Physics 110: Practice Mid-term

1.

Which form of radiation corresponds to electromagnetic waves with higher frequencies than those of visible light?

(a)

radio.

(b)

microwave.

(c)

infrared.

(d)

X-rays.

(e)

radar.

2.

What property of an electromagnetic wave does flux quantify?

(a)

the energy carried per unit area per unit time.

(b)

the rate at which wave crests pass a fixed point in space.

(c)

the speed of light.

(d)

the change in wavelength of a wave emitted by a body in motion.

(e)

the amount by which light is dispersed by a prism or grating.

3.

Star X has a surface temperature of 2,000 K and star Y has a surface temperature of 4,000 K. The wavelength of the peak of the emission of X is

(a)

the same at that of Y.

(b)

1/4 that of Y.

(c)

4 times that of Y.

(d)

1/2 that of Y.

(e)

2 times that of Y.

4.

Objects at room temperature emit (ignore reflection)

(a)

no electromagnetic radiation.

(b)

all forms of electromagnetic radiation with equal intensity.

(c)

mostly ultraviolet radiation.

(d)

mostly visible light.

(e)

mostly infrared radiation.

5.

Hydrogen emits a spectral line with a wavelength of 21 cm. We receive radiation from some gas moving at a certain speed that appears to have a wavelength of 21.01 cm. Which of the following is FALSE?

(a)

Radiation from gas coming towards us at the same speed would seem to have a wavelength of 20.99cm.

(b)

Radiation from gas going away from us at twice the speed would seem to have a wavelength of 21.02cm.

(c)

A spacecraft moving towards the gas at the same speed as the gas would receive radiation with a wavelength of precisely 21 cm.

(d)

Radiation from gas going away from us at the same speed would seem to have a wavelength of 20.99cm.

(e)

The gas is moving away from us.

6.

Four emission lines in the spectrum of hydrogen have the colors: red, green, blue and purple. Which of the following statements is correct?

(a)

All photons emitted by hydrogen have the same energy.

(b)

The red line has photons of the highest energy.

(c)

The red line has photons of the lowest energy.

(d)

The green line has photons of the highest energy.

(e)

The hydrogen is in the ground state.

7.

Which statement best describes the formation of an emission line spectrum?

(a)

Atoms in the ground state are excited by the emission of photons.

(b)

Atoms in excited states emit photons as they transition to lower energy states.

(c)

Atoms in excited states absorb photons as they transition to lower energy states.

(d)

Atoms in the ground state are excited by the absorption of photons.

(e)

Gas near a hot star glows by emitting thermal radiation.

8.

Suppose we observe a hot, tenuous gas with a cooler ``blackbody'' behind it. The combined spectrum will be

(a)

absorption lines only.

(b)

emission lines only.

(c)

continuous radiation only.

(d)

continuous radiation with emission lines superimposed.

(e)

continuous radiation with absorption lines superimposed.

9.

Two stars appear from Earth to be equally bright, i.e., they have the same apparent magnitude. Star X is three times further away than is star Y. Which of the following is TRUE?

(a)

Star X is three times brighter and has a larger parallax than star Y.

(b)

Star X is nine times brighter and has a smaller parallax than star Y.

(c)

Star X is nine times fainter and has a smaller parallax than star Y.

(d)

Star X is equally as bright but has a smaller parallax than star Y.

(e)

Star X is three times fainter and has a smaller parallax than star Y.

10.

Two stars have no radial motion towards or away from us but move with the same transverse speed across our line-of-sight. Star X is ten times further away than star Y.

(a)

Star X has a higher Doppler shift than star Y.

(b)

Star Y has a higher Doppler shift than star X.

(c)

Star Y has a higher proper motion than star X.

(d)

Both stars have the same proper motion.

(e)

Both stars have large Doppler shifts.

11.

The spectral type of a star can be interpreted as a measure of its

(a)

chemical composition

(b)

radial velocity

(c)

temperature

(d)

rate of rotation

(e)

distance

12.

How do the Sun, a white dwarf and a red giant rank in ascending order of luminosity?

(a)

Sun, white dwarf, red giant

(b)

red giant, white dwarf, Sun

(c)

red giant, Sun, white dwarf

(d)

white dwarf, Sun, red giant

(e)

white dwarf, red giant, Sun

13.

Star X is 4 parsecs from the Sun and star Y is 8 parsecs from the Sun. The parallaxes of the two stars are

(a)

1/4 second of arc for X and 1/8 second of arc for Y.

(b)

1/4 second of arc for X and 1/2 second of arc for Y.

(c)

4 seconds of arc for X and 8 seconds of arc for Y.

(d)

1 second of arc for X and 1 second of arc for Y.

(e)

8 seconds of arc for X and 4 seconds of arc for Y.

14.

In order to place a star in the Hertzsprung-Russell diagram we must measure its:

(a)

age, temperature and mass.

(b)

apparent brightness, age and distance.

(c)

mass, temperature and distance.

(d)

apparent brightness, temperature and distance.

(e)

apparent brightness, mass and age.

15.

Which fundamental property of main sequence stars determines where they will be located in the Hertzsprung-Russell diagram?

(a)

rotation

(b)

mass

(c)

Doppler shift

(d)

proper motion

(e)

distance

16.

If binary stars did not exist, we would have little information about stellar

(a)

distances

(b)

magnetic fields

(c)

ages

(d)

masses

(e)

temperatures

17.

Why do we know that gravitational contraction is not the source of the Sun's energy?

(a)

we do not detect the required shrinkage of the Sun.

(b)

gravitational contraction is known to be an insignificant source of energy.

(c)

if the Sun were shrinking the length of the year would be increasing, which we don't observe.

(d)

trick question, gravitational contraction is the source of the Sun's energy

(e)

the Kelvin-Helmholtz time is too small by a factor of 200.

18.

The helium nucleus that results from nuclear fusion is less massive that the four protons (hydrogen nuclei) that form it. What happens to the remaining mass?

(a)

it is converted into energy.

(b)

it makes the Solar wind.

(c)

it reappears later as neutrinos.

(d)

it makes an extra proton once in a while.

(e)

it sinks to the center of the Sun.

19.

Nuclear reactions take place in about what fraction of the volume of the Sun?

(a)

1.5%

(b)

10%

(c)

100%

(d)

90%

(e)

50%

20.

Which of the following statements is FALSE?

(a)

If the core of the Sun were to get hotter, it would expand.

(b)

The Sun has maintained roughly the same brightness for more than 4 billion years.

(c)

Nuclear fusion reactions require both high temperature and high density.

(d)

The Sun's source of energy is the nuclear fusion of hydrogen into helium.

(e)

If nuclear reactions in the Sun's core stopped suddenly, the Sun would go dark immediately.

21.

The Solar neutrino problem is

(a)

a substance abuse issue.

(b)

the anomalously low flux of neutrinos detected on Earth.

(c)

that Earth based detectors find more neutrinos than expected.

(d)

the Sun is not expected to radiate neutrinos.

(e)

caused by neutrinos getting caught inside the Sun.

22.

Clouds of interstellar gas contract because of

(a)

rotation

(b)

magnetic fields

(c)

pressure

(d)

nuclear reactions

(e)

gravity

23.

Dense interstellar clouds are thought to be supported by a combination of

(a)

thermal pressure and dust

(b)

dust and rotation

(c)

thermal pressure and magnetic fields

(d)

dust, magnetic fields and rotation

(e)

thermal pressure, magnetic fields and rotation

24.

Observations at which wavelengths best allow us to probe the interiors of dark molecular clouds?

(a)

visible light and x-rays

(b)

radio and infrared

(c)

visible and radio

(d)

infrared and x-rays

(e)

ultraviolet and gamma rays

25.

What happens to a rotating cloud core as it collapses to form a star?

(a)

the rotation rate gradually drops to zero.

(b)

conservation of angular momentum forces the protostar to assume a spherical shape.

(c)

whether or not the core rotates has no effect on the star formation process.

(d)

gravity forces the material to accumulate in a rotating, nebular disk.

(e)

rotating cloud cores cannot collapse because angular momentum must be conserved.

26.

As an interstellar cloud collapses to become a star, its central region eventually becomes opaque. Why is this a significant development in the formation of the star?

(a)

temperature can now begin to rise.

(b)

the star now begins to rotate.

(c)

the star now begins to radiate infrared radiation.

(d)

magnetic pressure now prevents further collapse.

(e)

it signals the onset of electron degeneracy.

27.

What happens to the gas that is left over when a star forms?

(a)

it gets blown away by a strong wind from the young star.

(b)

it is blasted away by an explosion in the young star.

(c)

it shrouds the star in a dust cocoon forever.

(d)

it gets swept away by passing stars.

(e)

it falls onto the star at a later stage.

28.

Which of the following is NOT evidence to support the idea that at least some stars are surrounded by disks when they form?

(a)

All planets in our Solar system orbit the Sun in the same direction and about in the same plane.

(b)

We can see disks in high resolution observations of nearby young stars.

(c)

The winds from young stars often appear to be collimated into bipolar jets.

(d)

Many young stars emit an excess of cool infrared radiation.

(e)

The best theories of star formation include surrounding disks.

29.

The time a forming star takes to reach the main sequence is

(a)

shorter, the smaller its mass

(b)

shorter, the larger its mass

(c)

shorter, the larger its helium content

(d)

shorter, the smaller its helium content

(e)

the same for all stars

30.

The expected future lifetime of the Sun is

(a)

infinite

(b)

several thousand years

(c)

more than twenty billion years

(d)

several billion years

(e)

several million years

31.

What is the key difference between a degenerate gas and a normal gas?

(a)

pressure in a degenerate gas does not depend on temperature

(b)

pressure in a degenerate gas varies all the time

(c)

a degenerate gas has loose moral values

(d)

a degenerate gas always has a low density

(e)

pressure in a degenerate gas does not depend on density

32.

The Vogt-Russell theorem contains a caveat for members of close binary star systems because stars in a close binary system

(a)

can transfer mass to/from the companion

(b)

exert strong forces on each other that inhibit nuclear reactions

(c)

lose mass into interstellar space at a high rate

(d)

always explode when their main sequence evolution ends

(e)

burn fuel at a higher rate than do isolated stars of the same mass

33.

Stars with masses less than about 0.08  do not stay on the main sequence because

(a)

they explode very soon after they form.

(b)

they quickly grow to larger masses by absorbing their planets.

(c)

the central temperature and density never becomes high enough for nuclear reactions to start.

(d)

they collapse straight to black holes before nuclear fusion can start.

(e)

they swell to become red giants as soon as they have finished forming.

34.

The main sequence lifetime of an O-type star compared with an M-type star is

(a)

much longer because there is more fuel available.

(b)

much shorter because the O star burns fuel at a much higher rate.

(c)

about the same because all stars live for about 10 billion years.

(d)

much shorter because there is less hydrogen in an O star.

(e)

much longer because an O star has many more types of fuel available.

35.

As soon as its core hydrogen is exhausted, a star like the Sun begins to evolve

(a)

toward the red giant stage.

(b)

toward the zero-age main sequence stage.

(c)

up the main sequence to become an O star.

(d)

to become a binary star system.

(e)

none of the other answers is correct.

36.

Star clusters are very useful when trying to understand stars because all the stars in a single cluster have the same

(a)

mass and distance.

(b)

age and color.

(c)

mass and chemical composition.

(d)

age and distance.

(e)

color and chemical composition.

37.

The age of a cluster of stars can be estimated from

(a)

counting the number of stars because older clusters have fewer stars.

(b)

plotting the stars on an H-R diagram to find the brightest surviving main sequence star.

(c)

measuring the chemical composition because older clusters have more helium.

(d)

measuring the mass of the stars because massive stars are older.

(e)

measuring the distance of the cluster from the nearest molecular cloud.

38.

The evolution of massive stars after the main sequence stage is best described as

(a)

increasing compression of the core and the fusion of successively heavier nuclei

(b)

a steady increase in luminosity

(c)

a steady decrease in surface temperature

(d)

a helium flash that causes the star to explode

(e)

a steady increase in radius

39.

The presence of hot, blue O stars in a cluster implies that the cluster must

(a)

be old

(b)

be young

(c)

be on the main sequence

(d)

have more oxygen than other clusters

(e)

be far away

40.

A star on the horizontal branch is best described as:

(a)

A giant that is converting carbon to helium in a shell surrounding the core.

(b)

A normal star that is burning hydrogen to helium in its core.

(c)

A giant that is burning helium to carbon in its core.

(d)

A dwarf star that has exhausted all nuclear energy sources.

(e)

A pulsating star that experiences regular helium flashes.

41.

The gas in a planetary nebula is

(a)

left over from the formation of a main sequence star

(b)

a disk surrounding a red giant star

(c)

falling towards the central star which will soon become more massive

(d)

composed mostly heavy elements formed in the core of a massive star

(e)

the former outer layers of an evolved intermediate mass star

42.

The fusion of iron nuclei does not occur in stable stars. Why not?

(a)

no star ever gets hot enough to fuse iron.

(b)

elements more massive than iron are not stable.

(c)

iron is destroyed in stars as soon as it is produced

(d)

iron fusion uses up energy instead of producing it

(e)

stars do not live long enough to fuse iron.

43.

What supports a white dwarf star against collapse?

(a)

thermal pressure.

(b)

gravity.

(c)

electron degeneracy pressure.

(d)

its magnetic field.

(e)

the White Dwarf Benevolent Society.

44.

Which of the following best describes the relationship between mass and radius for white dwarf stars?

(a)

radius is independent of mass.

(b)

more massive white dwarfs are larger.

(c)

more massive white dwarfs are smaller.

(d)

it is not possible to predict the radius of a white dwarf of known mass.

(e)

white dwarfs are all roughly the same size.

45.

Observations of Supernova 1987A confirmed that most of the energy released by Type II supernovae appears as

(a)

visible light

(b)

radioactive elements

(c)

expanding debris

(d)

x-rays

(e)

neutrinos

46.

A few weeks after outburst the luminosity of a supernova is powered by

(a)

residual fusion reactions in the ejected stellar material.

(b)

infall of matter onto the central neutron star.

(c)

decay of radioactive nickel and cobalt.

(d)

ionization by the central object, like a planetary nebula.

(e)

neutrinos depositing their energy in the dense stellar material.

47.

Which of the following is NOT true of supernova remnants?

(a)

they can emit visible light.

(b)

they emit synchrotron radiation.

(c)

they are falling toward the neutron stars at their centers.

(d)

their age can be estimated.

(e)

they contain elements heavier than hydrogen.

48.

Pulsars are

(a)

white dwarf stars with pulsating stellar winds.

(b)

spinning neutron stars with beamed radiation.

(c)

black holes surrounded by disks.

(d)

binary stars with regular eclipses.

(e)

unstable red giant stars that pulsate in size.

49.

An ``event horizon'' is

(a)

the depth to which we can see into the Sun.

(b)

a solid surface surrounding a black hole.

(c)

an imaginary surface enclosing the region from which not even light can escape.

(d)

the farthest any current telescope can see.

(e)

the surface of a dark molecular cloud.

50.

If the Sun were replaced by a black hole of the same mass, the Earth would

(a)

spiral inward to a smaller orbit.

(b)

quickly be swallowed by the black hole.

(c)

spiral outward to a larger orbit.

(d)

orbit inside the Schwarzschild radius.

(e)

pursue the same orbit as now.

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