Exam 2

Question 1

Seismic ________ waves cannot propagate through the Earth’s core, while seismic ________ waves can.
a) shear; pressure
b) electromagnetic; shear
c) pressure; shear
d) tidal; pressure
e) shear; tidal

Answer 1

a) shear; pressure *

Question 2

The theoretical resolution of a telescope in arc-seconds is given by
a = 2.5 x 10^-5 (lambda / D)
where lambda is the wavelength of the radiation in Angstroms and D is the diameter of the primary mirror or lens in meters. If we ________ the value of lambda while leaving D fixed, then the resolution angle a is ________
a) double; halved.
b) tripled; halved.
c) tripled; doubled.
d) double; doubled.
e) cannot be determined from the information given

Answer 2

d) double; doubled. *

Question 3

The theoretical resolution in arc-seconds for a radio telescope with a diameter D = 10^4 meters observing at a wavelength of 10^8 Angstroms is ________. This telescope would make observations with a resolution of ________ if it were located on the Moon’s surface, and observations with a resolution of ________ if it were located on the Earth’s surface.

a) 0.05 arc-seconds; 0.05 arc-seconds; 1 arc-second
b) 0.25 arc-seconds; 0.25 arc-seconds; 1 arc-second
c) 12.5 arc-seconds; 12.5 arc-seconds; 1 arc-second
d) 12.5 arc-seconds; 0.25 arc-seconds; 1 arc-second
e) 0.25 arc-seconds; 0.25 arc-seconds; 0.25 arc-seconds

Answer 3

e) 0.25 arc-seconds; 0.25 arc-seconds; 0.25 arc-seconds *

Question 4

The theoretical resolution in arc-seconds for an optical telescope with a diameter D = 5 meters observing at a wavelength of 10^4 Angstroms is ________. This telescope would make observations with a resolution of ________ if it were located on the Moon’s surface, and observations with a resolution of ________ if it were located on the Earth’s surface.

a) 0.05 arc-seconds; 1 arc-second; 0.05 arc-seconds
b) 1 arc-second; 0.025 arc-seconds; 1 arc-second
c) 10 arc-seconds; 0.025 arc-seconds; 1 arc-second
d) 0.05 arc-seconds; 0.05 arc-seconds; 1 arc-second
e) 1 arc-second; 1 arc-second; 1 arc-second

Answer 4

d) 0.05 arc-seconds; 0.05 arc-seconds; 1 arc-second *

Question 5

Interferometry is the process that uses two or more small telescopes separated by a large distance to

a) improve the resolution to that of a telescope with a size equal to the distance between the telescopes.
b) measure the spectra of dim stars and planets.
c) improve the light gathering power to that of a telescope with a size equal to the distance between the telescopes.
d) improve the sensitivity of photographic plates.

Answer 5

a) improve the resolution to that of a telescope with a size equal to the distance between the telescopes. *

Question 6

Atmospheric blurring is due to

a) turbulent motions in the Earth’s atmosphere
b) radioactive decay
c) photon collisions
d) atomic annihilation

Answer 6

a) turbulent motions in the Earth’s atmosphere *

Question 7

If the concentration of CO2 in a planet’s atmosphere increases,

a) the surface temperature drops until heating balances cooling.
b) the CO2 must escape into space.
c) the planet explodes.
d) the surface temperature increases because the additional CO2 acts like an extra blanket.
e) a solar eclipse occurs.

Answer 7

d) the surface temperature increases because the additional CO2 acts like an extra blanket. *

Question 8

A hot, dense gas at high pressure produces

a) a continuous spectrum.
b) an emission spectrum.
c) an absorption spectrum.
d) no spectrum

Answer 8

a) a continuous spectrum. *

Question 9

Atoms can be excited as a result of ________.

a) collisions with other atoms
b) emission of radiation
c) absorption of radiation
d) all of the above
e) both (a) and (c)

Answer 9

e) both (a) and (c) *

Question 10

The energy values for the electronic excitation levels in an atom are determined by

a) the radiation absorbed by it.
b) the number of electrons in its orbitals.
c) the number of protons in its nucleus.
d) none of the above

Answer 10

c) the number of protons in its nucleus. *

Question 11

A hot, thin gas produces ________. Emission and absorption lines can be broadened due to ________.

a) absorption lines; high temperature and strong rotation
b) emission lines; low temperature and slow rotation
c) emission lines; high temperature and strong rotation
d) a continuum spectrum; strong gravity
e) absorption lines; strong gravity

Answer 11

c) emission lines; high temperature and strong rotation *

Question 12

We know that according to the Doppler formula, the observed wavelength lambda_observed for a spectral line is related to the emitted wavelength lambda_emitted by the formula

(lambda_observed / lambda_emitted) = 1 + (v / c)

where c = 3 x 10^10 cm per second is the speed of light and v is the velocity of recession. If the emitted and observed wavelengths of the lines are given by

lambda_observed = 5,360 Angstroms
lambda_emitted = 4,102 Angstroms
lambda_observed = 5,672 Angstroms
lambda_emitted = 4,341 Angstroms
lambda_observed = 6,352 Angstroms
lambda_emitted = 4,861 Angstroms
lambda_observed = 8,576 Angstroms
lambda_emitted = 6,563 Angstroms
then the velocity v equals

a) away from us; 9.2 x 10^9 cm per second
b) towards us; 9.2 x 10^9 cm per second
c) towards us; 5.4 x 10^10 cm per second
d) towards us; 2.5 x 10^10 cm per second
e) away from us; 2.5 x 10^10 cm per second

Answer 12

a) away from us; 9.2 x 10^9 cm per second *

Question 13

The highest high tides occur when

a) the planets are all aligned.
b) the Earth, Moon and Sun form a right triangle.
c) the Earth, Moon and Sun form a square.
d) the Earth, Moon, and Sun form a line.

Answer 13

d) the Earth, Moon, and Sun form a line. *

Question 14

The lowest high tides occur when

a) the planets are all aligned.
b) the Earth, Moon and Sun form a right triangle.
c) the Earth, Moon and Sun form a square.
d) the Earth, Moon, and Sun form a line

Answer 14

b) the Earth, Moon and Sun form a right triangle. *

Question 15

Uranium 238 has a half-life of about 4 billion years and decays into lead. How long will it take for 7/8 of the Uranium to change into lead?

a) 2 billion years
b) 4 billion years
c) 6 billion years
d) 8 billion years
e) 12 billion years

Answer 15

e) 12 billion years *

Question 16

If two planets with the same mass but different temperatures start off with the same amount of hydrogen gas in their atmospheres, then the ________ planet will retain ________ of its hydrogen than the ________ planet.

a) hotter; more; cooler
b) cooler; more; hotter
c) cooler; less; hotter
d) hotter; less; cooler
e) both (b) and (d)

Answer 16

e) both (b) and (d) *

Question 17

Radio astronomy can be performed

a) in poor weather.
b) during the day.
c) from the surface of the Earth.
d) at night.
e) all of the above

Answer 17

e) all of the above *

Question 18

X-ray and gamma-ray astronomy

a) can be performed only from the ground.
b) provide no useful information.
c) are performed using the Hubble Space Telescope.
d) can be performed only from space.
e) all of the above

Answer 18

d) can be performed only from space. *

Question 19

Most of the oxygen in Earth’s current atmosphere was

a) created by plants.
b) outgassed from volcanoes.
c) formed at the same time the Earth formed.
d) released from silicon dioxide.
e) bound into rocks after being absorbed by the oceans.

Answer 19

a) created by plants. *

Question 20

(i) small, dense, rocky, with solid surfaces
(ii) clustered close to the Sun and each other
(iii) large and gaseous, with no solid surfaces
(iv) spread throughout the outer solar system
(v) weak magnetic fields, slow rotation
(vi) strong magnetic fields, rapid rotation

Which of the items on the list apply to the terrestrial planets?

a) (i), (ii), (iii)
b) (i), (iii), (v)
c) (i), (v), (vi)
d) (i), (ii), (v)
e) None of the items on the list apply to the terrestrial planets.

Answer 20

d) (i), (ii), (v) *

Question 21

(i) small, dense, rocky, with solid surfaces
(ii) clustered close to the Sun and each other
(iii) large and gaseous, with no solid surfaces
(iv) spread throughout the outer solar system
(v) weak magnetic fields, slow rotation
(vi) strong magnetic fields, rapid rotation

Which of the items on the list apply to the Jovian planets?

a) (iii), (iv), (vi)
b) (i), (ii), (iii)
c) (ii), (iv), (vii)
d) (i), (iii), (vi)
e) None of the items on the list apply to the Jovian planets

Answer 21

a) (iii), (iv), (vi) *

Question 22

We do not detect shear waves on the side of the Earth opposite from an earthquake. From this evidence we can deduce that

a) the Earth has a hollow center.
b) the Earth is composed mainly of light elements like hydrogen.
c) the Earth has the same density throughout the interior.
d) there is a solid core in the Earth.
e) there is a liquid core in the Earth.

Answer 22

e) there is a liquid core in the Earth. *

Question 23

Consider two telescopes, one with a 3 inch diameter and one with a 15 inch diameter. The 15 inch telescope receives ________ times more light energy per second than the 3 inch telescope.

a) 1/25
b) 9
c) 1/9
d) 25
e) 10

Answer 23

d) 25 *

Question 24

(i) Searches for periodic Doppler shifts of lines in the spectrum of the central star.
(ii) Searches for periodic changes in the pulsations observed from a pulsar
(iii) Searches for planets with satellites in orbit around them.
(iv) Searches for periodic drops in the light from the central star due to transits by a planet.
(v) Searches for perodic, bright flashes from the central star.
(vi) Searches for supernova explosions.

Which methods on the list can be used to detect extra-solar planets?
a) (ii), (iii), (iv)
b) (i), (ii), (iv)
c) (i), (v), (vi)
d) (i), (ii), (v)
e) None of the methods on the list can be used.

Answer 24

b) (i), (ii), (iv) *

Question 25

(i) Searches for periodic Doppler shifts of lines in the spectrum of the central star.
(ii) Searches for periodic changes in the pulsations observed from a pulsar
(iii) Searches for planets with satellites in orbit around them.
(iv) Searches for periodic drops in the light from the central star due to transits by a planet.
(v) Searches for perodic, bright flashes from the central star.
(vi) Searches for supernova explosions.

Which methods on the list have been used to detect planets with masses similar to that of the Earth?
a) (ii), (iv)
b) (i), (iii), (v)
c) (i), (v), (vi)
d) (iii), (iv), (v)
e) None of the methods on the list can be used.

Answer 25

a) (ii), (iv) *

Question 26

When white light is passed through a cool, thin gas, the photons that are absorbed

a) cause collisional excitation.
b) are re-emitted in the same direction they were originally moving in.
c) cause radiative excitation.
d) are re-emitted in random directions.
e) both (c) and (d)

Answer 26

e) both (c) and (d) *

Question 27

Most of the hydrogen in Earth’s early atmosphere has

a) escaped into space.
b) been absorbed by animals.
c) been removed by aliens.
d) been used by plants.
e) been bound into rocks or absorbed by the oceans

Answer 27

a) escaped into space. *

Question 28

Most large telescopes are reflectors and not refractors because

a) large lenses are difficult to support.
b) large lenses are difficult to fabricate.
c) large lenses suffer from chromatic aberration.
d) all of the above

Answer 28

d) all of the above *

Question 29

Based on the best available evidence, the amount of CO2 in the Earth’s atmosphere seems to be

a) decreasing continuously over the last 100 years, mainly due to pollution, and ocean damage.
b) remaining the same.
c) increasing over the last 100 years, due to air pollution, destruction of rain forest, and ocean damage.
d) rising and falling by a factor of 10 every year.
e) unmeasurable.

Answer 29

c) increasing over the last 100 years, due to air pollution, destruction of rain forest, and ocean damage. *

Question 30

The ________ force causes hurricanes to spin ________ in the Northern hemisphere.

a) coriolis; clockwise
b) coriolis; counterclockwise
c) gravitational; clockwise
d) gravitational; counterclockwise
e) none of the above

Answer 30

b) coriolis; counterclockwise *

Question 31

What is an absorption spectrum? An emission spectrum? How are they related?

Answer 31

1. A continuous spectrum is a ``rainbow’’ like that produced by a white light bulb. An absorption spectrum is a continuous spectrum with dark lines appearing at certain wavelengths, so that certain colors are missing from the spectrum. An emission spectrum shows bright lines located at the exact same wavelengths as the dark lines in an absorption spectrum.

Question 32

What is spectroscopy? How can spectroscopy be used to infer the composition and temperature of a star?

Answer 32

2. Spectroscopy is the study of the spectra emitted by astronomical objects like stars and planets. It is important because it allows the determination of the composition of the object being studied, as well as its temperature and density.

Question 33

Describe the basic components of a simple spectroscope and the additional components in spectrographs used by astronomers for modern observations.

Answer 33

3. A spectroscope is composed of a slit that allows light to shine onto a prism or grating. Light is diffracted (bent) by the prism or grating through an angle that depends on the wavelength of the light. The spectrum is projected onto a screen or piece of film.

Question 34

In the particle description of light, what is color?

Answer 34

5. In the particle description of light, the color corresponds to the energy contained in each photon.

Question 35

Give a brief description of a hydrogen atom.

Answer 35

7. A hydrogen atom has a single proton in the nucleus and a single electron in one energy level (orbital).

Question 36

What is the normal condition for atoms? What is an excited atom? What are orbitals?

Answer 36

9. Atoms are normally in the ground state, meaning that each electron is in the lowest possible energy level. This is the lowest energy state that the atom can be in. When one or more electrons are not in the lowest possible energy level, the energy of the atom is greater than that for the ground state, and the atom is said to be excited. An orbital is a sharply defined energy state that an electron can occupy in an atom.

Question 37

Why do excited atoms absorb and reemit radiation at characteristic frequencies?

Answer 37

10. Atoms emit and absorb radiation at characteristic frequencies that are determined by the energy levels the electrons can occupy. The energy levels in turn are determined by the structure of the nucleus of the atom. The difference between two electron energy levels is equal to the energy of the photon that must be emitted or absorbed by the atom when the electron makes that transition. With the photon’s energy determined in this way, we can also calculate its frequency and wavelength.

Question 38

How do molecules produce spectral lines unrelated to the movement of electrons between energy levels?

Answer 38

13. Molecules can produce spectral lines unrelated to the movement of electrons between energy levels in two ways, via either rotational or vibrational transitions.

Question 39

List three advantages of reflecting telescopes over reflectors.

Answer 39

1. Mirrors can be made larger than lenses because the full back of the mirror can be supported, whereas a lens can only be supported from its edges.
2. Lenses suffer from chromatic aberration, whereas mirrors do not.
3. Lenses absorb some of the light passing through them, while mirrors don’t absorb any light because the light doesn’t pass through the glass.

Question 40

Explain two reasons why astronomers are continually building larger and larger telescopes.

Answer 40

2. (i) Larger telescopes have more light gathering power, resulting in brighter images. (ii) Larger telescopes have better angular resolution, resulting in the ability to see smaller details in the source.

Question 41

How does Earth’s atmosphere affect what is seen
through an optical telescope?

Answer 41

4. Due to the turbulence in the Earth’s atmosphere, the best angular resolution that can be achieved from the ground is about 1 second of arc. The effects of atmospheric turbulence on the resolution limit are called ``seeing.’’

Question 42

What advantages does the Hubble Space Telescope (HST) have over ground-based telescopes? List some disadvantages.

Answer 42

5. The biggest advantage that the Hubble Space Telescope has over ground-based telescopes is that the ``seeing’’ is ideal in space, due to the lack of atmospheric gases. The Hubble can therefore make observations down to the theoretical resolution limit of its design. A disadvantage of the Hubble is that it is not as large as some of the telescopes on Earth.

Question 43

Why do radio telescopes have to be very large?

Answer 43

8. Radio telescopes have to be large because they detect radio waves, which can be hundreds of meters in wavelength.

Question 44

What is interferometry, and what problem in radio astronomy does it address? Is it limited to radio astronomy?

Answer 44

10. Interferometry is the combination of data from several small telescopes to produce angular resolution equivalent to a single telescope with effective size equal to the distance between the two small telescopes. In radio astronomy, this yields angular resolution in the best cases of 0.001 seconds of arc, which is the best resolution achievable by any observing technique.

Question 45

What are the main advantages of studying objects at many different wavelengths of radiation?

Answer 45

14. Studying objects at different wavelengths allows us to discover the temperatures of very hot and very cold objects, which don’t emit primarily in the optical region of the electromagnetic spectrum. We can also learn about the properties of the distributions of particles, atoms, and molecules that make up the object.

Question 46

A 2-m telescope can collect a given amount of light in 1 hour. Under the same observing conditions, how much time would be required for a 6-m telescope to perform the same task? A 12-m telescope?

Answer 46

3. A 6-m telescope has 3 times the radius of a 2-m telescope, and therefore it has 9 times the collecting area. Hence it requires only 1/9 hour (6 minutes and 40 seconds) to collect the same amount of light collected by a 2-m telescope in 1 hour. Similarly, a 12-m telescope has 6 times the radius of a 2-m telescope, and therefore it has 36 times the collecting area. Hence it requires only 1/36 hour (1 minute and 40 seconds) to collect the same amount of light collected by a 2-m telescope in 1 hour.

Question 47

A space-based telescope can achieve a diffraction-limited angular resolution of 0.05” for red light (wavelength 700 nm). What would the resolution of the instrument be
(a) in the infrared, at 3.5 .m, and
(b) in the ultraviolet, at 140 nm?

Answer 47

4. The formula for the angular resolution of a telescope is A=lambda / (40000 * D), where A is the angular resolution in arc seconds, D is the diameter of the telescope in meters, and lambda is the wavelength of the radiation in Angstroms. Note that according to this formula, A is proportional to lambda. Remember that in doing the calculations, you must express the wavelength in Angstroms!
   In the problem, we are told that the angular resolution of the spaced-based telescope is 0.05 arc seconds for a wavelength of 700 nm. Since A is proportional to lambda, it follows that
   (i) A = 0.25 arc seconds for infrared radiation with a wavelength of 3.5 micrometers;
   (ii) A = 0.01 arc seconds for ultraviolet radiation with a wavelength of 140 nm.

Question 48

Name three important differences between the terrestrial planets and the jovian planets.

Answer 48

4. The terrestrial planets are Mercury, Venus, Earth, and Mars. They are clustered in the inner solar system relatively close to the Sun, and are rocky in composition, with densities similar to that of Earth. The jovian planets are Jupiter, Saturn, Uranus, and Neptune. They are located in the outer solar system, and have densities much lower than Earth’s. The jovian planets are composed mostly of gases. Pluto doesn’t fit neatly into either category.

Question 49

Describe how the terrestrial planets formed.

Answer 49

9. The terrestrial planets formed via “accretion” of the rocky material that was located in the inner region of the nebula from which the Sun and the planets formed 4.5 billion years ago. Accretion is a process in which larger and larger bodies form, and them join together to create even larger proto-planets. The process can also go in the other direction, when collisions between large proto-planets causes them to fragment. After millions of years, the number of terrestrial planets has stabilized to give us the set of planets that see in the solar system today.

Question 50

By comparison with Earth’s average density, what do the densities of the water and rocks in Earth’s crust tell us about Earth’s interior?

Answer 50

1. The average density of Earth is higher than the density of the surface rocks and higher than the density of water. Hence the interior of the Earth must contain a high-density core.

Question 51

How do geologists use earthquakes to obtain information about Earth’s interior?

Answer 51

2. A primary, pressure wave (P-wave) causes material to vibrate in a direction parallel to the motion of the wave. A secondary, shear wave (S-wave) causes material to vibrate in a direction perpendicular to the motion of the wave. P-waves can travel through both liquids and solids, but S-waves cannot travel through liquids.

Question 52

Give two reasons geologists think that part of Earth’s core is liquid.

Answer 52

4. By observing the distribution of P-waves and S-waves after an earthquake, scientists are able to deduce the existence of a liquid core inside the Earth.

Question 53

What process is responsible for surface mountains, oceanic trenches, and other large-scale features on Earth’s surface?

Answer 53

8. Motions of the Earth’s plates (continental drift) gives rise to surface features such as mountains and oceanic trenches.