of the Milky Way and Andromeda has overcome Hubble’s la
D. None of the answers are correct

I observe two spiral galaxies. Galaxy A has a rotational velocity of 100 km/s. Galaxy B has a rotational velocity of 200 km/s. The radii of the galaxies are the same. What is the ratio of the masses of the two galaxies?

A. galaxy B is 2 times more massive than galaxy A
B. galaxy B is 4 times more massive than galaxy A
C. galaxy A is 4 times more massive than galaxy B
D. galaxy A is 2 times more massive than galaxy B

Let’s say there is a universe that has a matter density of 40% of the critical density, a radiation density of 70% of the critical density and no dark energy. Does this universe expand forever?

A. It stops expanding at time=infinity
B. Yes
C. No
D. You cannot determine the answer from the given information

NASA needs to put a new satellite in orbit around the Sun. The satellite will go in the point between Jupiter and the Sun where the gravitational pulls of the Sun and Jupiter cancel each other out. The radius of Jupiter's orbit is 5 AU. The Sun is 1000 times more massive than Jupiter. Disregard the other planets in the solar system for this problem.

The radius of the satellite's orbit around the Sun will be:

A. Closer to Jupiter than to the Sun (>2.5 AU)

B. Closer to the Sun than to Jupiter (<2.5 AU)

C. Halfway between the Sun and Jupiter (=2.5 AU)

D. There is not enough information to solve the problem.

Which rung of the distance ladder allows us to measure distances to nearby galaxies? (le, the "middle rung" of the distance ladder.)

A. Supernovae
B. Parallax
C.Variable stars
D. None of the answers listed

I see two stars, both have the same red color. I measure the brightness of these stars to be the same. The stars are not moving with respect to me, the observer, I have no other information about these stars. I can conclude:

A. The stars are the same distance away from us.

B. The two stars are different distances away from us.

C. You cannot determine anything else from the given information.

D. Travis Kelce is the best tight end of all time

On earth, I weight 100 pounds. Then I go to the planet Zurkon. Zurkon has a mass that is four times that of the Earth, and a radius that is twice as large as the earth. On Zurkon, what do I weigh?

A. 200 pounds

B. 100 pounds

C. 50 pounds

D. 25 pounds

A 100 kg object is moving with a speed of 10 m/s. If a force of 10 N is applied to the object (in the oppositte direction as the motion), how much time will it take the object to stop moving (velocity=0 m/s)?

A. 10 seconds

B. 100 seconds

C. 200 seconds

D. 1000 seconds

I see two stars. Both have the same red color and are the same apparent brightness. Both are variable stars, meaning that they get periodically brighter and fainter in a repeated pattern. They have different periods of variability, though. One star varies with a period of 4 days, while the other varies with a period of 10 days. You can conclude:

A. The longer period star is farther away

B. The longer period star has a larger radius (on average).

C. The shorter period star is further away.

D. Both A and B

You observe three blackbodies. They are all spherical with the same surface area.

You measure their spectra. Object A has a spectrum that peaks in the ultraviolet. Object B has a spectrum that peaks in red light. Object C has a spectrum that peaks in the microwaves. Which object emits the most microwaves?

A. Object A

B. Object B

C. Object C

D. Not able to determine from the given information

You observe a globular cluster that was born about 6 billion years ago. You measure the temperatures and luminosities of all the stars and make an HR diagram from your data. Which part of the HR diagram will NOT have any stars in it?

A. The white dwarf region

B. The blue part of the main sequence

C. The giant region

D. All regions in the HR diagram will be populated with stars

You observe a cloud of gas. Off to the side of the cloud, outside the view of your telescope, there is a bright star. The spectrum you observe is:

A. An absorption line spectrum

B. An emission line spectrum

C. A continuum spectrum

D. You cannot determine from the given information

You observe a blue star and a red star. They have the same apparent brightness.

Through parallax, you determine that they are the same distance away from us. The blue star has a surface temperature of 10,000 K. The red star has a surface temperature of 5,000 K. What can you conclude about the two stars?

A. The two stars have the same surface area.

B. The two stars have different luminosities.

C. The red star has a surface area that is 16 times that of the blue star.

D. The red star has a surface area that is 8 times that of the blue star.

Let's say the mass of the sun was mysteriously reduced. In this scenario, the amount of energy being generated remains the same, however. What happens to the sun? Choose the best answer

A. The sun will collapse into a black hole

B. The core temperature goes down

C. The volume goes up

D. The volume goes down

You take a spectrum of a star. The star has strong Hydrogen lines, but they aren't exactly where you expect them to be. For example, the Hydrogen alpha line, which in the laboratory is at 656 nm, is observed to be at 660 nm. You can conclude:

A. You and the star are moving toward one another

B. You and the star are moving away from one another

C. There is no neutral Hydrogen in the atmosphere of the star

D. You cannot determine from the given information.

What takes more energy? For an electron in a Hydrogen atom to go from the ground away state (n=1) to the second excited state (n=3), or for an electron to start from n=3 and go all the way to n=100?

A. n=1 to n=3

B. n=3 to n=100

C. they require the same amount of energy.

D. you cannot determine from the given information

You observe a star that is in the middle of large cloud of fully ionized gas. Assume the star emits like a blackbody. What will the spectrum you take look like?

A. A continuous blackbody spectrum

B. An absorption line spectrum

C. An emission line spectrum.

D. You cannot determine from the given information.

How many photons with wavelength 500 nm (nanometers) does it take to equal the energy from one photon of 250 nm?

A. 1

B. 2

C. 5

D. you cannot determine from the given information

You observe two main sequence stars. They have the same colors (ie, the same temperature), and the same luminosity. From parallax, you know that one star is 3 times farther away than the other. Their relative apparent brightnesses are:

A. The closer one is 3 times brighter.

5

B. The farther one is 3 times brighter.

C. The closer one is 9 times brighter.

D. The farther one is 9 times brighter.

What will happen to the sun right when it runs out of Hydrogen fuel at its core?

Choose the best answer.

A. It will arrive on the main sequence.

B. It will become a white dwarf.

C. It will explode as a supernova.

D. It will leave the main sequence.

You see a blue star in the sky. You measure its parallax and determine it is quite close to us (within a few parsecs). You measure its brightness and also find that this value is well below average for the stars in the solar neighborhood. What are you looking at? (Choose the best answer).

A. A hot main sequence star.

B. A star on the supergiant branch.

C. A brown dwarf.

D. A white dwarf.

You observe a protostar that will eventually become a solar-like star on the main sequence. It is much brighter than the sun. Why?

A. It is much hotter than the sun.

B. It is much larger than the sun (by radius).

C. Both A and B

D. Neither A nor B

A car is moving and honking its horn. You know that the horn has an intrinsic frequency of 100 Hz, but you hear something with higher pitch, at 120 Hz. How fast (and in what direction) is car moving? Use 300 m/s for the speed of sound.

A. 20 m/s towards you.

B. 30 m/s away from you

C. 60 m/s away from you

D. 60 m/s toward you.