Deck 18: The Birth of Stars

Question 1

Modern astronomers have been observing stars for about a century. What fraction is this of the typical lifetime of a normal main sequence?

A)1 part in 10¹⁰
B)1 part in 10¹²
C)1 part in 10⁵
D)1 part in 10⁸

Answer 1

D

Question 2

A main-sequence star begins its life with a certain amount of mass, but it eventually evolves into a giant star hundreds of times as large as it was when it was on the main sequence. How is this possible?

A)Nuclear reactions result in additional mass.
B)Stellar evolution involves mergers of stars-and that provides the additional mass needed for expansion.
C)Additional mass is not needed. As the star expands its density decreases.
D)Stars are constantly being bombarded by dust, as are the planets; this provides the additional mass needed for expansion.

Answer 2

C

Question 3

An H II region looks red, just as the metal filaments in a toaster glow red when heated. Are these colors produced in the same way?

A)Yes, both come from the blackbody spectrum, described by Wien's law and the law of Stefan and Boltzmann.
B)Not exactly, the metal spectrum peaks in the infrared, but there is enough spillover into the visible to make the object look red. The H II region's blackbody curve actually peaks in the red.
C)No, the metal's red is produced by a blackbody curve. The H II region glows red because of a specific emission line in hydrogen.
D)No, the H II region's red is produced by a blackbody curve. The metal glows red because of a specific emission line in iron.

Answer 3

C

Question 4

Compared to the air we breathe, how dense is the matter within a typical nebula, such as that in Orion, where star formation is taking place?

A)about 10^-16 times as dense
B)about 10^-19 times as dense
C)about the same density-otherwise self-gravity could not form stars from this material
D)about 10^-10 times as dense

Question 5

What is the similarity in physical principle between the mechanism at work in an emission nebula and that in a household fluorescent light bulb?

A)Both depend on atomic ionization and subsequent recombination of atoms to produce light, mainly in specific spectral lines.
B)The spectral lines they produce from hydrogen gas are the same in both the nebula and the fluorescent tube.
C)Both depend on heating of a neutral gas to produce a continuum spectrum.
D)Both depend on electrical energy to heat a gas; in a fluorescent bulb electric energy comes from the household supply, whereas in the emission nebula electrical energy arises from rapid motion of ionized gases.

Question 6

We see an emission nebula predominantly in:

A)blue light, originally emitted by stars within the nebula but scattered by dust.
B)light emitted over a wide range of wavelengths by dust grains that have been heated by radiation from embedded stars.
C)light emitted by molecules in the dense clouds of gas surrounding the stars in the nebula.
D)the Balmer H_α red line, from recombination of electrons with nuclei in ionized hydrogen.

Question 7

Hot O and B stars embedded in an emission nebula emit a great deal of ultraviolet radiation. But UV radiation is not visible to the unaided eye. So where does the visible red color come from?

A)A small fraction of the radiation produced by O and B stars is in the visible. However, the total luminosity is so large that this small fraction gives color to the entire nebula.
B)The UV radiation is redshifted by the Doppler effect, so we see it as visible.
C)The UV radiation is reddened by passing through interstellar dust until it is visible.
D)The UV radiation causes hydrogen to be ionized. When the electron and proton reunite, visible light is produced.

Question 8

The predominant color of an emission nebula is:

A)red, from the hydrogen Balmer H_α line.
B)blue, from scattering of light from hot stars by dust particles.
C)green-yellow from the 530.3 nm emission line of ionized iron, equivalent to that from the hot solar corona.
D)a continuum of all colors, the combined light from all the stars in the nebula.

Question 9

What is the characteristic color of an emission nebula?

A)red
B)blue
C)yellow
D)green

Question 10

What is the dominant spectral emission line from an emission nebula in visible light?

A)H_α
B)There are no spectral line features emitted by this nebula, because the light is scattered continuum starlight from embedded stars.
C)the sodium D lines
D)Lyman_α

Question 11

What is the predominant atomic or molecular mechanism that produces the light we see from emission nebulae?

A)Radio energy from embedded stars excites atoms to high atomic states, and spectral lines are produced as these atoms return to their unexcited states.
B)Radiation from nearby stars heats the gas, and the gas then emits a continuum spectrum appropriate to its temperature.
C)Photons of UV and X radiation from very hot embedded stars accelerate electrons by collision, and these accelerating electrons radiate at all wavelengths.
D)UV light from hot stars ionizes atoms, and the subsequent recombination of electrons with these ions produces spectral lines.

Question 12

What process makes an emission nebula glow?

A)electric currents caused by the flow of ionized gas, heating dust particles
B)free electrons emitting light as they pass close to, and are accelerated by, positively charged ions
C)light emitted when electrons jump between energy states in hydrogen atoms
D)high-energy electrons spiraling along magnetic field lines

Question 13

What causes the characteristic red color of an emission nebula?

A)thermal (blackbody) radiation with its peak in the visible red part of the spectrum
B)electrons jumping from the n = 2 energy state to the n = 3 energy state in hydrogen atoms
C)electrons jumping from the n = 3 energy state to the n = 2 energy state in hydrogen atoms
D)electrons jumping from the n = 2 energy state to the n = 1 energy state in hydrogen atoms

Question 14

A large emission nebula contains about how many solar masses of material?

A)1
B)100
C)10,000
D)1 million

Question 15

A small emission nebula contains about how many solar masses of material?

A)1
B)100
C)10,000
D)1 million

Question 16

Long-exposure color photographs of the night sky often show regions that glow red, such as parts of the Orion Nebula. This distinctive red color is caused by:

A)ionization and subsequent recombination of hydrogen atoms.
B)the emission of red and infrared light by warm dust grains.
C)the collective glow of many red giant stars in the region.
D)scattering of starlight by dust grains in the nebula.

Question 17

The bright stars at the center of an H II region (an emission nebula) are mostly:

A)young O and B stars.
B)red supergiants.
C)hot white dwarfs.
D)T Tauri stars.

Question 18

The blue color of a reflection nebula is produced by:

A)light emitted by the gas cloud that is Doppler shifted as the cloud moves rapidly toward us.
B)the continuum emission of very hot gas and dust.
C)emission from specific transitions in hydrogen gas.
D)selective scattering from very small dust grains.

Question 19

In photographs, the Pleiades open star cluster is surrounded by a bluish haze (See Figure 18-23 of Universe, 11th ed.(in the text)). What causes this blue light?

A)starlight absorbed and reemitted by interstellar gas in the star cluster
B)shock waves losing energy to interstellar gas in the star cluster, causing the atoms to emit light
C)starlight scattered by the light-sensitive grains in the photographic plate when the picture was taken
D)starlight scattered from interstellar dust in the star cluster

Question 20

Star clusters of the same type and structure appear to become fainter than expected, on the basis of the inverse-square law alone, as distance from the Sun increases. This is because:

A)star clusters are systematically smaller and hence less bright, the farther they are from the galactic center and hence from the Sun.
B)photons of light become "tired" and appear less bright, the farther they travel.
C)some of the light is scattered and absorbed by interstellar dust and gas between distant clusters and Earth.
D)the cosmological redshift has moved some of the light into the infrared spectral region.

Question 21

The effect of interstellar dust on starlight is:

A)to dim and redden distant stars by preferentially scattering their blue light.
B)to scatter the red light from stars preferentially, making them appear more blue than expected.
C)almost nonexistent, because light does not interact with dust.
D)to make stars appear less bright than expected by absorbing light about equally at all wavelengths.

Question 22

Suppose the light leaving a star has equal intensities in the blue and the red. The starlight passes through a dust cloud on its journey to Earth. When you observe the light reaching Earth you will find the:

A)blue part of the spectrum will be more intense than the red.
B)red part of the spectrum will be more intense than the blue.
C)blue and red intensities will still be equal, and they will be undiminished from the intensities that left the star.
D)blue and red intensities will still be the same, although they will both be diminished from the intensities that left the star.

Question 23

Evidence for the interstellar medium has been provided by all of the following EXCEPT ONE. Which is the EXCEPTION?

A)the existence of interstellar clouds like the Horsehead Nebula in Orion
B)unshifted spectral lines observed when examining a binary star system
C)interstellar reddening
D)the neutrino flux

Question 24

Astronomers use the symbol H II to mean:

A)neutral hydrogen.
B)ionized hydrogen.
C)molecular hydrogen (with two atoms).
D)deuterium (hydrogen with a nucleus of two particles: one neutron and one proton).

Question 25

The ionization energy of hydrogen is 13.6 electron volts or 2.18 × 10^-18 J. A photon emitted from a hot O or B star can ionize hydrogen if it has a wavelength no greater than:

A)2.18 × 10^-18 m.
B)5.69 × 10^-11 m.
C)6.54 × 10^-10 m.
D)9.12 × 10^-8 m.

Question 26

Hydrogen atoms can be ionized by light with a wavelength of 9.12 × 10^-8 m or shorter. What approximate temperature would a star need in order to have the PEAK of its spectrum at this wavelength?

A)32,000 K
B)44,000 K
C)51 million K
D)4 × 10¹⁵ K

Question 27

What are the dimensions of a typical dark nebula of gas and dust within which a star might form?

A)a few thousand M ? inside a diameter of about 10 pc, or about 30 ly
B)about 10 M ? inside a diameter of about 10 pc
C)1 or 2 M ? inside a diameter of about 100 au, about the size of the solar system
D)a few thousand M ? inside about 1 au, Earth's orbit

Question 28

The average density in a dark nebula is about how many times the average density in the interstellar medium?

A)10
B)1000
C)100,000
D)10 million

Question 29

The temperature inside a Bok globule, where star formation may be taking place, is:

A)about 10,000 K.
B)about 100 K.
C)significantly less than 1 K.
D)about 10 K.

Question 30

Typical gas temperatures in dark Bok globules, the birthplaces of stars, are:

A)just below freezing, at -3°C or 270 K.
B)100 K.
C)10 K.
D)just above that of the cosmic background radiation, at 3.5 K.

Question 31

A cocoon nebula is a:

A)bright knot of ionized gas in the bipolar outflow from a young star.
B)cloud of dust hiding a young protostar from sight.
C)giant molecular cloud containing young pre-stellar objects.
D)dark globule of dust and gas silhouetted against a bright H II region.

Question 32

What does a cocoon nebula do to deserve its name?

A)Its gravity prevents the evolving protostar from expanding rapidly and dissipating as a consequence of its very high temperature.
B)It enhances the visibility of a faint protostar by scattering the light from it, much as a mist spreads and enlarges the light from street lamps.
C)It hides protostars by absorbing the very large amounts of visible light emitted by them but reemits this energy as infrared radiation.
D)It absorbs all the electromagnetic radiation from protostars, rendering them invisible until they have melted and evaporated the dust and ionized the gas in the nebula.

Question 33

What would be the temperature inside a dense core that is about to collapse to form a star?

A)10 K
B)1000 K
C)less than 1 K
D)100 K

Question 34

Early in its lifetime a 1-M ? protostar will have 100 times the Sun's luminosity. How is this possible?

A)The first nuclear reactions in a protostar are more energetic than the later hydrogen to helium fusion.
B)The initial temperature is low but the protostar's radius is large.
C)Although small, the protostar's initial temperature is high.
D)The Kelvin-Helmholtz contraction, the protostar's first energy generation mechanism, is more efficient than nuclear fusion.

Question 35

A particular giant molecular cloud has a mass of 400,000 M ? . What is the mass of hydrogen in this cloud?

A)100,000 M ?
B)4000 M ?
C)392,000 M ?
D)300,000 M ?

Question 36

A particular giant molecular cloud has a mass of 400,000 M ? . What is the mass of helium in this cloud?

A)4000 M ?
B)300,000 M ?
C)392,000 M ?
D)100,000 M ?

Question 37

Which of the following is NOT a required condition for gas and dust to begin to condense into a protostar?

A)low mutual rotation, to avoid spin-off of material from the forming star
B)low temperature, to maintain low gas pressure in the cloud
C)higher than average density, to ensure that gravitational attraction is enhanced
D)high temperature, to ensure that gas atoms and dust particles collide with sufficient energy to stick together

Question 38

An object that is formed by the gravitational collapse of an interstellar cloud, and is slowly contracting and heating up to become a star, is called a:

A)Herbig-Haro object.
B)red giant star.
C)cocoon nebula.
D)protostar.

Question 39

A particular, dense interstellar cloud is about 10 pc (30 ly) across and contains a few thousand M ? of material. This cloud is classified as a:

A)Barnard object.
B)Herbig-Haro object.
C)bipolar outflow.
D)giant molecular cloud.

Question 40

How large was the Sun when it first formed as a protostar, perhaps 1000 years after formation?

A)the same diameter as it is now
B)about 100 times its present diameter
C)about 1000 times its present diameter
D)about 20 times its present diameter

Question 41

How large and bright is a young protostar of 1 M ? , whose age is about 1000 years, compared to the radius R and luminosity L of the Sun?

A)1 × R, the same size as the Sun, and 100 × L
B)20 × R, 100 × L
C)1 × R, 1 × L, because a protostar of this mass has already evolved to a Sun-like main-sequence star
D)20 × R, but still much fainter than the Sun, at L/10

Question 42

The main factor that determines the rate at which a protostar evolves is its:

A)initial mass, smaller masses evolving faster.
B)initial mass, larger masses evolving faster.
C)initial composition.
D)environment, protostars with binary companions evolving much faster.

Question 43

If a protostar of 1 M ? has a luminosity 100 times that of the Sun and a radius 20 times that of the Sun, what is its surface temperature? (The Sun's surface temperature is 5800 K.)

A)2000 K
B)2900 K
C)4100 K
D)5800 K

Question 44

How do low-mass protostars (< 4 M ? ) evolve in the H-R diagram?

A)temperature decreasing at approximately constant luminosity
B)temperature increasing at approximately constant luminosity
C)luminosity decreasing at approximately constant temperature
D)luminosity increasing at approximately constant temperature

Question 45

Protostars with masses of less than about 4 times the mass of the Sun evolve at approximately constant surface temperature through most of their pre-main-sequence lifetimes. This is because:

A)they contract too rapidly for their temperature and luminosity to change significantly.
B)they are not massive enough to contract significantly and luminosity remains constant.
C)the luminosity increases as the surface area decreases.
D)the luminosity decreases as the surface area decreases.

Question 46

Protostars with masses of more than about 4 times the mass of the Sun maintain approximately constant luminosity through their pre-main-sequence lifetimes. This is because:

A)the star contracts as its surface temperature decreases, and the change in surface area compensates for the change in energy emitted per unit area.
B)the surface temperature increases as the star contracts, and the change in energy emitted per unit area compensates for the change in surface area.
C)they contract too rapidly for their temperature and luminosity to change significantly.
D)they are too massive to contract significantly, thus keeping luminosity constant.

Question 47

How does a 15-M ? star evolve in the first 10⁵ years after it becomes a protostar?

A)Its temperature remains relatively cool at about 4000 K, but its luminosity decreases by a factor of 100.
B)Its brightness and temperature remain approximately constant as it stabilizes into a nuclear-heated star on the main sequence.
C)Its brightness remains constant, while its surface temperature increases by a factor of 10.
D)Its brightness increases by about a factor of 10⁴, because its temperature increases by a factor of 10, moving it upward along the main sequence.

Question 48

The lowest mass that a protostar can have and still become a star (i.e., start thermonuclear reactions in its core) is:

A)about one-half M ?
B)slightly less than 1/100 m ?
C)slightly less than 1/10 m ?
D)8/10 M ?

Question 49

The smallest mass that a main-sequence star can have is about 0.08 M ? . The reason for this is that:

A)the temperature in the core of a contracting protostar of less than 0.08 M ? does not get high enough for nuclear reactions to start.
B)protostars cannot form with masses less than 0.08 M ?
C)thermonuclear reactions begin so suddenly in stars of less than 0.08 M ? that the star is disrupted by an explosion.
D)protostars of less than 0.08 M ? are not massive enough to contract.

Question 50

What is the lowest mass that an object can have and still be a star?

A)0.08 M ? (80 times Jupiter's mass)
B)0.02 M ? (20 times Jupiter's mass)
C)0.002 M ? (twice Jupiter's mass)
D)0.80 M ? (800 times Jupiter's mass)

Question 51

Main-sequence stars do not have masses larger than about 200 M ? The reason that stars of larger mass do not exist is that:

A)nothing can prevent such stars from collapsing directly into black holes.
B)the thermonuclear reactions in such stars proceed so rapidly that the star explodes.
C)such stars contract directly to become planet-like objects.
D)they rapidly become very luminous and are quickly disrupted by the resulting very high internal pressure.

Question 52

How does a massive star normally end its life?

A)It gradually shrinks to the size of Earth.
B)We don't know, because its lifetime is longer than the age of the universe.
C)It collapses inward, and the whole star becomes a black hole.
D)It explodes.

Question 53

What is believed to be the maximum mass that a star can have?

A)There appears to be no physical limit.
B)about 1000 M ?
C)about 200 M ?
D)about 5 M ?

Question 54

What prevents a star from having a mass greater than about 200 M ? ?

A)No interstellar clouds are found that contain more than 200 M ? .
B)A star of larger mass would collapse under its own gravity, and the whole star would become a black hole.
C)Gas pressure becomes so high as a consequence of high temperatures that the excess mass is pushed back into space.
D)The core of a larger-mass star would evolve rapidly and explode before the overall star finished contracting as a protostar.

Question 55

How long does it take for a 1-M ? star to pass through the pre-main-sequence phase?

A)100,000 years
B)10 million years
C)10,000 years
D)10 billion years

Question 56

At approximately what temperature will nuclear reactions begin in the core of a pre-main-sequence star?

A)50 million K
B)1 million K
C)500,000 K
D)5 million K

Question 57

What is the relationship between stellar mass and position on the main sequence of the Hertzsprung-Russell diagram?

A)The most massive stars appear in the center of the main sequence while less massive main-sequence stars are brighter or dimmer than these stars as a consequence of their higher or lower temperatures.
B)The more massive the star, the higher up on the main sequence the star will appear.
C)The main sequence defines a line of stars whose masses are about 1 Mε , while stars of different masses appear on either side of the main sequence.
D)The less massive the star, the higher up on the main sequence the star will appear.

Question 58

In the Hertzsprung-Russell diagram, how does the position of a typical star change while it is at the main-sequence phase of its evolution?

A)Stars move from upper right to lower left along the main sequence.
B)A star moves very little on the H-R diagram while it is at this phase of its life.
C)Massive stars ( > 4 M ? ) move toward the upper left as their luminosity increases, whereas lower-mass stars move toward the lower right as their temperature decreases along the main sequence.
D)Stars move from upper left to lower right along the main sequence.

Question 59

Some protostars, when they evolve onto the main sequence, employ convection to carry material outward from the core to some region midway through the star. Radiation then carries the energy outward to the surface. Which stars show this characteristic?

A)only the smallest, with masses less than about 0.4 M ?
B)intermediate masses, from about 0.4M ? up to about 4 M ?
C)the largest, with masses above about 4 M ?
D)all of them

Question 60

Some protostars, when they evolve onto the main sequence, employ radiation to carry material outward from the core to some region midway through the star. Convection then carries the energy outward to the surface. Which stars show this characteristic?

A)only the smallest, with masses less than about 0.4 M ?
B)intermediate masses, from about 0.4 M ? up to about 4 M ?
C)the largest, with masses above about 4 M ?
D)all of them

Question 61

Those interior regions of a protostar that are relatively cool are more likely to transport energy by:

A)conduction, because the atoms are close enough together to act as a solid.
B)convection, because the hydrogen tends to form the H^- ion, a good absorber of radiation.
C)radiation, because the atoms move too slowly at these temperatures for convection to be effective.
D)radiation, because low temperature atoms produce copious amounts of visible light.

Question 62

T Tauri stars are at what stage of stellar evolution?

A)early phase, just after the formation of the protostar
B)just after the red giant phase
C)post-main sequence or later phase
D)main sequence or "middle age"

Question 63

A T Tauri star is:

A)an intermediate mass protostar near the end of its pre-main-sequence lifetime.
B)a young, massive O or B star.
C)a low-mass protostar embedded in a cocoon of dust clouds.
D)a high-mass protostar surrounded by a rotationally flattened disk of gas and dust.

Question 64

A Herbig-Haro object is:

A)the gas cloud produced by a supernova explosion.
B)a dark nebula that obscures distant stars from our view.
C)a young protostar emerging from its nebula.
D)glowing interstellar gas, heated by a high-velocity jet of matter from an evolving star.

Question 65

Herbig-Haro objects are:

A)dense dust clouds surrounding and being heated by massive protostars.
B)higher density knots of matter in protoplanetary disks around very young, low-mass stars.
C)regions of ionized gas on the edges of giant molecular clouds.
D)luminous knots of material at either end of bipolar jets emerging from young T Tauri stars.

Question 66

Herbig-Haro objects, bright, variable regions within nebulae, are now thought to be the result of:

A)the initial condensation of matter into a protostar, producing an infrared and visible glow.
B)intense jets of material ejected from a young star, hitting parts of the nebula.
C)brightening of the gas surrounding a massive star as the precursor to a supernova explosion.
D)the hot atmosphere of a star as it is ejected in the dying phases of the star's life.

Question 67

Typical jets of bipolar outflow last for:

A)10 years.
B)1000 years.
C)100,000 years.
D)10 million years.

Question 68

What is a proplyd?

A)a protoplanetary disk around a young star
B)a protostar hidden inside a dense shell of dust
C)a bright nebula formed in the bipolar outflow from a T Tauri star
D)an amoeba-like creature thought to inhabit ethane lakes on Saturn's moon, Titan

Question 69

The Hubble Space Telescope has photographed disks of gas and dust around young, low-mass stars in the Orion Nebula. It is thought that planets may form from the material in disks such as these. What name has been given to these disks?

A)stellar accretions
B)proplyds
C)cocoon nebulae
D)Barnard objects

Question 70

A star-forming region is best viewed in what region of the electromagnetic spectrum?

A)visible
B)ultraviolet
C)infrared
D)radio

Question 71

In the magnetic model for the bipolar outflow from a protostar:

A)magnetic field lines emanate from the poles of the protostar and carry material outward from the stellar interior.
B)magnetic field lines through the circumstellar accretion disk become twisted and concentrated by the rotation of this disk.
C)magnetic field lines associated with the galaxy as a whole are concentrated and shaped by their interaction with the magnetosphere of the protostar.
D)the jets are formed along magnetic field lines, which go from the north magnetic pole of one protostar to the south magnetic pole of a second protostar that forms a close binary pair with the first.

Question 72

The jets of bipolar outflow associated with many protostars are formed by the protostar's:

A)magnetic field.
B)gravitational field.
C)nearby hot O and B stars.
D)central black hole.

Question 73

When a dark nebula collapses to form a group of stars, what is this stellar group called?

A)a galaxy
B)a constellation
C)an open cluster
D)a globular cluster

Question 74

The pillars of M16, shown in Figure 18-20 of Universe, 11th ed., are eroding from:

A)intense electromagnetic radiation from the newly forming stars within the cocoons.
B)ultraviolet radiation from nearby O and B stars.
C)radioactivity.
D)bipolar jets from within the pillars.

Question 75

We know that the Pleiades star cluster is relatively old, about 50 million years old. Each of the following helps us to date the Pleiades EXCEPT ONE. Which is the EXCEPTION?

A)The Pleiades is not surrounded by an H II region.
B)The stars are nearly all on the main sequence.
C)The most massive stars have not yet evolved off the main sequence.
D)The O and B star association originally formed as part of the cluster is still present.

Question 76

Where would you look to find a newly forming star?

A)a globular cluster
B)an open cluster
C)a planetary nebula
D)inside a black hole

Question 77

What is the ultimate fate of an open cluster (or galactic cluster) of stars?

A)It is torn apart by collisions with giant molecular clouds.
B)Its stars finish their lives and explode as supernovae, until eventually there are no stars left.
C)It gradually becomes more compact until the stars in it merge and collapse to become a supermassive black hole.
D)Its stars escape one by one until the cluster no longer exists.

Question 78

Which of the following is a tightly bound physical system, likely to endure for many billions of years?

A)a binary star system
B)an open cluster
C)a stellar association
D)an OB association

Question 79

Dark nebulae have temperatures around 10 K and emit mostly in the millimeter range. Are these values consistent with blackbody radiation?

A)Wien's law predicts a peak wavelength of a few millimeters, so this emission is consistent with blackbody radiation.
B)Wien's law predicts a peak wavelength of a few meters, so this emission is not consistent with blackbody radiation.
C)Wien's law predicts a peak wavelength of less than 1 mm, so this millimeter radiation must come from specific molecular sources.
D)Wien's law predicts a peak wavelength in the x-ray range, so this emission is not consistent with blackbody radiation.

Question 80

The most abundant material in a giant molecular cloud is:

A)carbon monoxide.
B)hydrogen.
C)dust.
D)water vapor.