Deck 16: Star Birth

Question 1

What prevents the pressure from increasing as a cloud contracts due to its gravity?

A)As the cloud becomes denser,gravity becomes stronger and overcomes the pressure buildup.
B)The pressure is transferred from the center of the cloud to its outer edges where it can dissipate.
C)Thermal energy is converted to radiative energy via molecular collisions and released as photons.
D)Excess pressure is released in jets of material from the young stars.
E)Once the cloud reaches a critical density,the pressure becomes degenerate and independent of temperature.

Answer 1

C

Question 2

The typical density and temperature of molecular clouds are

A)100 molecules per cubic centimeter,10-30 Kelvin.
B)300 molecules per cubic centimeter,10-30 Kelvin.
C)1000 molecules per cubic centimeter,10-30 Kelvin.
D)100 molecules per cubic centimeter,100-300 Kelvin.
E)300 molecules per cubic centimeter,100-300 Kelvin.

Answer 2

B

Question 3

What is interstellar reddening?

A)Interstellar dust absorbs more red light than blue light,making stars appear redder than their true color.
B)Interstellar dust absorbs more red light than blue light,making stars appear bluer than their true color.
C)Interstellar dust absorbs more blue light than red light,making stars appear redder than their true color.
D)Interstellar dust absorbs more blue light than red light,making stars appear bluer than their true color.
E)The spectral line shift due to a star's motion through the interstellar medium.

Answer 3

C

Question 4

What is the minimum temperature for a cloud to excite emission lines from H₂?

A)10 K
B)30 K
C)100 K
D)300 K
E)1000 K

Question 5

What happens to the visible radiation produced by new stars within a molecular cloud?

A)It escapes the cloud completely.
B)It is absorbed by dust grains and heats up the cloud.
C)It is reflected back onto the protostar,heating it up further.
D)The blue light is absorbed and the red light transmitted.
E)It shoots out in bright jets.

Question 6

What happens to the rotation of a molecular cloud as it collapses to form a star?

A)The rotation rate remains the same and results in stellar rotation.
B)The rotation dissipates and any residual is left in small overall rotation of the star.
C)The rotation rate increases and results in fast rotation of the star.
D)The rotation rate increases and results in a flattened disk of material around a protostar.
E)The rotation increases the speed of collapse and produces more massive stars.

Question 7

The thermal pressure of a gas depends on

A)density only.
B)temperature only.
C)density and temperature.
D)composition.
E)gravity.

Question 8

Calculations show that gravity begins to overcome thermal pressure in clouds that are

A)less massive than the Sun.
B)more massive than the Sun.
C)more massive than ten times the Sun.
D)more massive than a hundred times the Sun.
E)more massive than a thousand times the Sun.

Question 9

If you wanted to observe stars behind a molecular cloud,in what wavelength of light would you most likely observe?

A)ultraviolet
B)visible
C)infrared
D)X-ray
E)gamma-ray

Question 10

What percentage of a molecular cloud's mass is interstellar dust?

A)1%
B)2%
C)28%
D)50%
E)1-50%,depending on the mass of the molecular cloud

Question 11

Why do we think the first generation of stars would be different from stars born today?

A)Without heavy elements,the clouds could not reach as low a temperature as today and had to be more massive to collapse.
B)Without heavy elements,the nuclear reactions at the center of the stars would be very different.
C)Without heavy elements,there was no dust in the clouds and they collapsed faster.
D)The Universe was much denser when the first stars were born.
E)There were no galaxies when the first stars were born.

Question 12

By mass,the interstellar medium in our region of the Milky Way consists of

A)70% Hydrogen,30% Helium.
B)70% Hydrogen,28% Helium,2% heavier elements.
C)70% Hydrogen,20% Helium,10% heavier elements.
D)50% Hydrogen,50% Helium.
E)50% Hydrogen,30% Helium,20% heavier elements.

Question 13

The gravitational force in a molecular cloud depends on

A)density only.
B)temperature only.
C)density and temperature.
D)composition.
E)thermal pressure.

Question 14

The typical size of an interstellar dust grain is

A)1 angstrom.
B)1 nanometer.
C)1 micrometer.
D)1 millimeter.
E)1 centimeter.

Question 15

When is thermal energy trapped in the dense center of a cloud?

A)when the gravity becomes so strong that photons cannot escape
B)when excited molecules collide with other molecules before they can release a photon
C)when the cloud becomes so hot and dense that nuclear fusion begins
D)when magnetic fields trap the radiation
E)when the cloud cools down so much that less light escapes than is produced by contraction

Question 16

What is the likely reason that we cannot find any examples of the first generation stars?

A)The first generation stars are too faint to be visible now.
B)The first generation stars formed such a long time ago that the light from them has not yet had time to reach us.
C)The first generation stars were all very massive and exploded as supernova.
D)The first generation stars formed with only H and He and therefore have no spectral features.
E)We do not know how the first generation stars were formed.

Question 17

How do astronomers infer the presence of magnetic fields in molecular clouds?

A)by measuring the amount of interstellar reddening
B)by measuring the Doppler shifts of emission lines from gas clumps in the cloud
C)by measuring the infrared light emitted by the cloud
D)by measuring the polarization of starlight passing through the cloud
E)by measuring the amount by which gravity is reduced

Question 18

What property of a molecular cloud does not counteract gravitational contraction?

A)thermal pressure
B)turbulent motions
C)magnetic fields
D)fragmentation

Question 19

Astronomers estimate that new stars form in our galaxy at the rate of about

A)one per year.
B)a few (2-3)per year.
C)ten per year.
D)20-30 per year.
E)100 per year.

Question 20

The most abundant molecule in molecular clouds is

A)H₂.
B)He₂.
C)CO.
D)H₂O.
E)HHe.

Question 21

When does a protostar become a true star?

A)when the star is 1 million years old
B)when the central temperature reaches 1 million Kelvin
C)when nuclear fusion begins in the core
D)when the thermal energy becomes trapped in the center
E)when the stellar winds and jets blow away the surrounding material

Question 22

What happens to the surface temperature and luminosity when gravity first assembles a protostar from a collapsing cloud?

A)Its surface temperature and luminosity increase.
B)Its surface temperature remains the same and its luminosity decreases.
C)Its surface temperature and luminosity decrease.
D)Its surface temperature decreases and its luminosity increases.
E)Its surface temperature and luminosity remain the same.

Question 23

Where would a brown dwarf be located on an H-R diagram?

A)upper right
B)on the lower part of the main sequence
C)below and to the right of the lowest part of the main sequence
D)lower left
E)above and to the left of the main sequence

Question 24

What happens to the surface temperature and luminosity when a protostar undergoes convective contraction?

A)Its surface temperature and luminosity increase.
B)Its surface temperature remains the same and its luminosity decreases.
C)Its surface temperature and luminosity decrease.
D)Its surface temperature decreases and its luminosity increases.
E)Its surface temperature and luminosity remain the same.

Question 25

What are the letters that follow the spectral sequence OBAFGKM?

A)NP
B)YZ
C)LT
D)CD
E)UV

Question 26

What is the range of timescales for star formation?

A)from 1 million years for the most massive stars up to 10 million years for the least massive stars
B)from 1 million years for the most massive stars up to 100 million years for the least massive stars
C)from 1 million years for the least massive stars up to 10 million years for the most massive stars
D)from 1 million years for the least massive stars up to 100 million years for the most massive stars
E)about 30 million years for all stars,whatever mass

Question 27

What prevents a brown dwarf from undergoing nuclear fusion?

A)Degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.
B)There is not enough mass to maintain nuclear reactions in a self-sustaining way.
C)The surface temperature never rises high enough for the radiation to be trapped and heat their interior to the temperatures required for nuclear fusion.
D)Radiation pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.
E)There are too many heavy elements and not enough hydrogen for fusion to occur in a self-sustaining way.

Question 28

About how many times more luminous than our Sun is a young solar mass protostar just beginning convective contraction?

A)2-5
B)5-10
C)10-100
D)100-1000
E)a million

Question 29

What is the smallest mass a newborn star can have?

A)8 times the mass of Jupiter
B)80 times the mass of Jupiter
C)800 times the mass of Jupiter
D)about 1/80 the mass of our Sun
E)about 1/800 the mass of our Sun

Question 30

When does hydrogen first begin to fuse into helium in the star formation process?

A)when the cloud first begins to contract
B)when the thermal pressure is trapped at the center of the cloud
C)when the protostars undergoes convective contraction
D)when the protostar undergoes radiative contraction
E)only when the star reaches the main-sequence

Question 31

For every star with a mass greater than 10 solar masses,about how many stars are there with masses less than a solar mass?

A)1
B)3
C)10
D)30
E)200

Question 32

What is the eventual fate of a brown dwarf?

A)It remains the same forever.
B)It gradually cools down and becomes ever dimmer.
C)It gradually contracts and heats up until nuclear fusion ignites in its interior and it becomes a faint star.
D)It becomes ever denser and hotter until it becomes a white dwarf.
E)Gravity ultimately "wins" and it becomes a small black hole.

Question 33

What is the greatest mass a newborn star can have

A)10 solar masses.
B)20 solar masses.
C)50 solar masses.
D)a few hundred solar masses.

Question 34

What happens to the surface temperature and luminosity when a protostar radiatively contracts?

A)Its surface temperature and luminosity increase.
B)Its surface temperature remains the same and its luminosity decreases.
C)Its surface temperature and luminosity decrease.
D)Its surface temperature decreases and its luminosity increases.
E)Its surface temperature and luminosity remain the same.

Question 35

No stars have been found with masses greater than 300 times our Sun because

A)molecular clouds do not have enough material to form such massive stars.
B)they would fragment into binary stars because of their rapid rotation.
C)they would generate so much power that they would blow themselves apart.
D)they shine exclusively at X-ray wavelengths and become difficult to detect.
E)they are not bright enough to be seen nearby.

Question 36

Which of the following may be caused by a protostellar disk?

A)protostellar jets
B)protostellar winds
C)accretion of material onto the star
D)relatively slow protostellar rotation
E)all of the above

Question 37

What species absorbs photons in a protostar's outer layers?

A)H
B)H₂
C)H⁺
D)H⁻
E)dust

Question 38

When does a star become a main-sequence star?

A)when the protostar assembles from a molecular cloud
B)the instant when hydrogen fusion first begins in the star's core
C)when the rate of hydrogen fusion within the star's core is high enough to maintain gravitational equilibrium
D)when a star becomes luminous enough to emit thermal radiation
E)when hydrogen fusion is occurring throughout a star's interior

Question 39

How long does the protostellar stage last for a star like our Sun?

A)1 million years
B)3 million years
C)10 million years
D)30 million years
E)100 million years

Question 40

Which of the following discoveries,if they existed,would necessitate a reevaluation of our ideas of stellar formation?

A)a cluster of stars that appeared to be 13 billion years old
B)a 100-solar-mass star
C)a 0.01-solar-mass star
D)a molecular cloud without any stars
E)planetary systems around other stars than our own

Question 41

Jupiter is a failed star.

Question 42

Explain how the balance of gravity versus thermal pressure predicts that a cloud should fragment into many stars.

Question 43

Photographs of many young stars show long jets of material apparently being ejected from their poles.

Question 44

In any star cluster,stars with lower masses greatly outnumber those with higher masses.

Question 45

Explain how gas in a protostellar disk spirals onto the central star.

Question 46

The most common constituent of molecular clouds,H₂,is rarely detected within them.

Question 47

There is no limit to the mass with which a star can be born.

Question 48

Protostars start off more luminous than the main sequence stars they become.

Question 49

Molecular clouds appear more transparent at longer wavelengths.

Question 50

Explain why stars form only in molecular clouds,the coldest,densest parts of the interstellar medium.

Question 51

No stars have been found composed solely of Hydrogen and Helium (and no heavier elements).

Question 52

Stars only form in molecular clouds that contain more than 100 times the mass of our Sun.

Question 53

Most stars are born in clusters containing thousands of stars.

Question 54

Using Mathematical Insight 16.1,calculate the minimum mass at which gravity and pressure balance for a cloud composed only of hydrogen and helium that cannot cool below 100 K.Assume that density is the same,300 particles per cubic centimeter,as molecular clouds in the Milky Way.

Question 55

Clouds that appear dark in visible light often glow when observed at long infrared wavelengths.

Question 56

Why does a cloud collapse rapidly at first,and then slow down as the it gets denser?

Question 57

What is interstellar reddening and explain how it can be used to map out the distribution of dust in a cloud.

Question 58

Briefly describe how a star forms.

Question 59

Describe the four distinct stages in the life track of a solar-mass protostar on the H-R diagram and explain why the track is the shape it is.

Question 60

Although some photographs show what looks like jets of material near many young stars,we now know that these "jets" actually represent gas from the surrounding nebula that is falling onto the stars.

Question 61

What are the mechanisms that restrict the mass range of stars to about 0.1 to 100 solar masses?

Question 62

Interstellar dust consists mostly of

A)ozone "smog."
B)microscopic particles of carbon and silicon.
C)hydrogen and helium atoms.
D)tiny grains of water ice.
E)the same tiny particles found in household dust.

Question 63

What do we mean by the interstellar medium?

A)The gas and dust that lies in between the stars in the Milky Way Galaxy.
B)The dust that fills the halo of the Milky Way Galaxy.
C)The middle section of the Milky Way Galaxy.
D)The name of an oracle who can channel messages from beings that live near the star called Vega.

Question 64

Process of Science: The existence of brown dwarfs was predicted for decades before their discovery in 1995.Why did it take so long?

Question 65

Which of the following phenomena is not commonly associated with the star formation process?

A)the formation of a spinning disk of material around a protostar
B)powerful "jets" shooting out along the rotation axis of a protostar
C)strong winds of particles blowing out into space from a protostar
D)intense ultraviolet radiation coming from a protostar

Question 66

Which of the following statements is probably true about the very first stars in the universe?

A)They were made only from hydrogen and helium.
B)They were made from pure energy.
C)They were probably orbited only by terrestrial planets,but no jovian planets.
D)They were made approximately of 98% hydrogen and helium,and 2% of heavier elements.

Question 67

The interstellar clouds called molecular clouds are

A)the clouds in which elements such as carbon,nitrogen,and oxygen are made.
B)clouds that are made mostly of complex molecules such as carbon dioxide and sulfur dioxide.
C)the hot clouds of gas expelled by dying stars.
D)the cool clouds in which stars form.

Question 68

What kind of gas cloud is most likely to give birth to stars?

A)a hot,dense gas cloud
B)a cold,dense gas cloud
C)a cold,low-density gas cloud
D)a hot,low-density gas cloud

Question 69

Which of the following types of molecule is the most abundant in an interstellar molecular cloud?

A)CO
B)H₂O
C)H₂
D)NH₃

Question 70

Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?

A)visible light
B)ultraviolet
C)infrared
D)blue light

Question 71

What law explains why a rotating,collapsing cloud forms a flat protostellar disk around a protostar?

A)Kepler's third law
B)the universal law of gravitation
C)Wien's law
D)conservation of angular momentum

Question 72

When does a protostar become a main-sequence star?

A)when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
B)when a piece of a molecular cloud first begins to contract into a star
C)when it becomes luminous enough to emit thermal radiation
D)at the instant that the first hydrogen fusion reactions occur in the protostar's core

Question 73

What effect are magnetic fields thought to have on star formation in molecular clouds?

A)They can help resist gravity,so that more total mass is needed before the cloud can collapse to form stars.
B)They accelerate the star formation process.
C)They allow small stars to form in isolation within gas clouds.
D)None-there are no magnetic fields in interstellar space.

Question 74

What is a protostar?

A)a star that has planets
B)an intermediate-mass star
C)a star that is still in the process of forming
D)a star in its final stage of life

Question 75

Process of Science: Astronomers have not found any stars without some heavy elements.How does this constrain theories for the formation of the first generation of stars in the Universe?

Question 76

Suppose you look by eye at a star near the edge of a dusty interstellar cloud.The star will look ________ than it would if it were outside the cloud.

A)dimmer and bluer
B)more redshifted
C)brighter and redder
D)dimmer and redder

Question 77

Process of Science: Since scientists can't follow the stellar formation process of a single star from start to finish,how do they study solar life cycles?

Question 78

Most interstellar clouds remain stable in size because the force of gravity is opposed by ________ within the cloud.

A)degeneracy pressure
B)radiation pressure
C)stellar winds
D)thermal pressure

Question 79

What can we learn about a star from a life track on an H-R diagram?

A)the star's age
B)the surface temperature and luminosity the star will have at each stage of its life
C)the star's current stage of life
D)how the star's distance from Earth varies at different times in its life

Question 80

Why are brown dwarfs easier to spot in a young,star forming region like Orion rather than an old group of stars like a globular cluster?