TEST BANK 21ST CENTURY ASTRONOMY THE SOLAR SYSTEM 5TH EDITION BY KAY
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Chapter 17: Evolution of
High-Mass Stars
Learning
Objectives
Define the bold-faced vocabulary terms within the chapter.
17.1 High-Mass Stars Follow Their Own Path
Illustrate the process of the CNO cycle.
Short Answer: 1
Describe the evolutionary sequence of a high-mass star once
it leaves the main sequence.
Multiple Choice: 16
Short Answer: 5, 6
Differentiate between the evolutionary stages of low,
intermediate, and high-mass stars.
Describe the physical mechanism in the star’s envelope that
leads to stellar pulsations.
Multiple Choice: 12, 13
Compare and contrast the evolutionary stages of low-,
intermediate-, and high-mass stars.
Short Answer: 2, 3, 4
Explain how Cepheid variables are used as standard candles.
Multiple Choice: 5
Short Answer: 7, 8
17.2 High-Mass Stars Go Out with a Bang
Relate binding energy to the release of energy via fission
and fusion.
Multiple Choice: 19, 21, 26, 27, 28, 35, 36
Short Answer: 10
Explain why iron cannot undergo nuclear fusion.
Multiple Choice: 18, 29, 30
Short Answer: 12
Illustrate the stages of core collapse in a massive star.
Multiple Choice: 22, 23, 24, 33
Explain how the implosion of a
massive star leads to a Type II supernova explosion.
Multiple Choice: 31
Short Answer: 11, 13
Differentiate Type Ia and Type II supernovae.
Multiple Choice: 20, 25, 32, 34
17.3 The Spectacle and Legacy of
Supernovae
Explain how supernovae produce heavy elements.
Multiple Choice: 39, 52, 56
Short Answer: 14, 15, 16
Describe the components of a Type II supernova remnant.
Summarize the physical characteristics of a neutron star.
Multiple Choice: 40, 41, 42, 43, 44, 46, 53, 55, 57
Short Answer: 17, 20
Explain the behavior and observable signals from a pulsar.
Multiple Choice: 47, 48, 54
Short Answer: 18, 21
Show how the observed characteristics of the Crab Nebula
indicate it is the remnant of a Type II supernova.
Multiple Choice: 45, 49, 50, 51
Short Answer: 19
17.4 Star Clusters Are Snapshots of
Stellar Evolution
Compare and contrast the properties of open and globular
clusters.
Multiple Choice: 58, 64
Short Answer: 27, 28, 29, 30
Relate the location of the cluster’s main-sequence turnoff on
an H-R diagram to the cluster’s age.
Multiple Choice: 60, 61, 62, 63, 66
Short Answer: 24, 25
Explain how the color and composition of a cluster yield
clues about its origin and age.
Multiple Choice: 59, 65, 67, 68, 69, 70
Short Answer: 22, 23, 26
Working It Out 17.1
Calculate the binding energy of an atomic nucleus.
Short Answer: 9
Working It Out 17.2
Calculate the strength of gravity and escape speed from a
neutron star.
Multiple Choice: 37, 38
MULTIPLE CHOICE
1.
The fundamental stellar
property that determines the major evolutionary differences in the life history
of stars is
a.
distance from Earth.
b.
mass.
c.
location within a
galaxy.
d.
rotation.
e.
presence/absence of
planets around stars.
ANS: B DIF: Easy REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
2.
The dominant mechanism
by which high-mass stars generate energy on the main sequence is called
a.
the proton-proton
chain.
b.
the carbon-carbon
reaction.
c.
the triple-alpha
process.
d.
the CNO cycle.
e.
neutrino cooling.
ANS: D DIF: Easy REF: 17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
3.
What causes massive
stars to lose mass at a high rate through stellar winds?
a.
radiation pressure
b.
high magnetic fields
c.
rapid rotation
d.
carbon fusion
e.
emission of neutrinos
ANS: A DIF: Easy REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
4.
As a high-mass
main-sequence star evolves off the main sequence, it follows a __________ on
the H-R diagram.
a.
nearly vertical path
b.
path of constant radius
c.
nearly horizontal path
d.
path of declining
luminosity
e.
path of increasing
temperature
ANS: C DIF: Easy REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
5.
If you measure the
average brightness and pulsation period of a classical Cepheid variable star,
you can also determine its
a.
age.
b.
rotation period.
c.
distance.
d.
mass.
e.
composition.
ANS: C DIF: Easy REF:
17.1
MSC: Applying
OBJ: Explain how Cepheid variables are used as standard
candles.
6.
The CNO cycle is the
more efficient mechanism for hydrogen fusion only in stars more massive than
1.3–1.5 M⊙ because of the greater __________ their cores.
a.
concentration of heavy
elements like carbon in
b.
turbulence in
c.
abundance of hydrogen
in
d.
temperature of
e.
rotation speed of
ANS: D DIF: Easy REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
7.
In the CNO cycle,
carbon is used as a catalyst for the fusion of hydrogen into helium. This means
that
a.
three helium nuclei
fuse to form carbon.
b.
carbon facilitates the
reaction but is not consumed in it.
c.
carbon boosts the
energy from the reaction, which is why massive stars are luminous.
d.
carbon breaks apart
into three helium nuclei.
e.
the reaction produces
carbon nuclei in addition to helium.
ANS: B DIF: Medium REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
8.
What is one way in
which high-mass stars differ from low-mass stars?
a. They
are found at cooler temperatures on the main sequence.
b. They fuse carbon through silicon without leaving the main
sequence.
c. Convection is important in their cores, which mixes up
helium throughout the core.
d. They turn into red giants explosively.
e. Most of their energy is produced by fission rather
than fusion.
ANS: C DIF:
Medium REF: 17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
9.
The nuclear fusion
reactions involved in the CNO cycle require much higher temperatures than the
reactions within the p-p chain because
a. carbon nuclei can exist only at extremely high
temperatures.
b. reactions involving a catalyst can only occur at
higher temperatures.
c. low mass stars do not have convective cores.
d. neutrinos can only survive at temperatures where the
CNO cycle is possible.
e. nuclei with higher number of protons have a greater
repulsive barrier against protons, so particles need to move fast to overcome
it.
ANS: E DIF: Medium REF:
17.1
MSC: Applying
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
10.
How does the formation
of elements by nuclear fusion depend on the mass of the star?
a. With increasing mass, progressively heavier nuclei are
forged deeper and deeper inside the star.
b. With increasing mass, the heaviest elements can form
only in the outermost layers of the star.
c. With increasing mass, elements between helium and gold
are formed in the cores.
d. With increasing mass, only elements between helium and
carbon can form in the cores.
e. All stars more massive than 8 solar masses create
elements from helium through uranium in their cores.
ANS: A DIF: Medium REF:
17.1
MSC: Applying
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
11.
Massive stars
synthesize chemical elements going from helium up to iron
a. throughout the interior.
b. primarily at the surface.
c. only in the core of the star.
d. along the equator of the star.
e. in a deep convection zone in the interior of the star.
ANS: C DIF: Medium REF:
17.1
MSC: Applying
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
12.
The luminosity of a
Cepheid star varies in time because
a. the entire star pulsates from its core to its surface.
b. the outer envelope of the star contracts and expands
radially.
c. the star rotates too quickly.
d. the star is too massive to be stable.
e. the star undergoes large surface temperature
fluctuations.
ANS: B DIF: Medium REF:
17.1
MSC: Understanding
OBJ: Describe the physical mechanism in the star’s envelope
that leads to stellar pulsations.
13.
What mechanism drives
the pulsations in Cepheid variables?
a. changes in the rate of core nuclear reactions
b. the formation and destruction of sunspots
c. the ionization and recombination of hydrogen
d. the ionization and recombination of helium
e. large rates of mass loss
ANS: D DIF: Medium REF:
17.1
MSC: Understanding
OBJ: Describe the physical mechanism in the star’s envelope
that leads to stellar pulsations.
14.
The main difference
between classical Cepheids and RR Lyrae stars is
a. their masses.
b. that Cepheids form at much greater distances from
Earth.
c. that RR Lyrae were discovered much earlier than
classical Cepheids.
d. their pulsation mechanisms.
e. that classical Cepheids obey a period-luminosity
relation, but RR Lyrae do not.
ANS: A DIF: Medium REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
15.
If a 25 M⊙ main-sequence star
loses mass at an average rate of 10−6 M⊙/yr while on the main sequence, then how much mass will it
lose in its lifetime of about 7 million years?
a. 3 M⊙
b. 5 M⊙
c. 7 M⊙
d. 10 M⊙
e. 12 M⊙
ANS: C DIF: Medium REF:
17.1
MSC: Applying
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
16.
In the post-main
sequence stages, high-mass stars follow quasi-horizontal paths in the H-R
diagram. This means that
a. their luminosity increases because they expand.
b. their color gets redder because they expand.
c. their surface temperature stays about the same because
their luminosity stays about the same.
d. they actually become supernovae immediately after
leaving the main sequence.
e. they collapse and become very small in size.
ANS: B DIF: Difficult REF:
17.1
MSC: Understanding
OBJ: Describe the evolutionary sequence of a high-mass star
once it leaves the main sequence.
17.
A main-sequence star of
25 solar masses has about 25 times the luminosity of a 10-solar-mass star
(recall the mass-luminosity relation presented in the previous chapter). This
is because the more massive star
a. has a hotter core, and therefore nuclear burning
proceeds more rapidly.
b. has more convection in its core, which heats up the
material there.
c. has more hydrogen to burn.
d. has more carbon available, which speeds up the CNO
cycle.
e. is probably younger than the 10 solar mass star.
ANS: A DIF: Difficult REF:
17.1
MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
18.
An iron core cannot
support a massive main-sequence star because iron
a. has low nuclear binding energy.
b. is not present in stellar interiors.
c. supplies too much pressure.
d. fusion occurs only in a degenerate core.
e. cannot fuse to make heavier nuclei and produce energy.
ANS: E DIF: Easy REF: 17.2
MSC: Understanding
OBJ: Explain why iron cannot undergo nuclear fusion.
19.
Once silicon burning
initiates in the core of a high-mass star, the star has only a few __________
left to live.
a. seconds
b. days
c. months
d. years
e. million years
ANS: B DIF: Easy REF:
17.2
MSC: Remembering
OBJ: Relate binding energy to the release of energy via
fission and fusion.
20.
Type I and Type II
supernovae can be distinguished by what combination of observations?
a. light curves and the detection of energetic cosmic
rays
b. light curves and the detection of neutrons
c. light curves and the detection of radio pulses
d. spectra and light curves
e. spectra and X-ray emission
ANS: D DIF: Easy REF:
17.2
MSC: Remembering
OBJ: Differentiate Type Ia and Type II supernovae.
21.
Which of these fusion
reactions begins first in the core of a massive star?
a. silicon fusion to iron
b. neon fusion to magnesium
c. carbon fusion to neon
d. helium fusion to carbon
e. hydrogen fusion to helium
ANS: E DIF: Easy REF:
17.2
MSC: Understanding
OBJ: Relate binding energy to the release of energy via
fission and fusion.
22.
The collapse of the
core of a high-mass star at the end of its life lasts approximately
a. one second.
b. one minute.
c. one hour.
d. one week.
e. one year.
ANS: A DIF: Easy REF:
17.2
MSC: Remembering
OBJ: Illustrate the stages of core collapse in a massive
star.
23.
Each stage of nuclear
burning in a 25 M⊙star is __________ in duration than in a star of 15 M⊙.
a. much shorter
b. a little shorter
c. equally long
d. a little longer
e. much longer
ANS: A DIF: Easy REF:
17.2
MSC: Understanding
OBJ: Illustrate the stages of core collapse in a massive
star.
24.
When the core of a massive
star collapses, a neutron star forms because
a. all the charged particles are ejected in the resulting
explosion.
b. protons and electrons combine to make neutrons.
c. iron nuclei disintegrate into neutrons.
d. neutrinos escaping from the core carry away most of
the electric charge.
e. the collapse releases a large number of protons, which
soon decay into neutrons.
ANS: B DIF: Easy REF:
17.2
MSC: Understanding
OBJ: Illustrate the stages of core collapse in a massive
star.
25.
Type Ia and Type II
supernovae are respectively caused by what types of stars?
a. white dwarfs; Cepheid variables
b. white dwarfs; pulsars
c. massive stars; white dwarfs
d. massive stars; neutron stars
e. white dwarfs; massive stars
ANS: E DIF: Easy REF:
17.2
MSC: Remembering
OBJ: Differentiate Type Ia and Type II supernovae.
26.
The stellar energy is
carried away primarily in the form of neutrinos when
a. an iron core builds up.
b. carbon burning begins.
c. stars are on the main sequence.
d. stars become unstable and pulsate.
e. stars acquire mass from close companions.
ANS: B DIF: Medium REF:
17.2
MSC: Understanding
OBJ: Relate binding energy to the release of energy via
fission and fusion.
27.
During the
main-sequence evolution of a massive star, progressively more massive elements
are fused in the core, giving the core support for
a. longer and longer times.
b. shorter and shorter times.
c. an approximately equal amount of time.
d. approximately 10,000 years.
e. only a few days.
ANS: B DIF: Medium REF:
17.2
MSC: Understanding
OBJ: Relate binding energy to the release of energy via
fission and fusion.
28.
Massive stars explode
soon after fusion to iron initiates because
a. iron has the smallest binding energy of all elements.
b. neutrinos emitted during the fusion to iron are
captured by the star’s lighter elements.
c. fusion of elements heavier than iron requires energy,
so the star runs out of fuel and cannot hold itself up against gravity.
d. stars do not contain elements heavier than iron; these
are made in supernovae explosions.
e. iron nuclei are unstable and rapidly break apart into
lighter elements.
ANS: C DIF: Medium REF:
17.2
MSC: Applying
OBJ: Relate binding energy to the release of energy via
fission and fusion.
29.
Massive stars explode
when they
a. accrete mass from their binary star companion.
b. generate uranium in their cores.
c. merge with another massive star.
d. run out of nuclear fuel in their core, and the cores collapse.
e. lose a lot of mass in a stellar wind.
ANS: D DIF: Medium REF:
17.2
MSC: Applying
OBJ: Explain why iron cannot undergo nuclear fusion.
30.
In their
post-main-sequence stages of evolution, the high-mass stars would spend
progressively shorter and shorter periods of time in part because
a. they are losing energy faster as neutrino cooling
becomes more important.
b. stellar cores become permanently inert once the stars
leaves the main sequence.
c. the stars switch from fusion to fission to produce
energy.
d. massive stars lose most of their nuclear fuel through
powerful stellar winds.
e. the energy is now produced only via violent
gravitational contractions.
ANS: A DIF: Medium REF:
17.2
MSC: Understanding
OBJ: Explain why iron cannot undergo nuclear fusion.
31.
A few hours before a
high-mass star is blasting its outer layers in a colossal explosion,
specialized detectors on Earth would be able to reveal a spike in the number of
a. iron nuclei.
b. carbon nuclei.
c. protons.
d. neutrinos.
e. aurorae.
ANS: D DIF: Medium REF:
17.2
MSC: Understanding
OBJ: Explain how the implosion of a massive star leads to a
Type II supernova explosion.
32.
Supernovae are very
energetic events and for several weeks within their occurrence they can
outshine all but which one of the following objects?
a. Our entire Milky Way galaxy
b. An entire globular cluster of stars
c. The high-mass star Eta Carinae
d. A 1000 R⊙ red supergiant star
e. A 50-day period classical Cepheid
ANS: A DIF: Medium REF:
17.2
MSC: Applying
OBJ: Differentiate Type Ia and Type II supernovae.
33.
Where did the iron in
your blood come from?
a. Nuclear reactions on the surfaces of neutron stars
b. Nuclear reactions that took place in supernova
explosions
c. Nuclear reactions in the cores of low-mass stars
d. Nuclear reactions in the cores of massive stars
e. Nuclear reactions in red giant and horizontal-branch
stars
ANS: D DIF: Medium REF:
17.2
MSC: Understanding
OBJ: Illustrate the stages of core collapse in a massive
star.
34.
How does the light
energy emitted by a supernova within the first few days compare with the energy
emitted by the Sun during its entire lifetime?
a.
The supernova emits far
less energy.
b.
The energy output of a
supernova is impossible to estimate.
c.
Both emit about the
same energy.
d.
It is impossible to
estimate the total energy output of the Sun over its entire lifespan.
e.
The supernova emits far
more energy.
ANS: C DIF: Difficult REF:
17.2
MSC: Applying
OBJ: Differentiate Type Ia and Type II supernovae.
35.
Each kilogram of
hydrogen that fuses into helium releases about 6 × 1014 Joules
of energy. How many tons of hydrogen are fused each second to power a massive
main-sequence star with a luminosity of 100 L⊙? (Note that 1 L⊙ = 4 × 1026
Joule/second and 1 ton = 103 kg.)
a. 2 × 106 tons
b. 7 × 107 tons
c. 2 × 109 tons
d. 7 × 1010 tons
e. 2 × 1011 tons
ANS: D DIF: Difficult REF:
17.2
MSC: Applying
OBJ: Relate binding energy to the release of energy via
fission and fusion.
36.
Why does the luminosity
of a high-mass star remain nearly constant as the star burns elements heavier
than helium in its core, even though it is producing millions of times more
energy per second than it did on the main sequence?
a. Most of the energy is trapped in the core, increasing
the core’s temperature.
b. All of the extra energy goes into heating the shells
of fusion surrounding the core.
c. Most of the energy is absorbed by the outer layers of
the star, increasing the star’s radius but leaving its luminosity unchanged.
d. Most of the energy is carried out of the star by
escaping neutrinos.
e. All of the energy goes into breaking apart light elements
like helium and carbon.
ANS: D DIF: Difficult REF:
17.2
MSC: Applying
OBJ: Relate binding energy to the release of energy via
fission and fusion.
37.
The weight of an
average human of 70 kg on a typical 2 M⊙, 15-km-wide neutron
star would be about ______ times greater than here on Earth.
a. 1027
b. 1011
c. 105
d. 10
e. 1018
ANS: B DIF:
Difficult REF: Working It out 17.2
MSC: Applying
OBJ: Calculate the strength of gravity and escape speed from
a neutron star.
38.
Using the formula g
= GMNS/R2NS,
calculate the acceleration of gravity on a 3 M⊙ neutron and radius 10
km, and express this in terms of the acceleration of gravity on the surface of
the Earth (g = 9.8 m/s2).
a. 4 × 104
b. 4 × 105
c. 4 × 108
d. 4 × 1011
e. 4 × 1014
ANS: D DIF: Difficult REF:
Working It Out 17.2
MSC: Applying
OBJ: Calculate the strength of gravity and escape speed from
a neutron star.
39.
Essentially all the
elements heavier than iron in our Milky Way were formed
a. by supernovae.
b. during the formation of black holes.
c. by fusion in the cores of the most massive
main-sequence stars.
d. during the formation of planetary nebulae.
e. during the initial stages of the Big Bang.
ANS: A DIF: Easy REF:
17.3
MSC: Understanding
OBJ: Explain how supernovae produce heavy elements.
40.
Neutron stars have
masses that range from
a. 3.5 M⊙ to 25 M⊙.
b. 1.2 M⊙ to 30 M⊙.
c. 2.5 M⊙ to 10 M⊙.
d. 1.4 M⊙ to 3 M⊙.
e. 0.1M⊙ to 1.4 M⊙.
ANS: D DIF: Easy REF:
17.3
MSC: Remembering
OBJ: Summarize the physical characteristics of a neutron
star.
41.
A neutron star contains
a mass of up to 3 M⊙ in a sphere with a diameter approximately the size of
a. an atomic nucleus.
b. an apple.
c. a school bus.
d. a small city.
e. the Earth.
ANS: D DIF: Easy REF:
17.3
MSC: Remembering
OBJ: Summarize the physical characteristics of a neutron
star.
42.
Which of the following
is not a common characteristic of a neutron star?
a. extremely high density
b. enormous magnetic field
c. short rotation period
d. large radius
e. source of pulsars
ANS: D DIF: Easy REF:
17.3
MSC: Remembering
OBJ: Summarize the physical characteristics of a neutron star.
43.
What mechanism provides
the internal pressure inside a neutron star?
a. ordinary pressure from hydrogen and helium gas
b. degeneracy pressure from neutrons
c. degeneracy pressure from electrons
d. rapid rotation
e. strong magnetic fields
ANS: B DIF: Easy REF:
17.3
MSC: Understanding
OBJ: Summarize the physical characteristics of a neutron
star.
44.
A neutron star in a
mass-transfer binary system is called
a. a quasar.
b. a double star.
c. an X-ray binary.
d. a Cepheid variable.
e. a white dwarf star.
ANS: C DIF: Easy REF:
17.3
MSC: Understanding
OBJ: Summarize the physical characteristics of a neutron
star.
45.
Scientists estimate
that the stellar shells of the Crab nebula, blasted outward in a Type II
supernova explosion, have slowed their expansion to about ___________ of the
initial speed in about 1000 years.
a. 50 percent
b. 90 percent
c. 75 percent
d. 5 percent
e. 33 percent
ANS: D DIF: Easy REF:
17.3
MSC: Remembering
OBJ: Show how the observed characteristics of the Crab Nebula
indicate it is the remnant of a Type II supernova.
46.
How do we understand
the very fast rotation of neutron stars?
a. It is a consequence of the conservation of angular
momentum applied to collapsing objects.
b. The convection in the cores of high-mass stars is
responsible for this.
c. The high temperature in the cores of high-mass stars
imprints fast rotations.
d. The degenerate iron core leads to fast spins.
e. The fast rotation is due to their huge gravity on
their surface.
ANS: A DIF: Easy REF:
17.3
MSC: Understanding
OBJ: Summarize the physical characteristics of a neutron
star.
47.
We can identify only a
fraction of all the radio pulsars that exist in our Galaxy because
a. gas and dust efficiently block radio photons.
b. few swing their beam of synchrotron emission in our
direction.
c. most have evolved to become black holes, which emit no
light.
d. massive stars are very rare.
e. neutron stars have tiny radii and are hard to detect
even with large telescopes.
ANS: B DIF: Easy REF:
17.3
MSC: Applying
OBJ: Explain the behavior and observable signals from a
pulsar.
48.
When the first pulsar
was discovered, scientists thought it might be a signal from a distant
extraterrestrial civilization. However, this idea was quickly discarded because
a. they realized the signals were interference from cars
and trucks passing by the radio observatory.
b. the government made the scientists hide their original
finding.
c. they realized that Cepheid variables could produce the
detected radio signals.
d. more pulsars were discovered, which meant that these
were natural phenomena.
e. the technology required to create pulsed signals is
beyond the power of any civilization.
ANS: D DIF: Easy REF:
17.3
MSC: Applying
OBJ: Explain the behavior and observable signals from a
pulsar.
49.
The Crab Nebula is an
important test of our ideas about supernova explosions because
a. people saw the supernova and later astronomers found a
pulsar inside the nebula.
b. the system contains an X-ray binary.
c. the nebula is expanding slowly, as expected from mass
loss rates in massive stars.
d. the original star must have been like the Sun before
it exploded.
e. astronomers observed the merger of the two stars.
ANS: A DIF: Medium REF:
17.3
MSC: Understanding
OBJ: Show how the observed characteristics of the Crab Nebula
indicate it is the remnant of a Type II supernova.
50.
One reason why we think
neutron stars were formed in supernova explosions is that
a. all supernova remnants contain pulsars.
b. pulsars are made of heavy elements, such as those
produced in supernova explosions.
c. pulsars are often found near Cepheids and Wolf-Rayet
stars, which are also signs of massive star formation.
d. pulsars spin very rapidly, as did the massive star
just before it exploded.
e. pulsars sometimes have material around them that looks
like the ejecta from supernovae.
ANS: E DIF: Medium REF:
17.3
MSC: Applying
OBJ: Show how the observed characteristics of the Crab Nebula
indicate it is the remnant of a Type II supernova.
51.
The Type II supernova
that created the Crab Nebula was seen by Chinese and Arab astronomers in the
year 1054 CE. Assuming that the nebula is about 6,500 light-years away from us,
we know the star must have exploded in the year
a. 554 CE.
b. 1054 CE.
c. 1054 BCE.
d. 5447 BCE.
e. 7555 BCE.
ANS: D DIF: Medium REF:
17.3
MSC: Applying
OBJ: Show how the observed characteristics of the Crab Nebula
indicate it is the remnant of a Type II supernova.
52.
Iron has 26 protons in
its nucleus, and gold has 79 protons. Where did all the gold on the Earth come
from?
a. nucleosynthesis on the surfaces of neutron stars
b. nucleosynthesis that took place in supernova
explosions
c. nucleosynthesis in the cores of low-mass stars
d. nucleosynthesis in the cores of massive stars
e. nucleosynthesis in red giant and horizontal-branch
stars
ANS: B DIF: Medium REF:
17.3
MSC: Applying
OBJ: Explain how supernovae produce heavy elements.
53.
In a rescaled model in
which the Crab Nebula (10 ly across in reality) would only be about 10 km
across, how big would the central neutron star (about 10 km in reality) be?
a. the size of football field
b. the size of an aspirin
c. the size of a football
d. the size of an atom
e. the size of the tip of a ballpoint pen
ANS: D DIF: Medium REF:
17.3
MSC: Applying
OBJ: Summarize the physical characteristics of a neutron
star.
54.
The particles that
circle around the magnetic-filed lines of neutron stars producing radiation are
a. neutrinos.
b. ions.
c. electrons and positrons.
d. neutrons.
e. photons.
ANS: C DIF: Medium REF:
17.3
MSC: Understanding
OBJ: Explain the behavior and observable signals from a
pulsar.
55.
The collapse of the
core in high-mass stars naturally explains all but which one of the following
neutron stars properties?
a. high density
b. small size
c. high magnetic field
d. fast rotation
e. large distance from Earth
ANS: E DIF: Medium REF:
17.3
MSC: Understanding
OBJ: Summarize the physical characteristics of a neutron
star.
56.
Nucleosynthesis refers
to the formation of
a. massive atoms from less massive ones.
b. less massive atoms by fragmentation of more massive
ones.
c. various isotopes of the same element.
d. a star at the center of a collapsing nebula.
e. first life forms on planets.
ANS: A DIF: Medium REF:
17.3
MSC: Understanding
OBJ: Explain how supernovae produce heavy elements.
57.
Which of the following
is not true about neutron stars?
a. Their existence had been predicted 30 years before
they were discovered.
b. All neutron stars are observed as pulsars from Earth.
c. Neutron stars often show fast expanding debris around
them.
d. Neutron stars can be members of X-ray binaries.
e. Their densities are comparable to that of atomic
nuclei.
ANS: B DIF: Difficult REF:
17.3
MSC: Evaluating
OBJ: Summarize the physical characteristics of a neutron
star.
58.
Which of the clusters
in the figure shown below are open clusters?
a. NGC 290 and M53
b. NGC 290 and M55
c. NGC 290 and NGC 6530
d. M53 and M55
e. M53 and NGC 290
ANS: C DIF: Easy REF:
17.4
MSC: Applying
OBJ: Compare and contrast the properties of open and globular
clusters.
59.
In young clusters the
light is dominated by
a. luminous hot, blue, and some red supergiants.
b. luminous red giants and red dwarfs.
c. numerous Sun-like stars.
d. large numbers of protostars.
e. accreting white dwarfs.
ANS: A DIF: Easy REF:
17.4
MSC: Evaluating
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
60.
What characteristic of
a star cluster is used to determine its age?
a. the chemical composition of stars in the cluster
b. the luminosity of the faintest stars in the cluster
c. the color of the main-sequence turnoff in the cluster
d. the total number of stars in the cluster
e. the apparent diameter of the cluster
ANS: C DIF: Easy REF:
17.4
MSC: Applying
OBJ: Relate the location of the cluster’s main-sequence
turnoff on an H-R diagram to the cluster’s age.
61.
You observe three
different star clusters and find that the main-sequence turnoff stars in
Cluster 1 have spectral type F, the main-sequence turnoff stars in Cluster 2
have spectral type A, and the main-sequence turnoff stars in Cluster 3 have
spectral type G. Which star cluster is the youngest and which one is the
oldest?
a. Cluster 1 is the youngest and Cluster 2 is the oldest.
b. Cluster 2 is the youngest and Cluster 1 is the oldest.
c. Cluster 2 is the youngest and Cluster 3 is the oldest.
d. Cluster 3 is the youngest and Cluster 1 is the oldest.
e. Cluster 3 is the youngest and Cluster 2 is the oldest.
ANS: C DIF: Easy REF:
17.4
MSC: Applying
OBJ: Relate the location of the cluster’s main-sequence
turnoff on an H-R diagram to the cluster’s age.
62.
List the H-R diagrams
in the figure shown below from oldest to youngest.
a. 2, 1, 3, 4
b. 1, 4, 3, 2
c. 4, 3, 1, 2
d. 1, 2, 4, 3
e. 3, 1, 4, 2
ANS: A DIF: Easy REF:
17.4
MSC: Applying
OBJ: Relate the location of the cluster’s main-sequence
turnoff on an H-R diagram to the cluster’s age.
63.
In the figure shown
below, the H-R diagram for the very old cluster M55 shows the presence of an
intriguing type of objects, namely main-sequence stars bluer and more luminous
than the turn-off stars.
These stars have been called “blue stragglers,” and an
attractive explanation might be that they are
a. newly formed stars within in the very gas-rich
globular cluster environment.
b. high-mass stars, recently formed by mergers of old,
lower mass stars in the densely packed cluster.
c. very luminous Type Ia supernovae at their peak
luminosity.
d. exotic stars that produce energy by mechanisms other
than nuclear fusion.
e. stars that actually do not belong to M55.
ANS: B DIF: Medium REF:
17.4
MSC: Evaluating
OBJ: Relate the location of the cluster’s main-sequence
turnoff on an H-R diagram to the cluster’s age.
64.
The figure below shows
the H-R diagram for the very young cluster NGC 6530. Notice that the main
sequence is missing the red, low-luminosity tail. At the same time, there are
several reddish and yellowish objects to the right of the main sequence
(clearly marked). What could these objects represent?
a. evolved Sun-like stars
b. planets orbiting other stars
c. red dwarfs
d. red supergiants
e. protostars in the pre-main sequence stages
ANS: E DIF: Medium REF:
17.4
MSC: Applying
OBJ: Compare and contrast the properties of open and globular
clusters.
65.
What might be true
about the oldest stars in the Milky Way?
a. They would have lots of heavy elements, since they
have been around for a long time and have undergone a lot of nucleosynthesis in
their cores.
b. They would be seen as supergiants.
c. They would have few heavy elements, since there was
not much chance for earlier generations of stars to explode as supernovae
before these stars were formed.
d. They would be massive, since they were among the first
stars formed.
e. They would likely be seen as pulsars.
ANS: C DIF: Difficult REF:
17.4
MSC: Analyzing
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
66.
Of all the
main-sequence stars ever formed with a mass equal to 25 percent of the Sun’s
mass, how many are still on the main sequence today?
a. none
b. 1 percent
c. 10 percent
d. 50 percent
e. 100 percent
ANS: E DIF: Difficult REF:
17.4
MSC: Evaluating
OBJ: Relate the location of the cluster’s main-sequence
turn-off on an HR diagram to the cluster’s age
67.
Which objects would
give us the best clues about the age of a galaxy like ours?
a. the periods of the fastest spinning pulsars and
Cepheid variables
b. the numbers of planetary nebulae
c. the observed frequency of high-mass-star supernovae
d. the H-R diagrams of globular clusters of stars
e. the luminosity of the most massive stars
ANS: D DIF: Difficult REF:
17.4
MSC: Evaluating
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
68.
Recent research
revealed that stars like our Sun slow down their rotation as they age, while on
the main sequence. Assuming that you find a Sun-like star with a measured
rotational period that is only 4-5 days long, it is likely that, compared with
our Sun, the star will also have
a. higher abundance of massive elements than our Sun.
b. a very blue color.
c. a much smaller size.
d. no planets around it.
e. very weak gravity.
ANS: A DIF: Difficult REF:
17.4
MSC: Evaluating
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
69.
Which of the following
arguments would render the discovery of a 100 M⊙star with less than 0.5
percent massive elements most intriguing?
a. Such high-mass stars shine mostly in the UV domain of
the spectrum.
b. Young stars typically have much higher massive-element
abundances than older stars.
c. Stars of such high mass very rarely form within
interstellar clouds.
d. Stars like this one are most likely found in open
clusters.
e. It would be the first star of such mass ever
discovered.
ANS: B DIF: DIF REF:
17.4
MSC: Evaluating
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
70.
The determination of
colors and ages of clusters could be complicated by the fact that
a. we cannot always resolve individual stars.
b. color is also affected by chemical composition of
stars.
c. members of clusters have a large variety of ages.
d. stars in clusters are at very different distances from
us.
e. there are too many stars in a cluster.
ANS: B DIF: DIF REF:
17.4
MSC: Evaluating
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
SHORT ANSWER
1.
Why does the CNO cycle
happen only in high-mass stars?
ANS: The CNO cycle requires protons
to collide and react (i.e., fuse) with nuclei of carbon, nitrogen, and oxygen.
These nuclei have a strong electrical repulsion, since they have six protons
(carbon), seven protons (nitrogen), or eight protons (oxygen). High
temperatures, which mean high particle velocities, are required to overcome
this electrical repulsion. The corresponding high temperatures are achieved
only in the cores of higher-mass stars, which have a greater internal pressure
to their high mass.
DIF: Easy REF:
17.1 MSC: Analyzing
OBJ: Illustrate the process of the CNO cycle.
2.
How do Classical
Cepheid variable stars differ from RR Lyrae variable stars in their masses,
luminosities, and periods?
ANS: Classical Cepheids are
supergiant, massive stars, whereas RR Lyrae stars are horizontal branch stars
with masses more like that of our Sun. Cepheids are also much more luminous and
have longer periods (1 to 100 days) than RR Lyrae stars, which have periods of
less than one day.
DIF: Easy REF:
17.1 MSC: Understanding
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
3.
At what rate would a 25
M⊙ star lose mass via stellar winds (in M⊙/yr), in order to
reduce its mass by 20 percent in its lifetime? The main-sequence lifespan is
about 7 million years.
ANS: The stars would reduce its mass
by 5 M⊙ in 7 million
years. This means an average loss of about 7 × 10-7 M⊙/yr.
DIF: Easy REF:
17.1 MSC: Applying
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
4.
Why do main-sequence
high-mass stars lose so much mass compared with low-mass stars?
ANS: They have very strong winds
because their luminosities (proportional to the number of photons liberated by
the stars per unit time) are so high that the radiation pressure (i.e.,
pressure exerted by photons on massive particles) in their outermost layers is
stronger than their surface gravity.
DIF: Medium REF:
17.1 MSC: Applying
OBJ: Compare and contrast the evolutionary stages of low,
intermediate, and high-mass stars.
5.
An O-type star can
expand into a supergiant that could exceed 1,000 R⊙. If such a star would
be dropped in the place of our Sun, how far would its “surface” extend?
ANS: The radius of the Sun is about R⊙ = 0.00465 A.U. The orbital radius of Jupiter around the
Sun is about 5.2 A.U., which is the equivalent of more than 1100 R⊙. Thus, if dropped at the center of the solar system,
a supergiant star of this size could fill the whole orbit of Jupiter.
DIF: Medium REF:
17.1 MSC: Applying
OBJ: Describe the evolutionary sequence of a high-mass star
once it leaves the main sequence.
6.
Betelgeuse is a star
whose luminosity exceeds 1.2 × 105 L⊙, but its measured surface temperature is quite low for a
star, about 3500K. What would be its estimated size and where would it fall in
the H-R diagram shown in the figure below? What would be the final fate of
Betelgeuse?
ANS: As we learnt previously, stellar
luminosity is given by two measures: size and surface temperature. L = 4πR2 σT4, which implies that L(L⊙)=[R(R⊙)]2 × [T(T⊙)]4. Assuming T⊙ = 5800 K, it follows that [R(R⊙)]2 = 1.2 × 105 × 0.64. The corresponding radius of Betelgeuse
would be R(R⊙) = 951. Its fate
is to go supernova in the near astronomical future. Or maybe it has already
exploded and the information is on its way to us.
DIF: Medium REF:
17.1 MSC: Understanding
OBJ: Describe the evolutionary sequence of a high-mass star
once it leaves the main sequence.
7.
The figure below shows
three distinct types of pulsating stars: Type I (classical) Cepheids, type II
Cepheids and RR Lyrae. Assume that you find a variable star whose period of
pulsation is several days. You accurately measure its apparent brightness, but
you misidentify its type, labeling it “classical” instead of Type II, which
would be its correct type in fact. What would be the effect in the
determination of distance?
ANS: For the same apparent magnitude
(i.e., brightness) and period, a Classical Cepheid is much more luminous and
more distant than its Type II cousin. Therefore utilizing the period-relation
linear relationship for Type I Cepheids would lead to a severe overestimation
of the luminosity, with further implications into measuring a much larger
distance to the star. For example, overestimating the luminosity by a factor of
100 would mean a derived distance 10 times larger. (Note the logarithmic y-axis
scale.)
DIF: Difficult REF:
17.1 MSC: Applying
OBJ: Explain how Cepheid variables are used as standard
candles.
8.
With the Hubble Space
Telescope, you discover a Cepheid variable star in a nearby galaxy that has a
period of 30 days. If nearby Cepheids follow a period- luminosity that relates
the absolute magnitude to the period of pulsation via M = −2.76logP(days)
− 1.4, then what is this Cepheid’s luminosity? Recall that the
Sun’s absolute magnitude is M = 4.83. If the apparent
magnitude of the Cepheid is 25, what is this galaxy’s distance in Mpc?
ANS: The Cepheid’s absolute magnitude
is M = −2.76log 30
− 1.4 = −5.48.
Therefore, its luminosity is obtained considering that the Cepheid is 10.31
magnitudes brighter than our Sun. This means that its luminosity is 
10.31
=
13,300 times greater than our
Sun’s.
10.31
=
13,300 times greater than our
Sun’s.
Absolute magnitude M, apparent magnitude m, and
distance d are related by the equations m − M = 5 log (d/10
pc) and 
pc.
pc.
Thus, the distance of the Cepheid is 
pc = 107.1
pc = 12.5 Mpc.
pc = 107.1
pc = 12.5 Mpc.
DIF: Difficult REF: 17.1 MSC: Analyzing
OBJ: Explain how Cepheid variables are used as standard
candles.
9.
The first step shown in
the CNO cycle diagram in the figure below shows the fusion of one proton and
one nucleus of 12C. What are the frequency and wavelength of photon
emitted in the process?
ANS: The mass of a 12C
atom is m(12C) = 12.0000 amu (atomic mass units). The mass of one
proton is m (1H) = 1.0073 amu. The mass of 13N is m(13N)
= 13.0057 amu.
There is a mass deficit of 13.0057 − (12.0000 + 1.0073) = 0.0016 amu = 2.66 × 10-30 kg. This corresponds to an
equivalent energy of E = mass deficit × c2 = 2.39 × 10-13 J. A photon of such energy would
have a frequency f = E/h = 3.6 × 1020 Hz and a wavelength λ = c/f = 8.3 × 10-13 m, which means a gamma ray. (Note
that a fraction of this energy could be in fact become kinetic energy of the 13N.)
DIF: Difficult REF:
Working It Out 17.1
MSC: Applying
OBJ: Calculate the binding energy of an atomic nucleus.
10.
What is the meaning of
nuclear binding energy?
ANS: The binding energy of a
nucleus is the energy required to tear it apart and separate the constituents
(nucleons, i.e., protons and neutrons). Consequently, it is also equal to the
energy released during the nuclear reactions that lead to the
formation/assembling of that nucleus (fusing protons and neutrons together).
DIF: Easy REF:
17.2 MSC: Understanding
OBJ: Relate binding energy to the release of energy via
fission and fusion.
11.
Name three processes
that speed the collapse of the core of a dying high-mass star.
ANS: (1) Photodisintegration
uses up the thermal energy of the core and speeds the collapse (highly
energetic gamma ray photons are consumed into breaking iron nuclei apart into helium
nuclei).
(2) Electrons and protons combine to form neutrons and
neutron-rich isotopes. This becomes possible in the overcompressed stellar
core. This process uses thermal energy and thus speeds the collapse.
(3) Neutrinos continue to escape, carrying away energy and
speeding the collapse.
DIF: Medium REF:
17.2 MSC: Analyzing
OBJ: Explain how the implosion of a massive star leads
to a Type II supernova explosion.
12.
Why do very massive
stars explode once they generate an iron core?
ANS: Fusion of light elements
into more massive ones releases energy provided that it leads to the production
of nuclei with increased binding energy (per nucleon). This is the case when
hydrogen is converted into helium, helium into carbon, etc. Iron, however, is
at the peak of the binding energy curve shown in the figure below. Therefore,
energy can be released only in the synthesis of elements up to iron. Using iron
itself to form heavier elements absorbs rather than liberates
energy, because the nuclear reactions lead to nuclei less tightly bound than
iron. As a result, the star cannot generate energy to hold itself up against
its self-gravity and will experience a catastrophic core-collapse followed by
an explosive blast of the outer layers.
DIF: Difficult REF:
17.2 MSC: Analyzing
OBJ: Explain why iron cannot undergo nuclear fusion.
13.
Although a Type II
supernova shines with a luminosity of one billion Suns (109 L⊙) in light, most of the
energy in the explosion is emitted in another way. What is it, and how much
more energy does it carry compared with that of the light?
ANS: The overwhelming energy
of a Type II supernova is carried away mostly by neutrinos. Neutrinos remove
about 100 times more energy than the fast moving (ejected) outer stellar layers
have in the form of kinetic energy and about 10,000 times more energy that the
photons that we see as light.
DIF: Medium REF:
17.3 MSC: Understanding
OBJ: Explain how the implosion of a massive star leads
to a Type II supernova explosion.
14.
Why do free neutrons
released in supernovae create new heavy elements?
ANS: Neutrons have no electric
charge, so there is no electric repulsion between neutrons and atomic nuclei,
irrespective of the proton content of those nuclei. Free neutrons are produced
in very large numbers in certain nuclear reactions in the interiors of evolved
stars. Free neutrons are captured and incorporated by nuclei during supernova
events, and subsequently they decay into protons within those nuclei. This
permits the formation of elements with atomic numbers higher than iron.
DIF: Medium REF:
17.3 MSC: Applying
OBJ: Explain how supernovae produce heavy elements.
15.
In the universe, as a
general trend, more massive atoms are less prevalent than lighter ones. There
is, however, a severe exception to this pattern in the domain of low-mass elements.
Identify the low mass nuclei related to the aforementioned “exception,” and
briefly indicate how nuclear physicists explain this apparent conundrum.
ANS: The stellar
nucleosynthesis reveals that elements like lithium (Li), beryllium (Be) and
boron (B) are actually consumed (destroyed) in nuclear reactions inside stars,
whereas heavier elements like carbon (C), nitrogen (N), oxygen (O) and even
iron (Fe) have a much higher likelihood of being produced and scattered back
into the interstellar medium when stars die.
DIF: Medium REF:
17.3 MSC: Understanding
OBJ: Explain how supernovae produce heavy elements.
16.
Briefly explain why
planets like ours could not have formed at about the same time as the very
first generations of stars in the universe.
ANS: Planets like Earth
incorporated, at formation, a large fraction of heavy elements that had been
synthesized and released by generations of massive stars that existed before
our solar system. The primordial composition of the material from which the
very first generations of stars formed did not include heavy elements.
DIF: Medium REF:
17.3 MSC: Understanding
OBJ: Explain how supernovae produce heavy elements.
17.
Explain why neutron
stars are fast-spinning objects and why they have very strong magnetic fields.
ANS: As the core of a
high-mass star collapses, its spinning speed increases due to the conservation
of angular momentum. The collapse also leads to high magnetic field density and
strength, because it becomes concentrated in a small volume.
DIF: Medium REF:
17.3 MSC: Applying
OBJ: Summarize the physical characteristics of a neutron
star.
18.
Explain why the pulsars
we see might be only a fraction of the neutron stars in the Milky Way.
ANS: Pulsars are rapidly
spinning neutron stars with very strong magnetic fields. Subatomic particles,
such as electrons, are stripped off the surface of the star and are accelerated
along the magnetic lines. As a consequence, two beams of electromagnetic
radiation are delivered away from the magnetic poles. As the object rotates,
the beam of light sweeps out the same way a lighthouse beam does.
If that light beam happens to intercept our line of sight, we
could detect the typical sharp pulses of emission in various domains (radio,
optical, X-ray). It is likely that most neutron star beams are aimed in random
directions, so they do not intersect Earth’s direction and so go undetected.
DIF: Medium REF:
17.3 MSC: Applying
OBJ: Explain the behavior and observable signals from a
pulsar.
19.
The Crab Nebula is
about 10 light-years across and its glow is energized by the pulsar at its
center, about 10 km in size. The shell of this nebula is currently expanding at
almost 1500 km/s. Estimate the age of the Crab Nebula. How does it reconcile
with the documented records of a “guest star” from Chinese astronomers dating
to 1054 CE?
ANS: If the “radius” of about
5 ly (recall that 1 ly = 9.46 × 1012
km) has been covered at a rate of about 1500 km/s in about (5 × 9.46 × 1012
km)/(1500 km/s) = 3.15 × 1010
s, that would translate to about 1,000 years of expansion, in agreement with
the time elapsed since 1054.
A caveat should be pointed out, however. Considering that at
the moment of the stellar explosion the ejected layers could have started at
30,000 km/s and scientists presently measure about 1500 km/s, the average speed
could be quite uncertain (significantly larger than 1500 km/s) and hence our
rough estimate a nice coincidence.
DIF: Difficult REF:
17.3 MSC: Evaluating
OBJ: Show how the observed characteristics of the Crab Nebula
indicate it is the remnant of a Type II supernova.
20.
Show that the average
density of a typical neutron star is comparable to the typical densities of
atomic nuclei.
ANS: Taking as typical a
neutron stars of 15 km in size (R = 7.5 km) and a mass of 2 M⊙(the solar mass M⊙ = 2 × 1030 kg), one would estimate an average
density of ρ = M/V
= 2.3 × 1018 kg.
Different experiments suggest that atomic nuclei are quite spherical and all
have essentially the same density, 2.3 × 1017 kg/m3.
DIF: Difficult REF:
17.3 MSC: Evaluate
OBJ: Summarize the physical characteristics of a neutron
star.
21.
It is known that
pulsars tend to slow down their rotation as they age, albeit at a very, very
slow rate (they are considered by far the most accurate clocks in the universe
in fact). Some extreme pulsars, however, rotate hundreds of times per second
(they are hence call millisecond pulsars) and were discovered in
globular clusters. They spin faster than most of the usual kitchen gadgets we
use. What would be a reasonable explanation that you could infer using as a
hint in the figure shown below?
ANS: Very old pulsars can be
revived and spun-up if they get gravitationally bound to a companion star and
syphon material from that. The material would form an accretion disk with a
direction of rotation that matches the direction of rotation of the pulsar
itself. The neutron star could gain angular momentum from the material that
falls onto it. This could be a reasonable explanation.
DIF: Difficult REF:
17.3 MSC: Evaluate
OBJ: Explain the behavior and observable signals from a
pulsar.
22.
Explain why a blue star
cluster is likely younger than a red star cluster.
ANS: The blue color of a star
cluster is dominated by the higher-energy photons liberated by the most massive
stars within it. Because such massive stars are also short-lived, the cluster
must be young. On the other hand, if the light of a star cluster appears red,
its light output must be dominated by evolved stars (post main sequence, red giants)
and low-mass main-sequence stars. The light from such stars would easily be
outshone by the presence of high-mass stars in the cluster. The fact that we
see mostly the light from the fainter, low-mass stars means the high-mass stars
must have already “died,” which indicates that the red cluster is older than
the blue cluster. (Note that further complications could come from the fact
that composition can also affect color, with a lower fraction of heavy elements
being related to bluer stars.)
DIF: Easy REF:
17.4 MSC: Applying
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
23.
About 2 percent of the
mass of the Sun is in elements heavier than helium. Where did these elements
come from?
ANS: The Sun is only one third
of the estimated age of the universe. It formed as a large clump of gas and
dust in the interstellar medium (a.k.a. solar nebula) collapsed under
its own gravity. That original cloud had been previously seeded with elements
heavier than helium synthesized in earlier generations of stars and then
released into interstellar space mostly through supernovae explosions.
DIF: Easy REF:
17.4 MSC: Applying
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
24.
Suppose you observe
three star clusters and construct their respective H-R diagrams. Cluster 1 has
a main-sequence turnoff point at spectral type G, Cluster 2 has a turnoff point
at spectral type A, and Cluster 3 has a turnoff point at spectral type B. Which
cluster is the youngest and which is the oldest? Explain why. What is the
approximate age of the oldest cluster?
ANS: For stars along the main
sequence, spectral type B indicates the highest mass, while type G is
indicative of lower, Sun-like stellar masses, with A-type falling in an
intermediate domain. Since more massive stars have shorter main-sequence
lifetimes, Cluster 3 is the youngest and Cluster 1 is the oldest. One could
infer that Cluster 1 is about 10 billion years old because the Sun itself is a
G-type star and has an estimated main-sequence lifetime of about 10 billion
years.
DIF: Easy REF:
17.4 MSC: Applying
OBJ: Relate the location of the cluster’s main-sequence
turnoff on an H-R diagram to the cluster’s age.
25.
Why are star clusters
helpful for testing our ideas about star formation and stellar evolution?
ANS: Stellar clusters
constitute ideal “laboratories” for testing our understanding about the
formation and evolution of stars, because one can make a few powerful
assumptions about the stars within the same cluster: (1) they all formed at
about the same time; (2) they all started out from the same original cloud of
interstellar medium, thus all have almost identical initial composition; and
(3) they all are about the same distance from us, thus differences in apparent
brightness are solely caused by luminosity differences. This allows us to
directly explore the effect of stellar mass on the evolution of individual
stars, in particular to verify that high-mass stars evolve more quickly than
low-mass stars.
DIF: Medium REF:
17.4 MSC: Applied
OBJ: Relate the location of the cluster’s main-sequence
turnoff on an H-R diagram to the cluster’s age.
26.
Give two major
arguments for the scarcity of stars in the high-mass, high-luminosity,
main-sequence blue tail of the H-R diagram.
ANS: Firstly, only a very
small number of high-mass stars form to begin with. Secondly, stars of high
mass go through all the stages of their formation and evolution (pre-main
sequence, main sequence and post-main sequence) very fast, finding their demise
in supernovae events.
DIF: Medium REF:
17.4 MSC: Applying
OBJ: Explain how the color and composition of a cluster yield
clues about its origin and age.
27.
Explain why spectra of
stellar photospheres (for main sequence stars in particular) give us insights
into the chemical composition of the original clumps of interstellar material
that formed the stars in the first place.
ANS: In main-sequence stars
the matter synthesized or “processed” in nuclear reactions doesn’t quite mix
with the material in the stellar atmospheres. One can say that stellar surfaces
and atmospheres still retain the least-altered information about the
interstellar material from which those stars formed. Thus, the spectra
revealing the abundance of massive elements give clues about the composition of
the original nebula and reflect the preexisting, cumulative amount of star
formation.
DIF: Medium REF:
17.4 MSC: Applying
OBJ: Compare and contrast the properties of open and globular
clusters.
28.
Justify with numerical
arguments the statement that “every star less massive than about 0.8 solar mass
that ever formed is still around today.”
ANS: We learned previously
that the main-sequence lifespan is estimated by tMS = 1010 yr × (M/M⊙)−2.5. In our context tMS = 1010
yr × (0.8)−2.5 = 17.4 Gyr, which is longer than the estimated age of
the universe itself.
DIF: Medium REF:
17.4 MSC: Applying
OBJ: Compare and contrast the properties of open and globular
clusters.
29.
Explain the likelihood
of Type Ia and Type II supernovae occurrence in open and globular clusters of
stars.
ANS: Type Ia supernovae
involve an accreting white dwarf, stealing material from a companion star. The
very presence of a white dwarf requires a rather long time for Sun-like stars
to reach that stage. Thus, Type Ia should be far more prevalent in globular
clusters, which are in fact some of the best indicators for the age of the
galaxy itself. Type II supernovae represent the typical fate of high-mass
stars, which are the more frequently members of open clusters in Milky Way and
other star forming galaxies. (Note that Type Ia could also occur in parts of a
galaxy other than its globular clusters.)
DIF: Medium REF:
17.4 MSC: Evaluate
OBJ: Compare and contrast the properties of open and globular
clusters.
30.
As star formation leads
to large numbers of low-mass stars and rare cases of high-mass stars, what
would be the ultimate fate of galaxies like our Milky Way?
ANS: The interstellar material
is being “locked” in stars faster that it is recycled back into it. Therefore,
in the distant astronomical future, galaxies will cease their star formation
and slowly become dimmer and dimmer.
DIF: Medium REF:
17.4 MSC: Evaluate
OBJ: Compare and contrast the properties of open and globular
clusters.
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