ch.14

Question 1

What holds a star together?

Answer 1

Gravity

Gravity holds a star together while the pressure of its gases supports it against gravity’s pull.

Question 2

How does a star generate the pressure that supports it?

Answer 2

By the conversion of hydrogen into helium in its core.

Question 3

What happens to a star when its hydrogen fuel is exhausted?

Answer 3

It dies quietly into a white dwarf or violently into a neutron star or black hole.

Question 4

What role does gravity play in stellar evolution?

Answer 4

It drives stellar evolution from formation to final death.

Question 5

What drives the duration of a star’s evolution?

Answer 5

The amount of mass (gravity).

Question 6

What are the two ways to study a star’s evolution?

Answer 6

• Stellar models via computer calculations
• Observations of different stars

Question 7

What is the mass division that separates low-mass and high-mass stars?

Answer 7

About 10 solar masses.

Question 8

What characterizes a star during its time on the main sequence?

Answer 8

It is in equilibrium with any fluctuations quickly restored.

Question 9

What occurs as hydrogen in the core is consumed?

Answer 9

The star begins to leave the main sequence.

Question 10

How do low-mass stars die?

Answer 10

Quietly.

Question 11

How do high-mass stars die?

Answer 11

With a bang.

Question 12

What happens to a star’s core as the fuel is used up?

Answer 12

The core contracts.

Question 13

What stage does a sun-like star enter after the red giant branch?

Answer 13

Helium fusion.

Question 14

What happens during the helium flash?

Answer 14

Helium begins to fuse extremely rapidly.

Question 15

Why do giant stars pulsate?

Answer 15

Their atmospheres trap some of their radiated energy.

Question 16

What is the Period-Luminosity Law?

Answer 16

The longer the period of pulsation, the more luminous the star.

Question 17

What are two important groups of variable stars?

Answer 17

• Cepheid
• RR Lyrae

Question 18

What is formed in stellar cores or supernovae?

Answer 18

All heavy elements.

Question 19

What is a nova?

Answer 19

A star that flares up very suddenly and then returns slowly to its former luminosity.

Question 20

What happens to a white dwarf in a binary system?

Answer 20

It can undergo repeated novas.

Question 21

What is the remnant of a supernova explosion called?

Answer 21

Supernova remnant.

Question 22

What type of supernova occurs from the death of a high-mass star?

Answer 22

Type II supernova.

Question 23

What is a Type I supernova?

Answer 23

A carbon-detonation supernova.

Question 24

What marks the end of a star’s life cycle for high-mass stars?

Answer 24

The core collapses and rebounds in a supernova.

Question 25

What is a white dwarf?

Answer 25

The remaining core of a low-mass star after the outer layers are expelled.

Question 26

How does a Type II supernova occur?

Answer 26

When a high-mass star's core collapses after reaching iron.

Question 27

What happens to the core of a white dwarf when it exceeds the Chandrasekhar limit?

Answer 27

It can explode as a Type I supernova.

Question 28

What is the Crab Nebula?

Answer 28

A remnant from a supernova explosion witnessed in 1054.

Question 29

What is the lifetime of a star dependent on?

Answer 29

Its mass.

Question 30

Fill in the blank: A star will spend most of its 'shining' lifetime as a ______.

Answer 30

main-sequence star.

Question 31

True or False: A supernova is a one-time event.

Answer 31

True.

Question 32

What occurs when the outer envelope of a red giant is ejected?

Answer 32

The remaining core is called a white dwarf.

Question 33

What is the process called when lighter elements combine to form heavier elements?

Answer 33

Nucleosynthesis.

Question 34

What is the main sequence?

Answer 34

The stable phase of a star's life cycle where it fuses hydrogen into helium.

Question 35

What is the remaining core of a low mass star called after the red giant is ejected?

Answer 35

White dwarf

Question 36

Which statement is correct about the 'helium flash'?

Answer 36

It marks the end of the red giant stage for a star like the Sun ## Footnote The helium flash does not occur in high-mass stars and releases enough energy to change the degenerate gas in the core to normal gas.

Question 37

What is a characteristic of degenerate gas?

Answer 37

It exists at very high densities

Question 38

Which statement correctly describes pulsating stars?

Answer 38

Their temperature and radius change rhythmically

Question 39

What makes variable stars useful in determining star distances?

Answer 39

Their luminosity can be determined from their pulsation period

Question 40

Which statement about planetary nebulas is correct?

Answer 40

They are shells of glowing gas and dust ejected by dying Sun-like stars

Question 41

What happens in the final moments of the life of high-mass stars?

Answer 41

They have iron cores

Question 42

Which statement about the evolution of high-mass stars is not correct?

Answer 42

High-mass stars have an intense helium flash

Question 43

How do most of the heavier elements up to iron form in the Universe?

Answer 43

By nucleosynthesis in the cores of massive stars

Question 44

Where do most of the elements heavier than iron form?

Answer 44

In supernova explosions

Question 45

How do astronomers test models of stellar evolution?

Answer 45

By comparing the paths in the H-R diagram predicted by models with the H-R diagrams of star clusters

Question 46

Which statement regarding the H-R diagrams of star clusters is not correct?

Answer 46

A very young cluster will have stars that lie in the right hand side and a little above the main-sequence

Question 47

The Sun formed from the gravitational collapse of a cold _____

Answer 47

Interstellar cloud

Question 48

For a star like the Sun, the gravitational collapse of the interstellar cloud lasted for about ____

Answer 48

100 million years

Question 49

_____ is the most significant force that determines the evolution of stars.

Answer 49

Gravity

Question 50

The main source of energy in main-sequence stars is ____.

Answer 50

Fusion of hydrogen to helium

Question 51

A protostar becomes a main-sequence star when it ____.

Answer 51

Starts fusing hydrogen to helium

Question 52

The typical temperature of an interstellar cloud that could collapse to a protostar is about ___.

Answer 52

10 K

Question 53

Protostars are observable by radio telescopes but hard to detect in visible light because they have _____ and are ____.

Answer 53

Low surface temperature; surrounded by dust

Question 54

The protostar stage is ___ for ___ stars.

Answer 54

Shorter; high-mass

Question 55

A ____ is a region where jets of gas from young stars impact and heat the gas surrounding the young star.

Answer 55

Bipolar outflow

Question 56

In the H-R diagram a T Tauri star will be located ___.

Answer 56

On the right-hand side and a little above the main sequence

Question 57

Hydrostatic equilibrium is the balance between _____ of a star and ____.

Answer 57

The internal pressure; gravity

Question 58

A protostar is not _____.

Answer 58

In hydrostatic equilibrium

Question 59

The CNO cycle produces energy by fusing _____ into ____.

Answer 59

Hydrogen; helium

Question 60

The triple alpha process produces energy by fusing ____ into ____.

Answer 60

Helium; carbon

Question 61

After the hydrogen in the star's core is depleted, the core ____ and ____.

Answer 61

Shrinks; heats up

Question 62

High-mass stars become ____ variables when they cross the instability strip, while low-mass stars become ____ variables.

Answer 62

Cepheid; RR Lyrae

Question 63

The light curve of a large Cepheid variable would typically show pulsations in luminosity ____.

Answer 63

Every 60 days

Question 64

The _____ of planetary nebulas is _____.

Answer 64

Shape; usually conical and irregular

Question 65

The last fusion product possible in stars is _________ because _________.

Answer 65

Iron; fusing iron nuclei absorbs energy, it does not liberate it.

Question 66

Supernova remnants:

Answer 66

Both A and C ## Footnote A. Initially expand at thousands of km/s. C. Are important for mixing material into the interstellar medium.

Question 67

Massive stars burn their fuel more rapidly than low-mass stars.

Answer 67

True

Question 68

The gravitational collapse of an interstellar cloud can be triggered by a collision with another cloud.

Answer 68

True

Question 69

When a star like the Sun exhausts its nuclear fuels, it ultimately leaves behind a white dwarf and an expanding planetary nebula.

Answer 69

True

Question 70

Before turning into a white dwarf, a star like the Sun will spend most of its life as a main sequence star.

Answer 70

True

Question 71

Before turning into a white dwarf, a star with a mass 10 times the mass of the Sun will spend most of its life as a main sequence star.

Answer 71

False

Question 72

Convection in the outer layers of a star like the Sun helps mixing and makes more hydrogen available for burning.

Answer 72

True

Question 73

The temperature of a protostar can reach 1500 K but they are very hard to detect in visible wavelengths because of their low luminosity and because they are surrounded by dust.

Answer 73

True

Question 74

Large interstellar clouds can collapse to form several stars approximately at the same time.

Answer 74

True

Question 75

Interstellar clouds usually form groups of protostars that reach the main sequence at the same time.

Answer 75

True

Question 76

Interstellar clouds usually form groups of stars that leave the main sequence at the same time.

Answer 76

True

Question 77

T Tauri stars are never seen in old star clusters.

Answer 77

True

Question 78

Main sequence stars are in hydrostatic equilibrium.

Answer 78

True

Question 79

The proton-proton chain and the CNO cycle produce energy by fusing hydrogen into helium.

Answer 79

True

Question 80

In the core of high-mass stars, the energy is transported by convection currents.

Answer 80

False

Question 81

As stars evolve their temperature and luminosity change, therefore their spectral type (OBAF..., etc.) changes as well.

Answer 81

True

Question 82

The triple alpha process requires higher temperatures to start and produces less energy for a given mass than the proton-proton chain.

Answer 82

True

Question 83

Red giants are huge because their outer layers have expanded to a very low density tenuous atmosphere.

Answer 83

True

Question 84

The larger pulsating stars have a longer pulsation period and higher luminosity.

Answer 84

True

Question 85

The pulsation in the luminosity of variable stars is driven by the changing opacity of their atmosphere.

Answer 85

True