Life and Death of Stars II

Question 1

Which of the following statements about the future fate of our Sun is FALSE?

A. The Sun will burn helium into carbon, but never get hot enough its core to burn carbon into oxygen.
B. At the end of its life, all that will remain of the Sun will be a white dwarf.
C. The Sun will turn into a red giant, expanding roughly to the size of Mars’s orbit and engulfing the Earth.
D. When the Sun can no longer burn the elements in its core to create energy, it will explode as a
supernova

Answer 1

D. When the Sun can no longer burn the elements in its core to create energy, it will explode as a
supernova

Question 2

Which of the following statements about Population II stars in the Milky Way is FALSE?

A. Population II stars are the oldest stars in the Milky Way
B. Population II stars are found in the Milky Way’s bulge
C. Population II stars are found in the Milky Way’s halo
D. Population II stars are formed in the disk of the Milky Way

Answer 2

D. Population II stars are formed in the disk of the Milky Way

Question 3

The Hertzprung-Russell Diagram for stars is a relation between:

A. Distance vs. spectral type or temperature
B. Luminosity vs. spectral type or temperature
C. Apparent brightness vs. distance
D. Radial velocity vs. distance

Answer 3

B. Luminosity vs. spectral type or temperature

Question 4

What is the single most important characteristic in determining the course of a star’s evolution?

Answer 4

Mass

Question 5

Nearly all the elements in the Universe are formed in the cores of massive stars, except:

A. Iron and nickel
B. Carbon and oxygen
C. Hydrogen and helium
D. Silicon and sulphur

Answer 5

C. Hydrogen and helium

Question 6

Why is a black hole called “black”?

A. Light coming from behind shines right through it
B. It is so small you can’t see it
C. Light can’t escape from it
D. Matter becomes highly absorbing when strongly compressed

Answer 6

C. Light can’t escape from it

Question 7

Which of the following statements about neutron stars is FALSE?

A. Neutron stars spin very fast
B. Neutron stars are supported by neutron degeneracy pressure
C. If a person could survive this experiment, a scientist would weigh more standing on a white dwarf than standing on a neutron star
D. Newly formed neutron stars are thought to have an active phase making them “blink” as pulsars

Answer 7

C. If a person could survive this experiment, a scientist would weigh more standing on a white dwarf than

Question 8

Stars ten times more massive than the Sun

A. Take longer to reach the main sequence.
B. Become black holes.
C. Become red dwarfs.
D. Are dimmer than the sun.

Answer 8

B. Become black holes.

Question 9

T Tauri stars are

A. Standard candles.
B. Optically visible in their cocoon.
C. Frequently strong infrared sources.
D. Remnant cores of dead stars.

Answer 9

C. Frequently strong infrared sources.

Question 10

Planetary nebulae are

Answer 10

Gas clouds ejected by dying stars.

Question 11

Briefly explain the changes that the Sun will go through after it exhausts its core hydrogen. Be
sure to explain both the changes occurring in the Sun’s core and the changes visible from outside
the Sun.

Answer 11

(PARAGRAPHS ARE WORTH 0.5 MARKS, UNTIL A MAXIMUM OF 5 MARKS IS ACHIEVED)
1. Hydrogen fusion supplies the thermal energy that keeps a main-sequence star in balance. But when the Sun’s core hydrogen is finally depleted, nuclear fusion will cease.

2. With no fusion to replace the energy the star radiates from its surface, the core will no longer be able to resist the inward pull of gravity, and it will begin to shrink.
3. Because gravity will shrink both the inert (non-fusing) helium core and the surrounding shell of hydrogen, the hydrogen shell will soon become hot enough for hydrogen shell fusion—hydrogen fusion in a shell around the core at a faster rate.
4. The higher fusion rate will generate enough energy to dramatically increase the Sun’s luminosity and enough pressure to push the surrounding layers of gas outward.
5. Newly produced helium keeps adding to the mass of the helium core, amplifying its gravitational pull and shrinking it further. The hydrogen-fusing shell shrinks along with the core, growing hotter and denser. The fusion rate in the shell consequently rises, feeding even more helium to the core.
6. The core and shell will therefore continue to shrink and heat up—with the Sun as a whole continuing to grow larger and more luminous—until the temperature in the inert helium core reaches about 100 million
   K. At that point, it will be hot enough for helium nuclei to begin to fuse together, and the Sun will enter the next stage of its life.
7. Theoretical models show that the thermal pressure in the inert helium core is too low to counteract gravity. Instead, the pressure fighting against gravity is degeneracy pressure. Because degeneracy pressure does not increase with temperature, the onset of helium fusion heats the core rapidly without causing it to inflate. The rising temperature causes the helium fusion rate to spike drastically in what is called a helium flash.
8. The helium flash releases an enormous amount of energy into the core. This core expansion pushes the hydrogen-fusing shell outward, lowering its temperature and its fusion rate. The result is that, even though
   helium core fusion and hydrogen shell fusion are taking place simultaneously in the star the total energy production falls from the peak it reached during the red giant stage, reducing the star’s luminosity and allowing its outer layers to contract somewhat. As the outer layers contract, the star’s surface temperature increases, so its colour turns back toward yellow from red.
9. When the core helium is exhausted, fusion will again cease. The core, now made of the carbon produced by helium fusion, will begin to shrink once more under the crush of gravity.
10. The exhaustion of core helium will cause the Sun to expand once again. This time, the trigger for the expansion will be helium fusion in a shell around the inert carbon core. Meanwhile, hydrogen fusion will continue in a shell atop the helium layer. The Sun will have become a double shell–fusion star. Both shells will contract along with the inert core, driving their temperatures and fusion rates so high that the Sun will expand to an even greater size and luminosity than in its first red giant stage.
11. Carbon fusion is possible only at temperatures above about 600 million K, and degeneracy pressure will halt the collapse of the Sun’s inert carbon core before it ever gets that hot. With the carbon core unable to undergo fusion and provide a new source of energy, the Sun will finally have reached the end of its life.
12. The huge size of the dying Sun will give it only a very weak grip on its outer layers. As the Sun’s luminosity and radius keep rising, its wind will grow stronger. At the point where the gas temperature has dropped to 1000–2000 K, some of the heavier elements in the wind begin to condense into microscopic clusters, forming small, solid particles of dust. Through winds and other processes, the Sun will eject its outer layers into space, creating a huge shell of gas expanding away from the inert carbon core.
13. The exposed core will still be very hot and will therefore emit intense ultraviolet radiation. This radiation will ionize the gas in the expanding shell, making it glow brightly as a planetary nebula. We have photographed many examples of planetary nebulae around other low-mass stars that have recently died in this very same way.

Question 12

Which stars have longer lifetimes: massive stars or less massive stars? Explain why. (2)

Question 13

What is degeneracy pressure, and how is it linked to white dwarfs and neutron stars? What is the
difference between electron degeneracy pressure and neutron degeneracy pressure? (3)

Answer 13

1. Degeneracy pressure is a type of pressure that does not depend on temperature at all. It depends
   instead on the laws of quantum mechanics that also give rise to distinct energy levels in atoms.
   [1]
2. The degeneracy pressure in white dwarfs arises from closely packed electrons, so we call it electron
   degeneracy pressure. A white dwarf exists in a state of balance because the outward push of electron
   degeneracy pressure matches the inward crush of gravity. [1]
3. Like white dwarfs, neutron stars resist the crush of gravity with degeneracy pressure that arises when particles are packed as closely as nature allows. In the case of neutron stars, however, it is neutrons rather than electrons that are closely packed, so we say that neutron degeneracy pressure supports neutron stars. [1]

Question 14

Which stars have longer lifetimes: massive stars or less massive stars? Explain why. (3)

Answer 14

Less massive stars. [1]
A star’s lifetime depends on both its mass and its luminosity. Its mass determines how much hydrogen fuel the star initially contains in its core. Its luminosity determines how rapidly the star uses up its fuel.

Massive stars start their lives with a larger supply of hydrogen, but they fuse this hydrogen into helium so rapidly that they end up with shorter lives. [1]

(AWARD 1 MARK IF STUDENTS GIVE EXAMPLES INSTEAD OF EXPLAINING – PROVIDING THE
SCORE IS 2 MARKS MAXIMUM)

For example, a 10-solar-mass star (10MSun) is born with 10 times as much hydrogen as the Sun.
However, its luminosity of 10,000LSun means that it uses up this hydrogen at a rate 10,000 times as fast as the rate in the Sun. Because a 10-solar-mass star has only 10 times as much hydrogen and consumes it 10,000 times faster, its lifetime is only 1/1000 as long as the Sun’s lifetime.

Question 15

Which wavelength regime contains potentially ionizing (electron-stripping) photons?

A. microwave
B. infrared
C. visible
D. ultraviolet

Answer 15

D. ultraviolet

Question 16

What nebula is the coolest and densest and might be the site of very early star formation?

A. absorption nebula
B. reflection nebula
C. emission nebula
D. planetary nebula

Answer 16

A. absorption nebula

Question 17

In addition to the law of gravity, which additional physical law means that flat, spinning gas clouds, and therefore flat, orderly planetary systems, must be common and almost inevitable?

A. conservation of energy
B. conservation of mass
C. conservation of angular momentum
D. conservation of time

Answer 17

C. conservation of angular momentum

Question 18

After the fusion of hydrogen (in the core of the sun) what is left over?

Answer 18

helium

Question 19

Nearby stars, and probably all stars, are composed mostly of

Answer 19

hydrogen

Question 20

A globular cluster in our Galaxy is

A. a constellation such as Orion
B. an asterism like the Pleiades
C. a group of very young stars
D. a group of very old stars

Answer 20

D. a group of very old stars

Question 21

The Chandrasekhar limit is

A. the radius of a black hole
B. around ten times the solar mass
C. the maximum mass of a white dwarf
D. the maximum radius of a red giant

Answer 21

C. the maximum mass of a white dwarf

Question 22

When the Sun becomes a Red Giant

A. it will eventually become a supernova
B. its surface will become hotter than it is now
C. it will produce iron and heavier elements in its core
D. hydrogen fusion in its core will have ceased

Answer 22

D. hydrogen fusion in its core will have ceased

Question 23

The dead remnant left over after the life of a 1 MSUN star is

A. a black hole
B. a neutron star
C. a white dwarf
D. a brown dwarf

Answer 23

C. a white dwarf

Question 24

Why does helium fusion require much higher temperatures than hydrogen fusion? Briefly explain
why helium fusion in the Sun will begin with a helium flash.

[4 marks]

Answer 24

Recall that fusion occurs only when two nuclei come close enough together for the attractive strong force to
overcome electromagnetic repulsion. Helium nuclei have two protons (and two neutrons) and hence a greater positive charge than a hydrogen nucleus with its single proton [1].

The greater charge means that helium nuclei repel one another more strongly than hydrogen nuclei. Helium fusion therefore occurs only when nuclei slam into one another at much higher speeds than those needed for hydrogen fusion, which means that helium fusion requires
much higher temperatures [1].

The ignition of helium fusion in a low-mass star like the Sun has one subtlety. Theoretical models show that the thermal pressure in the inert helium core is too low to counteract gravity. Instead, the pressure fighting against gravity
is degeneracy pressure—the same type of pressure that supports brown dwarfs. Because degeneracy pressure does not increase with temperature, the onset of helium fusion heats the core rapidly without causing it to inflate [1].

The rising temperature causes the helium fusion rate to spike drastically in what is called a helium flash [1].

Question 25

Which stars have longer lifetimes: massive stars or less massive stars? Explain why.

[3 marks]

Answer 25

Less massive stars have longer lifetimes [1].

A star’s mass is its most important property - it determines not only its luminosity and surface temperature during the main-sequence (hydrogen core fusion) life, but also its main sequence lifetime and what happens to it after it finally exhausts its core hydrogen.

Stars on the Main Sequence must be using the energy generated via nuclear fusion in their cores to create hydrostatic equilibrium. The condition of hydrostatic equilibrium is that the pressure is balancing gravity. Higher mass
stars, for instance, have a larger gravitational force which also means that a higher pressure is required to maintain
equilibrium [1].

So, the cores of massive stars have significantly higher temperatures than the cores of Sun-like stars. At higher temperatures, the nuclear fusion reactions generate energy much faster, so the hotter the core the shorter their lifespan as nuclear fuel is consumed faster [1].

Question 26

Explain why H-R diagrams look different for star clusters of different ages. How does the location of the main-sequence turnoff point tell us the age of the star cluster? Consider drawing a H-R diagram to aid in the explanation.

[3 marks]

Answer 26

H-R diagrams for different clusters with different ages will have different turnoff points [1].

Because all of a cluster’s stars were born at the same time, we can measure a cluster’s age by finding the main-sequence turnoff point of its stars on an H-R diagram [1].

The cluster’s age is equal to the core hydrogen fusion lifetime of the hottest, most luminous stars that remain on the main sequence. Different star clusters will, hence, have different turnoff points [1]

Question 27

The proton-proton chain

Answer 27

is a three step process which converts some mass to energy as helium nuclei are formed.

Question 28

Many things that we use today, such as automobiles, are made from iron. What was the original source of the iron mined from Earth?

Answer 28

the core of an exploding supernova.

Question 29

The age of a star cluster can be deduced from

Answer 29

the turn-off point of stars on its main sequence.

Question 30

In about 5 billion years, the sun will become

Answer 30

a red giant.

Question 31

When neutron degeneracy fails in a high-mass star, it becomes a

Answer 31

black hole.

Question 32

What is the Chandrasekhar limit?

Answer 32

The maximum possible mass of a white dwarf.

Question 33

For main sequence stars, the general rule is: the higher the surface temperature, the

Answer 33

greater the masses of the stars.

Question 34

The oldest star clusters are

Answer 34

globular clusters.

Question 35

The Sun derives its energy from

Answer 35

the fusion of hydrogen into helium.

Question 36

A Type I supernova occurs when

Answer 36

an accreting white dwarf exceeds the Chandrasekhar limit.

Question 37

What is a molecular cloud?
[1 mark]

Answer 37

Star-forming clouds are called molecular clouds, because their low temperatures allow hydrogen atoms to pair up to form hydrogen molecules. They typically have temperatures of only 10–30 K.

Question 38

Briefly describe the process by which a protostar forms from gas in a molecular cloud. [3 marks]

Answer 38

The molecular clouds that give birth to stars tend to be quite large, because more total mass helps gravity overcome gas pressure. [1]

Once a large molecular cloud begins to collapse, gravity pulls the gas toward the cloud’s densest regions, causing it to fragment into smaller pieces that each form one or more new stars. Each shrinking cloud fragment heats up as it contracts [1]

The rising pressure pushes back against the crush of gravity, slowing the contraction. The dense centre of the cloud fragment is now a protostar—the clump of gas that will become a new star. Gas from the surrounding cloud continues to rain down on the protostar, increasing its mass. [1]

The temperature of the cloud remains below 100 K, so it glows in long wavelength infrared light. As the cloud continues to contract, the growing density makes it increasingly difficult for radiation to escape, especially from the centre. [1]

Once the central region of the cloud fragment becomes dense enough to trap infrared radiation so that it can no longer radiate away its heat, the central temperature and pressure begin to rise dramatically. [1]

A typical star-forming cloud is thousands of times more massive than a typical star, and can give birth to many stars at the same time. [1]

The source of this heat is the gravitational potential energy released as gravity pulls each part of the cloud fragment closer to the centre of the fragment. [1]

Early in the process of star formation, the contracting gas quickly radiates away much of this energy, preventing the temperature and pressure from building enough to resist gravity. [1]

Question 39

What is a planetary nebula?
[1 mark]

Answer 39

A planetary nebula is the huge glowing cloud of gas ejected from a low-mass star at the end of its life, expanding away from the inert carbon core. The exposed core will still be very hot and will therefore emit intense ultraviolet
radiation. This radiation will ionize the gas in the expanding shell, making it glow brightly as an emission nebula.

Question 40

What happens to the core of a star after a planetary nebula occurs? [2 marks]

Answer 40

The core of the star is now what we call a white dwarf. [1]

We now see that white dwarfs are small in radius but high in mass because they are the compressed, exposed cores of dead stars, supported against the crush of gravity by degeneracy pressure. [1]

They are hot because they only recently were in the centre of a star and have not yet had time to cool much [1]

A white dwarf is little more than a decaying corpse that will cool for the indefinite future, eventually disappearing from view as it becomes too cold to emit any more visible light. [1]

Question 41

What event initiates a supernova? [3 marks]

Answer 41

The degeneracy pressure that briefly supports the inert iron core arises because the laws of quantum mechanics prohibit electrons from getting too close together (ELECTRON degeneracy pressure). [1]

Once gravity pushes the electrons past the quantum mechanical limit, however, they can no longer exist freely. In
an instant, the electrons disappear by combining with protons to form neutrons, releasing neutrinos in the
process. The degeneracy pressure provided by the electrons instantly vanishes, and gravity has free rein.
[1]
In a fraction of a second, an iron core with a mass comparable to that of our Sun and a size larger than that of
Earth collapses into a ball of neutrons just a few kilometres across. [1]
Collapse halts only because the neutrons have a degeneracy pressure of their own. The entire core then
resembles a giant atomic nucleus. (NEUTRON degeneracy pressure) [1]
The gravitational collapse of the core releases an enormous amount of energy—more than 100 times what the
Sun will radiate over its entire 10-billion-year lifetime. Where does this energy go? It drives the outer layers off into
space in a titanic explosion called a supernova. [1]