Chapter 12 Stellar Evolution

Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 12Stellar Evolution

Copyright © 2010 Pearson Education, Inc.

“We are stardustBillion year old carbonWe are golden”Woodstock by Joni Mitchell


Units of Chapter 12Leaving the Main SequenceEvolution of a Sun-like StarThe Death of a Low-Mass StarEvolution of Stars More Massive than the SunSupernova ExplosionsObserving Stellar Evolution in Star ClustersThe Cycle of Stellar Evolution


a) red giants.b) pulsars.c) black holes.d) white dwarfs.e) red dwarfs.

Question 1

Stars like our Sun will end their lives as


a) red giants.b) pulsars.c) black holes.d) white dwarfs.e) red dwarfs.

Question 1

Stars like our Sun will end their lives as

Low-mass stars eventually swell into red giants, and their cores later contract

into white dwarfs.


a) in the Big Bang.b) by nucleosynthesis in massive stars.c) in the cores of stars like the Sun.d) within planetary nebulae.e) They have always existed.

Question 2

Elements heavier than hydrogen and Helium were created


Question 2

Elements heavier than hydrogen and helium were created

Massive stars create enormous core

temperatures as red supergiants, fusing helium into carbon, oxygen, and even heavier elements.

a) in the Big Bang.b) by nucleosynthesis in massive stars. c) in the cores of stars like the Sun.d) within planetary nebula e) They have always existed.


Leaving the Main SequenceDuring its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium.


a) its core begins fusing iron.b) its supply of hydrogen is used up.c) the carbon core detonates, and it

explodes as a Type I supernova.d) helium builds up in the core, while the

hydrogen-burning shell expands.e) the core loses all of its neutrinos, so all

fusion ceases.

Question 3

The Sun will evolve away from the main sequence when


a) its core begins fusing iron.b) its supply of hydrogen is used up.c) the carbon core detonates, and it

explodes as a Type I supernova.d) helium builds up in the core, while the

hydrogen-burning shell expands.e) the core loses all of its neutrinos, so all

fusion ceases.

Question 3

The Sun will evolve away from the main sequence when

When the Sun’s core becomes unstable and contracts,

additional H fusion generates extra pressure, and the star will swell into a red giant.


Leaving the Main SequenceEventually, as hydrogen in the core is consumed, the star begins to leave the main sequence. Its evolution from then on depends very much on the mass of the star:Low-mass stars go quietly.High-mass stars go out with a bang!

End times 1End times 2


Evolution of a Sun-like Star

Even while on the main sequence, the composition of a star’s core is changing.


Evolution of a Sun-like StarAs the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse.

Hydrogen begins to fuse outside the core.


Evolution of a Sun-like StarStages of a star leaving the main sequence.


Evolution of a Sun-like StarStage 9: The red giant branch:

As the core continues to shrink, the outer layers of the star expand and cool. It is now a red giant, extending out as far as the orbit of Mercury.Despite its cooler temperature, its luminosity increases enormously due to its large size.


Evolution of a Sun-like StarThe red giant stage on the H–R diagram


a) when T-Tauri bipolar jets shoot out.b) in the middle of the main sequence stage.c) in the red giant stage.d) during the formation of a neutron star.e) in the planetary nebula stage.

Question 4

The helium flash occurs


a) when T-Tauri bipolar jets shoot out.b) in the middle of the main sequence stage.c) in the red giant stage.d) during the formation of a neutron star.e) in the planetary nebula stage.

Question 4

The helium flash occurs

When the collapsing core of a red giant reaches high

enough temperatures and densities, helium can fuse

into carbon quickly – a helium flash.


Evolution of a Sun-like StarStage 10: Helium fusionOnce the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse.The helium flash:Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium.


Evolution of a Sun-like StarStage 10 on the H–R diagramHorizontal branch lasts 10s of millions of years


Evolution of a Sun-like StarStage 11: Back to the giant branch:As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster.

The star is now similar to its condition just as it left the main sequence, except now there are two shells.


Evolution of a Sun-like StarThe star has become a red giant for the second time.


The Death of a Low-Mass StarThis graphic shows the entire evolution of a Sun-like star.Such stars never become hot enough for fusion past carbon to take place.


a) T-Tauri stage.b) emission nebula stage.c) supernova stage.d) nova stage.e) planetary nebula stage.

Question 5

Stars gradually lose mass as they become white dwarfs during the


a) T-Tauri stage.b) emission nebula stage.c) supernova stage.d) nova stage.e) planetary nebula stage.

Question 5

Stars gradually lose mass as they become white dwarfs during the

Low-mass stars forming white dwarfs slowly lose their outer atmospheres,

and illuminate these gases for a relatively short time.


The Death of a Low-Mass StarThere is no more outward fusion pressure being generated in the core, which continues to contract.Stage 12: The outer layers of the star expand to form a planetary nebula.


a) electron degeneracy.b) neutron degeneracy.c) thermal pressure from intense core

temperatures.d) gravitational pressure.e) helium-carbon fusion.

Question 6

The source of pressure that makes a white dwarf stable is


a) electron degeneracy.b) neutron degeneracy.c) thermal pressure from intense core

temperatures.d) gravitational pressure.e) helium-carbon fusion.

Question 6

The source of pressure that makes a white dwarf stable is

Electrons in the core cannot be squeezed infinitely close, and prevent a low-mass star

from collapsing further.


The Death of a Low-Mass StarThe star now has two parts:• A small, extremely dense carbon core• An envelope about the size of our solar

system.The envelope is called a planetary nebula, even though it has nothing to do with planets – early astronomers viewing the fuzzy envelope thought it resembled a planetary system.


The Death of a Low-Mass StarStages 13 and 14: White and black dwarfs:

Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small.It is luminous only due to its high temperature.


The Death of a Low-Mass StarThe small star Sirius B is a white dwarf companion of the much larger and brighter Sirius A.


a) an asteroid.b) a planet the size of Earth.c) a planet the size of Jupiter.d) an object the size of the Moon.e) an object the size of a sugar cube.

Question 7

In a white dwarf, the mass of the Sun is packed into the volume of


a) an asteroid.b) a planet the size of Earth.c) a planet the size of Jupiter.d) an object the size of the Moon.e) an object the size of a sugar cube.

Question 7

In a white dwarf, the mass of the Sun is packed into the volume of

The density of a white dwarf is about a million

times greater than normal solid matter.


The Death of a Low-Mass StarThe Hubble Space Telescope has detected white dwarf stars in globular clusters


The Death of a Low-Mass StarAs the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to glow.


The Death of a Low-Mass StarA nova is a star that flares up very suddenly and then returns slowly to its former luminosity.


The Death of a Low-Mass StarA white dwarf that is part of a semi-detached binary system can undergo repeated novae.


The Death of a Low-Mass StarMaterial falls onto the white dwarf from its main-sequence companion. When enough material has accreted, fusion can reignite very suddenly, burning off the new material. Material keeps being transferred to the white dwarf, and the process repeats.


Evolution of Stars More Massive than the Sun

It can be seen from this H–R diagram that stars more massive than the Sun follow very different paths when leaving the main sequence.



High-mass stars, like all stars, leave the main sequence when there is no more hydrogen fuel in their cores.The first few events are similar to those in lower-mass stars – first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells.



Stars with masses more than 2.5 solar masses do not experience a helium flash – helium burning starts gradually.A 4-solar-mass star makes no sharp moves on the H–R diagram – it moves smoothly back and forth.



The sequence below, of actual Hubble images, shows first a very massive star, then a very unstable red giant star as it emits a burst of light, illuminating the dust around it.

Eta Carinae ~ 100 solar masses



A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate.Its path across the H–R diagram is essentially a straight line – it stays at just about the same luminosity as it cools off.Eventually the star dies in a violent explosion called a supernova.




Supernova ExplosionsA supernova is incredibly luminous, as can be seen from these curves – more than a million times as bright as a nova.


Supernova ExplosionsA supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star.There are two different types of supernovae, both equally common:Type I, which is a carbon-detonation supernova;Type II, which is the death of a high-mass star.


Supernova ExplosionsCarbon-detonation supernova: White dwarf that has accumulated too much mass from binary companionIf the white dwarf’s mass exceeds 1.4 solar masses, electron degeneracy can no longer keep the core from collapsing.Carbon fusion begins throughout the star almost simultaneously, resulting in a carbon explosion.


Supernova ExplosionsThis graphic illustrates the two different types of supernovae.


Question 8

A Type II supernova occurs when

a) hydrogen fusion shuts off.b) uranium decays into lead.c) iron in the core starts to fuse.d) helium is exhausted in the outer layers.e) a white dwarf gains mass.


a) hydrogen fusion shuts off.b) uranium decays into lead.c) iron in the core starts to fuse.d) helium is exhausted in the outer layers.e) a white dwarf gains mass.

Question 8

A Type II supernova occurs when

Fusion of iron does not produce energy or provide pressure; the star’s core collapses immediately, triggering a supernova explosion.


Supernova ExplosionsSupernovae leave remnants – the expanding clouds of material from the explosion.

The Crab Nebula is a remnant from a supernova explosion that occurred in the year 1054.


Which is the Supernova?A or B


a) the number of main sequence stars.b) the ratio of giants to supergiants.c) the luminosity of stars at the turnoff

point.d) the number of white dwarfs.e) supernova explosions.

Question 9

Astronomers determine the age of star clusters by observing


a) the number of main sequence stars.b) the ratio of giants to supergiants.c) the luminosity of stars at the turnoff

point.d) the number of white dwarfs.e) supernova explosions.

Question 9

Astronomers determine the age of star clusters by observing

The H–R diagram of a cluster can indicate its approximate age.

Turnoff point from the main sequence


Observing Stellar Evolution in Star Clusters

The following series of H–R diagrams shows how stars of the same age, but different masses, appear as the cluster as a whole ages.After 10 million years, the most massive stars have already left the main sequence, whereas many of the least massive have not even reached it yet.



After 100 million years, a distinct main-sequence turnoff begins to develop. This shows the highest-mass stars that are still on the main sequence.After 1 billion years, the main-sequence turnoff is much clearer.


a) ending their main-sequence stage.b) also evolving into red giants.c) forming planetary nebulae.d) barely starting to fuse hydrogen.e) starting the nova stage.

Question 10

In a young star cluster, when more massive stars are evolving into red giants, the least massive stars are


a) ending their main-sequence stage.b) also evolving into red giants.c) forming planetary nebulae.d) barely starting to fuse hydrogen.e) starting the nova stage.

Question 10

In a young star cluster, when more massive stars are evolving into red giants, the least massive stars are

More massive stars form much faster, and have much shorter

main-sequence lifetimes. Low-mass stars form more

slowly.



After 10 billion years, a number of features are evident:The red giant, subgiant, asymptotic giant, and horizontal branches are all clearly populated.

White dwarfs, indicating that solar-mass stars are in their last phases, also appear.



This double cluster, h and Persei, must be quite young – its H-R diagram is that of a newborn cluster. Its age cannot be more than about 10 million years.



The Hyades cluster, shown here, is also rather young; its main-sequence turnoff indicates an age of about 600 million years.



This globular cluster, M80, is about 10-12 billion years old, much older than the previous examples.


The Cycle of Stellar EvolutionStar formation is cyclical: stars form, evolve, and die.In dying, they send heavy elements into the interstellar medium.These elements then become parts of new stars.And so it goes.


a) as a protostar.b) as a red giant.c) as a main-sequence star.d) as a white dwarf.e) evolving from type O to type M.

Question 11

A star will spend most of its “shining” lifetime


a) as a protostar.b) as a red giant.c) as a main-sequence star.d) as a white dwarf.e) evolving from type O to type M.

Question 11

A star will spend most of its “shining” lifetime

In the main-sequence stage, hydrogen fuses to helium. Pressure from light and

heat pushing out balances gravitational pressure

pushing inward.


a) they gradually become cooler and dimmer (spectral type O to type M).

b) they gradually become hotter and brighter (spectral type M to type O).

c) they don’t change their spectral type.

Question 12

As stars evolve during their main-sequence lifetime


a) they gradually become cooler and dimmer (spectral type O to type M).

b) they gradually become hotter and brighter (spectral type M to type O).

c) they don’t change their spectral type.

Question 12

As stars evolve during their main-sequence lifetime

A star’s main-sequence characteristics of surface temperature and brightness are based on its mass.

Stars of different initial mass become different spectral types on the main sequence.

Chapter 12 Stellar Evolution

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