Chapter 12 Stellar Evolution

Units of Chapter 12

Leaving the Main Sequence Evolution of a Sun-like Star The Death of a Low-Mass Star Evolution of Stars More Massive than the Sun Supernova Explosions Observing Stellar Evolution in Star Clusters The Cycle of Stellar Evolution Summary of Chapter 12

12.1 Leaving the Main Sequence

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

Eventually, as hydrogen in the core is consumed, the star begins to leave the main sequence. Its evolution from then on depends on the mass of the star: very much Low-mass stars go quietly.

High-mass stars go out with a bang!

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

12.2 Evolution of a Sun-like Star

As 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.

Stages of a star leaving the main sequence.

Stage 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.

The red giant stage on the H –R diagram

Stage 10 : Helium fusion Once 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.

Stage 10 on the H –R diagram

Stage 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.

The star has become a red giant for the second time.

12.3 The Death of a Low-Mass Star This graphic shows the entire evolution of a Sun-like star.

Such stars never become hot enough for fusion past carbon to take place.

There 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.

The 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.

Stages 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 small star Sirius B is a white dwarf companion of the much larger and brighter Sirius A.

The Hubble Space Telescope has detected white dwarf stars in globular clusters

As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to glow.

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

A white dwarf that is part of a semi-detached binary system can undergo repeated novas.

Material 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.

As the sun ages, the chemical composition of its core changes so that it contains a lower percentage of ______ and a greater percentage of ______.

A. helium, hydrogen B. hydrogen, helium C. uranium, lead D. oxygen, carbon

Which of the following is not true of red giants A. their average density is very low.

B. molecules are prominent in their spectra.

C. most are variable stars.

D. most are pre-main sequence stars.

As a one solar mass star evolves to the red giant stage: A. its surface temperature and its luminosity increase.

B. its surface temperature and its luminosity decrease.

C. its luminosity decreases and its surface temperature increases.

D. its luminosity increases and its surface temperature decreases.

After a star's core runs out of fuel, how does the core get to a high enough temperature to ignite the next stage of fusion reactions?

A. by chemical reactions.

B. by other fusion reactions.

C. by gravitational contraction.

D. none of these; the fusion reactions stop.

Which of the following are old stars with no current nuclear reactions?

A. red giants B. main sequence stars C. white dwarfs D. proto stars

12.4 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.

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 .

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

A 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.

Carbon-detonation supernova : White dwarf that has accumulated too much mass from binary companion If 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.

This graphic illustrates the two different types of supernovae.

Supernovae 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.

12.6 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.

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.

12.7 The Cycle of Stellar Evolution

Star 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.

Massive stars have short lifetimes because they A. have little available fuel.

B. can't sustain high enough temperatures.

C. are too large.

D. consume their fuel more rapidly.

Which of the following is the single most important indicator of how a star will evolve?

A. Radius (size).

B. Chemical composition.

C. Mass.

D. Surface temperature.

Which of the following stars is probably oldest?

A. A one solar mass main sequence star.

B. A one solar mass white dwarf.

C. A ten solar mass main sequence star.

D. A ten solar mass red giant.

Which of the following is not a necessary ingredient in the construction of a theoretical star model?

A. A balance between gravity and gas pressure.

B. A knowledge of the star's position and motion in space.

C. A knowledge of the star's mass and chemical composition.

D. A balance between the star's luminosity and the amount of energy generated.

The more massive a main sequence star is, then the A. redder it is.

B. more luminous it is.

C. more time it spends on the main sequence.

D. greater percentage of heavy elements it contains.

When a star dies, it becomes a supernova A. always.

B. only if it is a few times more massive than the sun.

C. only if it includes the whole galaxy.

D. never.

Type I supernovae occur in A. interstellar clouds.

B. binary star systems.

C. young star clusters.

D. globular clusters.

The crab nebula is A. a supernova remnant.

B. a newly forming star.

C. an h-2 region.

D. a black hole.

A type II supernova explosion A. involves a massive, population I star.

B. blows off a large fraction of the star's mass.

C. peaks about a month after the explosion begins.

D. all of the above.

E. none of the above.

Stellar remnants with masses between 1.4 and 3 solar masses will be A. white dwarfs.

B. neutron stars.

C. black holes.

D. planetary nebulae.

Summary of Chapter 12

•

Once hydrogen is gone in the core, a star burns hydrogen in the surrounding shell. The core contracts and heats; the outer atmosphere expands and cools.

•

Helium begins to fuse in the core, as a helium flash. The star expands into a red giant as the core continues to collapse. The envelope blows off, leaving a white dwarf to gradually cool.

•

Nova results from material accreting onto a white dwarf from a companion star.

Summary of Chapter 12, cont.

•

Massive stars become hot enough to fuse carbon, then heavier elements, all the way to iron. At the end, the core collapses and rebounds as a Type II supernova.

•

Type I supernova is a carbon explosion, occurring when too much mass falls onto a white dwarf.

•

All heavy elements are formed in stellar cores or in supernovae.

•

Stellar evolution can be understood by observing star clusters.