astronomy: horizons 10th edition

The Deaths of Stars

Astronomy: Horizons10th edition

Michael Seeds

The Deaths of Stars

• Gravity is patient—so patient it can kill stars. – All over the sky, astronomers find beautiful nebulae

puffed gently into space by dying stars.

The Deaths of Stars

• In contrast, astronomers occasionally see a new star appear in the sky, grow brighter, then fade away after a few weeks or a year. – A nova—what seems to be a new star in the sky—is

produced by the eruption of a very old, dying star.– A supernova—a particularly luminous and longlasting

nova—is caused by the violent, explosive death of a star.

– Modern astronomers find a few novae each year, but supernovae are so rare that there are only one or two supernovae each century in our galaxy.

The Deaths of Stars

• Astronomers know that supernovae occur because they occasionally flare in other galaxies and because telescopes reveal the shattered remains of these titanic explosions.

The Deaths of Stars

• The mass of a star is critical in determining its fate.– Massive stars use up their nuclear fuel at a furious rate

and die after only a few million years—whereas the lowest-mass stars use their fuel sparingly and may be able to live hundreds of billions of years.

– Massive stars can die in violent explosions that appear as supernovae—but lower-mass stars die quiet deaths.

The Deaths of Stars

• To follow the evolution of stars to their graves, you need to sort them into three categories according to their masses:– Low-mass red dwarfs– Medium-mass sunlike stars– Massive upper-main-sequence stars

• These stars lead dramatically different lives and die in different ways.

The Deaths of StarsGiant Stars

• A main-sequence star generates its energy by nuclear fusion reactions that combine hydrogen to make helium. – The period during which the star fuses hydrogen

lasts a long time, and the star remains on the main sequence for 90 percent of its total existence as an energy-generating star.

The Deaths of Stars

• When the hydrogen is exhausted, however, the star begins to evolve rapidly.– It swells into a giant star and then begins to fuse

helium.– However, it can remain in this giant stage for

only about 10 percent of its total lifetime—then, it must die.

– The giant-star stage is the first step in the death of a star.

Giant Stars Giant Stars

The Deaths of StarsExpansion into a Giant

• The nuclear reactions in a main-sequence star’s core fuse hydrogen to produce helium.– As the core is cooler than 100,000,000 K, helium

cannot fuse in nuclear reactions.– So, it accumulates at the star’s center—like ashes

in a fireplace.

The Deaths of Stars

• Initially, this helium ash has little effect on the star.

• As hydrogen is exhausted and the stellar core becomes almost pure helium, the star loses the ability to generate the nuclear energy that supports it.– As it is the energy generated at the center that

opposes gravity and supports the star, the core begins to contract as soon as energy generation starts to die down.

Expansion into a Giant Expansion into a Giant

The Deaths of Stars

• Although the core of helium ash cannot generate nuclear energy, it does grow hotter because it contracts and converts gravitational energy into thermal energy.– The rising temperature heats the unprocessed

hydrogen just outside the core—hydrogen that was never before hot enough to fuse.

Expansion into a Giant

The Deaths of Stars

• When the temperature of the surrounding hydrogen becomes high enough, hydrogen fusion begins in a spherical layer or shell around the exhausted core of the star.– Like a grass fire burning outward from an exhausted

campfire, the hydrogen-fusion shell creeps outward—leaving helium ash behind and increasing the mass of the helium core.


The Deaths of Stars

• The flood of energy produced by the hydrogen-fusion shell pushes toward the surface, heating the outer layers of the star and forcing them to expand dramatically.


The Deaths of Stars

• Stars like the sun become giant stars of 10 to 100 times the diameter of the sun.

• The most massive stars become supergiants some 1,000 times larger than the sun. – This explains the large diameters and low densities of

the giant and supergiant stars that you have learned about.

– Now, you understand that these stars were once normal main-sequence stars, but they expanded to large size and low density when hydrogen shell fusion began.


The Deaths of Stars

• The expansion of the envelope dramatically changes the star’s location location in the H–R diagram. – Just as contraction

heats a star, expansion cools it.


The Deaths of Stars

– As the outer layers of gas expand, energy is absorbed in lifting and expanding the gas.

– The loss of that energy lowers the temperature of the gas.

– Thus, the point that represents the star in the H–R diagram moves quickly to the right (in less than a million years for a star of 5 solar masses).


The Deaths of Stars

– A massive star moves to the right across the top of the diagram and becomes a supergiant—whereas a medium-mass star like the sun becomes a red giant.

– As the radius of a giant star continues to increase, the enlarging surface area makes the star more luminous, moving its point upward in the diagram.


The Deaths of StarsDegenerate Matter

• Although the hydrogen-fusion shell can force the envelope of the star to expand, it cannot stop the contraction of the helium core.– As the core is not hot enough to fuse helium, gravity

squeezes it tighter, and it becomes very small.– If you represent the helium core of a giant star with a

baseball, the outer envelope of the star would be about the size of a baseball stadium.

– Yet the core would contain about 12 percent of the star’s mass compressed to very high density.

The Deaths of StarsDegenerate Matter

• When gas is compressed to such extreme densities, it begins to behave in astonishing ways that can alter the evolution of the star. – To follow the story of stellar evolution, you must

consider the behavior of gas at extremely high densities.

The Deaths of Stars

• Normally, the pressure in a gas depends on its temperature.– The hotter the gas is, the faster its particles

move and the more pressure it exerts.• However, the gas inside a star is

ionized.– So, there are two kinds of particles—atomic

nuclei and free electrons.

Degenerate Matter

The Deaths of Stars

• When a gas is so dense that the electrons are not free to change their energy, astronomers call it degenerate matter.

• Although it is a gas, it has two peculiar properties that can affect the star.

Degenerate Matter

The Deaths of Stars

• First, the degenerate gas resists compression. – To compress the gas, you must push against the

moving electrons—and changing their motion means changing their energy.

– That is why degenerate matter, though still a gas, is harder to compress than the toughest hardened steel.

Degenerate Matter

The Deaths of Stars

• Second, the pressure of degenerate gas does not depend on temperature. – To know why, note that the pressure depends on the

speed of the electrons, which cannot be changed without tremendous effort.

– The temperature, however, depends on the motion of all the particles in the gas—both electrons and nuclei.

Degenerate Matter

The Deaths of Stars

• These two properties of degenerate matter become important when stars end their main-sequence lives and approach their final collapse. – Eventually, many stars collapse into white dwarfs—

and you will discover that these tiny stars are made of degenerate matter.

– Long before that, though, the cores of many giant stars become so dense that they are degenerate—a situation that can produce a cosmic bomb.

Degenerate Matter

The Deaths of StarsHelium Fusion

• Hydrogen fusion in main-sequence stars leaves behind helium ash, which cannot begin fusing because the temperature is too low.

• Helium nuclei have a positive charge twice that of a proton.– To overcome the repulsion between nuclei, they must

collide at a velocity higher than that at which hydrogen fuses.

The Deaths of Stars

• The temperature for hydrogen fusion is too cool to fuse helium.

• As a star becomes a giant and begins fusing hydrogen in a shell, the inert core of helium contracts and grows hotter.– It may even become degenerate.– However, when it finally reaches a temperature of

100,000,000 K, it begins to fuse helium nuclei to make carbon.

Helium Fusion

The Deaths of Stars

• How a star begins helium fusion depends on its mass. – Stars more massive than about 3 solar masses

contract rapidly, their helium-rich cores heat up, and helium fusion begins gradually.

– Less-massive stars evolve more slowly, and their cores contract so much the gas becomes degenerate.

– On Earth, a teaspoon of the gas would weigh more than an automobile.

Helium Fusion

The Deaths of Stars

• In this degenerate matter, the pressure does not depend on temperature.– That means the pressure–temperature thermostat does

not regulate energy production. – When the temperature becomes hot enough, helium

fusion begins to make energy and the temperature rises.– However, pressure does not increase because the gas

is degenerate.

Helium Fusion

The Deaths of Stars

• The higher temperature increases the helium fusion even further.– The result is a runaway explosion called the

helium flash in which, for a few minutes, the core of the star can generate more energy per second than does an entire galaxy.

Helium Fusion

The Deaths of Stars

• Although the helium flash is sudden and powerful, it does not destroy the star. – If you were observing a giant star as it experienced

the helium flash, you would probably see no outward evidence of the eruption.

– The helium core is quite small and all the energy of the explosion is absorbed by the distended envelope.

Helium Fusion

The Deaths of Stars

• In addition, the helium flash is a very short-lived event in the life of a star.– In a matter of minutes

to hours, the core of the star becomes so hot it is no longer degenerate, the pressure–temperature thermostat brings the helium fusion under control, and the star proceeds to fuse helium steadily in its core.

Helium Fusion

The Deaths of Stars

• The sun will experience a helium flash.• Stars less massive than about 0.4 solar

mass never get hot enough to ignite helium.

• Stars more massive than 3 solar masses ignite helium before their contracting cores become degenerate.

Helium Fusion

The Deaths of Stars

• Whether a star experiences a helium flash or not, the ignition of helium in the core changes the structure of the star. – The star now makes energy in its helium-fusion core

and in its hydrogen fusion shell. – The energy flowing outward from the core halts its

contraction of the core.– The distended envelope of the star contracts and

grows hotter.

Helium Fusion

The Deaths of Stars

– Thus, the point that represents the star in the H–R diagram moves downward and back to the left toward the hot side of the diagram.

Helium Fusion

The Deaths of Stars

• Helium fusion produces carbon and oxygen atoms in the core, atoms that require much higher temperatures to fuse. – Thus, as the helium fuel is used up, carbon and oxygen

atoms accumulate in an inert core. – Once again, the core contracts and heats up, and a

helium-fusion shell ignites below the hydrogen-fusion shell.

Helium Fusion

The Deaths of Stars

– The star now makes energy in two fusion shells, it quickly expands, and its surface cools once again.

– The point that represents the star in the H–R diagram moves back to the right, completing a loop.

Helium Fusion

The Deaths of Stars

• The stars in a star cluster all formed at about the same time and from the same cloud of gas.– So, they must be about the same age and

composition.• The differences you see among stars in

a cluster must arise from differences in mass.– That makes stellar evolution visible.

Star Clusters: Evidence of Evolution

The Deaths of Stars

• There are three points to note about star clusters.

• One, there are two kinds of star clusters.– However, they are similar in the way their stars evolve.


The Deaths of Stars

• You can estimate the age of a cluster by observing the distribution of the points that represent its stars in the H–R diagram.


The Deaths of Stars

• Finally, the shape of a cluster’s H–R diagram is governed by the evolutionary path the stars take. – By comparing clusters

of different ages, you can visualize how stars evolve almost as if you were watching a film of a star cluster evolving over billions of years.


The Deaths of Stars

• Were it not for star clusters, astronomers would have little confidence in the theories of stellar evolution. – Star clusters make evolution visible and assure

astronomers that they really do understand how stars are born, live, and die.


The Deaths of StarsBuilding Scientific Arguments

• How are giant stars different from main-sequence stars?

The Deaths of Stars

• A giant star has used up the hydrogen in its core, and it can no longer maintain main-sequence stability. – Hydrogen fusion continues in a shell—but the core of

helium contracts while the envelope of the star expands and cools.

– Later, helium fusion will begin in the core of the star and will produce an inert carbon-oxygen core and a helium-fusion shell below the hydrogen-fusion shell.

– Thus, the internal structure of a giant star is dramatically different from the simple structure of a main-sequence star.

Building Scientific Arguments

The Deaths of Stars

• Consider a 5-solar-mass star and the sun.– They are both spheres of hot gas held together by

gravity and supported by internal pressure. – They look quite similar, and both will become giant

stars.


The Deaths of StarsDeaths of Lower-Main-Sequence Stars

• Contracting stars heat up by converting gravitational energy into thermal energy.

• Low-mass stars have little gravitational energy.– So, when they contract, they can’t get very hot. – This limits the fuels they can ignite.

The Deaths of Stars

• You have learned that protostars less massive than 0.08 solar mass cannot even get hot enough to ignite hydrogen.– This section will concentrate on stars more

massive than 0.08 solar mass but no more than a few times the mass of the sun.

Deaths of Lower-Main-Sequence Stars

The Deaths of Stars

• Structural differences divide the lower-main-sequence stars into two subgroups:– Very-low-mass red dwarfs– Medium-mass stars such as the sun


The Deaths of Stars

• The critical difference between the two groups is the extent of interior convection. – If the star is convective, fuel is constantly

mixed.– The resulting evolution is drastically altered.


The Deaths of StarsRed Dwarfs

• Stars less massive than about 0.4 solar mass—the red dwarfs— have two advantages over more massive stars.

• First, they have very small masses.– Thus, they have very little weight to support. – Their pressure–temperature thermostats are set low,

and they consume their hydrogen fuel very slowly.

The Deaths of Stars

• Second, they are totally convective—that is, they are stirred by circulating currents of hot gas rising from the interior and cool gas sinking inward. – This means the stars are mixed like a pot of soup

that is constantly stirred as it cooks. – Hydrogen is consumed and helium accumulates

uniformly throughout the star.– Thus, the star is not limited by the fuel in its core—it

can use all its hydrogen to prolong its life on the main sequence.

Red Dwarfs

The Deaths of Stars

• As a red dwarf is mixed by convection, it cannot develop an inert helium core surrounded by unprocessed hydrogen. – It can never ignite a hydrogen shell and thus cannot

become a giant star.– Rather, nuclear fusion converts hydrogen into helium

—which cannot fuse because the star cannot get hot enough.

Red Dwarfs

The Deaths of Stars

• What astronomers know about stellar evolution indicates that these red dwarfs should use up nearly all their hydrogen and live very long lives on the lower main sequence.– They could survive for a hundred billion years or

more.

Red Dwarfs

The Deaths of Stars

• Of course, astronomers can’t test this part of their theories because the universe is only about 13.7 billion years old.– So, not a single red dwarf has died of old age

anywhere in the universe. – Every red dwarf that has ever been born is still

shining today.

Red Dwarfs

The Deaths of StarsMedium-Mass Stars

• Stars like the sun can ignite hydrogen and helium and become giants.

• However, if they contain less than 4 solar masses, they cannot get hot enough to ignite carbon—the next fuel after helium.– When they reach that impasse, they collapse and

become white dwarfs.

The Deaths of Stars

• There are two keys to the evolution of these sunlike stars:– Lack of complete mixing – Mass loss

Medium-Mass Stars

The Deaths of Stars

• The interiors of medium-mass stars are not completely mixed. – Stars of 1.1 solar masses or less have no convection

near their centers.– So, they are not mixed at all.

Medium-Mass Stars

The Deaths of Stars

– Stars with a mass greater than 1.1 solar masses have small zones of convection at their centers.

– However, this mixes no more than about 12 percent of the star’s mass.

Medium-Mass Stars

The Deaths of StarsPlanetary Nebulae

• When a medium-mass star like the sun becomes a distended giant, its atmosphere becomes so cool it becomes more opaque.– Light has to push against it to escape.

• At the same time, the fusion shells become so thin they are unstable and they begin to flare—which pushes the atmosphere outward.

The Deaths of Stars

• An aging giant can expel its outer atmosphere in repeated surges to form one of the most interesting objects in astronomy—a planetary nebula.– It is called so because it looks

like the greenish-blue disk of a planet such as Uranus or Neptune.

Planetary Nebulae

The Deaths of Stars

• In fact, a planetary nebula has nothing to do with a planet. – It is composed of ionized gases expelled by a

dying star.

Planetary Nebulae

The Deaths of Stars

• There are four points to note about planetary nebulae.

Planetary Nebulae

The Deaths of StarsPlanetary Nebulae

The Deaths of Stars

• Repeated expulsions of expanding shells and oppositely directed jets—much like bipolar flows—produce many of the asymmetries seen in planetary nebulae.

Planetary Nebulae

The Deaths of Stars

• Finally, the star itself must contract into a white dwarf.

Planetary Nebulae

The Deaths of Stars

• Most astronomy books give that the sun will form a planetary nebula.

• That may not happen, though. – To light up a planetary nebula, a star must become a

white dwarf with a temperature of at least 25,000 K. – Mathematical models show that a collapsing star of

less than 0.55 solar mass can take as long as a million years to heat up enough to ionize its nebulae.

– By that time, the expelled gases are long gone.

Planetary Nebulae

The Deaths of Stars

• Also, some research suggests that a star needs a binary companion to speed up its spin and make it eject a planetary nebula. – The sun, of course, has no binary companion. – This is an area of active research, and there are

no firm conclusions.

Planetary Nebulae

The Deaths of Stars

• The medium-mass stars die by ejecting gas into space and contracting into white dwarfs.

• This suggests another way you can search for evidence of the deaths of medium-mass stars.– You can study white dwarfs.

Planetary Nebulae

The Deaths of Stars

• White dwarfs are the second most common kind of star. –Only red dwarfs are more abundant.

White Dwarfs

The Deaths of Stars

• White dwarfs are the remains of medium-mass stars that fused hydrogen and helium, failed to ignite carbon, drove away their outer layers to form planetary nebulae, and collapsed and cooled to form white dwarfs. – The billions of white dwarfs in the Milky Way must

be the remains of medium-mass stars.

White Dwarfs

The Deaths of Stars

• The first white dwarf discovered was the faint companion to Sirius. – In that visual binary system, the bright star is Sirius A.– The white dwarf, Sirius B, is 10,000 times fainter than

Sirius A.

White Dwarfs

The Deaths of Stars

• The orbital motions of the stars inform you that the white dwarf ’s mass is 0.98 solar mass.

• Its blue-white color informs you that its surface is hot—about 45,000 K.

White Dwarfs

The Deaths of Stars

• Although it is very hot, it has a very low luminosity.

• So, it must have a small surface area.– It is about the size of Earth.

White Dwarfs

The Deaths of Stars

• Dividing its mass by its volume reveals that it is very dense—over 2 x 106 g/cm3. – On Earth, a teaspoonful of Sirius B material

would weigh more than 11 tons.– Thus, basic observations and simple physics lead

to the conclusion that white dwarfs are astonishingly dense.

White Dwarfs

The Deaths of Stars

• A normal star is supported by energy flowing outward from its core.

• A white dwarf, however, cannot generate energy by nuclear fusion.– It has exhausted its hydrogen and helium fuels

and produced carbon.

White Dwarfs

The Deaths of Stars

• As the star collapses into a white dwarf, it converts gravitational energy into thermal energy.

• Its interior becomes very hot.– Nevertheless, it cannot get hot enough to

ignite carbon fusion.

White Dwarfs

The Deaths of Stars

• The star contracts until it becomes degenerate.

• Although a tremendous amount of energy flows out of the hot interior, it is not the energy flow that supports the star. – It is supported against its own gravity by the inability

of its degenerate electrons to pack into a smaller volume.

White Dwarfs

The Deaths of Stars

• The interior of a white dwarf is mostly carbon and oxygen ions immersed in a whirling storm of degenerate electrons. – Theory predicts that, as the star cools, these ions will

lock together to form a crystal lattice.– So, there may be some truth in thinking of aging white

dwarfs as great crystals of carbon and oxygen.

White Dwarfs

The Deaths of Stars

• Near the surface, where the pressure is lower, a layer of ionized gases makes up a hot atmosphere.– A 150-lb human on the surface of a white dwarf would

weigh 50 million pounds.

• That strong gravity pulls the atmosphere down into a shallow layer. – If Earth’s atmosphere were equally shallow, people on

the top floors of skyscrapers would have to wear space suits.

White Dwarfs

The Deaths of Stars

• Clearly, a white dwarf is not a true star.– It generates no nuclear energy.– It is almost totally degenerate.– Except for a thin layer at its surface, it contains no

gas.

• Instead of calling it a ‘star,’ you can call it a ‘compact object.’

White Dwarfs

The Deaths of Stars

• A white dwarf ’s future is bleak.– As it radiates energy into space, its temperature

gradually falls.– It cannot shrink any smaller, though, because its

degenerate electrons cannot get closer together.

• This degenerate matter is a very good thermal conductor.– So, heat flows to the surface and escapes into space.– The white dwarf gets fainter and cooler, moving

downward and to the right in the H–R diagram.

White Dwarfs

The Deaths of Stars

• As the white dwarf contains a tremendous amount of heat, it needs billions of years to radiate that heat through its small surface area.

• Eventually, such objects may become cold and dark, so-called black dwarfs. – The Milky Way is not old enough to contain black

dwarfs. – The coolest white dwarfs in the galaxy are about the

temperature of the sun.

White Dwarfs

The Deaths of Stars

• Stars do lose mass. – Observations provide clear evidence that young stars

have strong stellar winds.– Aging giants and supergiants also lose mass.

White Dwarfs

The Deaths of Stars

• This suggests that stars bigger than the Chandrasekhar limit can eventually die as white dwarfs if they reduce their mass. – Stars more massive than 4 solar masses can ignite

carbon in their cores.– That, however, may not prevent them from becoming

white dwarfs.

White Dwarfs

The Deaths of Stars

• Theoretical models show that stars that begin life with as much as 8 solar masses should lose mass fast enough to reduce their mass so low they can collapse to form white dwarfs with masses below 1.4 solar masses. – With mass loss, a wide range of medium-mass stars

can eventually die as white dwarfs.

White Dwarfs

The Deaths of StarsBuilding Scientific Arguments

• What produced the energy that makes a planetary nebula glow?– To build this scientific argument, you must think

about the earlier history of the central star.

The Deaths of Stars

• The low-density gas in a planetary nebula emits light at certain wavelengths to produce an emission spectrum.

• The gas of the nebula is excited by ultraviolet radiation from a hot central object in the planetary nebula. – That object is hot because it was once the interior of

the star that ejected its surface to create the planetary nebula.


The Deaths of Stars

• Furthermore, the inside of the star was hot because nuclear fusion reactions in and around the core fused hydrogen into helium and then helium into carbon. – The light you see coming from a planetary nebula is

energy that was originally locked inside the nuclei of hydrogen and helium atoms.


The Deaths of StarsEvolution of Binary Systems

• Stars in binary systems can evolve independently of each other if they orbit at a large distance from each other.– One of the stars can swell into a giant and collapse

without disturbing its companion.

• However, some binary stars are as close to each other as 0.1 AU.– When one of those stars begins to swell into a giant, its

companion can suffer in peculiar ways.

The Deaths of Stars

• These interacting binary stars are interesting in their own right.– The stars share a complicated history and can

evolve to experience strange and violent phenomena.

• However, they are also important because they can help astronomers understand the ultimate fate of stars and certain observed phenomena—such as the temporary appearance of new stars in the sky.

Evolution of Binary Systems

The Deaths of StarsMass Transfer

• Binary stars can sometimes interact by transferring mass from one star to the other. – Of course, the gravitational field of each star

holds its mass together.

The Deaths of Stars

• However, the gravitational fields of the two stars, combined with the rotation of the binary system, define a dumbbell-shaped volume around the pair of stars called the Roche lobes.

• The surface of the volume is called the Roche surface.– Matter inside a star’s

Roche lobe is gravitationally bound to the star.

Mass Transfer

The Deaths of Stars

• The size of the Roche lobes depends on the masses of the stars and on the distance between the stars.– If the stars are far apart, the lobes are very large—

and the stars easily control their own mass.– If the stars are close together, the lobes are small and

can interfere with the evolution of the stars.

Mass Transfer

The Deaths of Stars

• The Lagrangian points are places in the orbital plane of a binary star system where a bit of matter can reach stability.

Mass Transfer

The Deaths of Stars

• For astronomers, the most important of these points is the inner Lagrangian point where the two Roche lobes meet.– If matter can leave a star and reach this point, it can

then flow into the other star. – Thus, the inner Lagrangian point is the connection

through which the stars can transfer matter.

Mass Transfer

The Deaths of Stars

• In general, there are only two ways matter can escape from a star and reach the inner Lagrangian point.

• First, if a star has a strong stellar wind, some of the gas blowing away from the star can pass through the inner Lagrangian point and be captured by the other star.– Mass transfer driven by a stellar wind tends to be

slow.

Mass Transfer

The Deaths of Stars

• Second, if an evolving star expands so far that it fills its Roche lobe—which can occur if the stars are close together and the lobes are small—then matter can overflow through the inner Lagrangian point onto the surface of the other star.– Mass can be transferred rapidly by an expanding

star.

Mass Transfer

The Deaths of StarsEvolution with Mass Transfer

• Mass transfer between stars can affect the evolution of the stars in surprising ways.

The Deaths of Stars

• In some binary systems, the less massive star has become a giant, whereas the more massive star is still on the main sequence.

Evolution with Mass Transfer

The Deaths of Stars

• This is called the Algol paradox—after the binary system Algol.


The Deaths of Stars

• Mass transfer explains how this could happen.

• Imagine a binary system that contains a 5-solar-mass star and a 1-solar-mass companion. – The two stars formed at the same time.– So, you might expect the higher-mass star to

evolve faster and leave the main sequence first.


The Deaths of Stars

– When it expands into a giant, however, it can fill its Roche lobe and transfer matter to the low-mass companion.

– The higher-mass star can lose mass and evolve into a lower-mass star.

– The companion can gain mass and become a higher-mass star still on the main sequence.

– Thus, you could find a system such as Algol containing a 5-solar-mass main-sequence star and a 1-solar-mass giant.


The Deaths of Stars

• The first four frames of the figure show how mass transfer produces a system like Algol.

• The last frame shows an additional stage in which the giant star has collapsed to form a white dwarf.– The more massive companion has

expanded and is transferring matter back to the white dwarf.


The Deaths of Stars

• Such systems can become the site of tremendous explosions. – To see how this can happen, you must consider

how mass falls into a star.


The Deaths of StarsAccretion Disks

• Matter flowing from one star to another cannot fall directly into the star.– Due to conservation of angular momentum, it must

flow into a whirling disk around the star.

• Angular momentum refers to the tendency of a rotating object to continue rotating.– All rotating objects possess some angular momentum.– In the absence of external forces, an object maintains

(conserves) its total angular momentum.

The Deaths of Stars

• An ice skater takes advantage of conservation of angular momentum by starting a spin slowly with arms extended and then drawing them in.– As her mass becomes concentrated closer to her axis

of rotation, she spins faster.

Accretion Disks

The Deaths of Stars

• Mass transferred through the inner Lagrangian point in a binary system toward a white dwarf must conserve its angular momentum. – Thus, it must flow into a rapidly rotating whirlpool

called an accretion disk around the white dwarf.

Accretion Disks

The Deaths of Stars

• Two important things happen in an accretion disk.

• First, the gas in the disk grows very hot due to friction and tidal forces, the same kind of gravitational forces that make Earth’s oceans ebb and flow.

Accretion Disks

The Deaths of Stars

• The disk also acts as a brake—ridding the gas of its angular momentum and allowing it to fall into the white dwarf.– The temperature of the gas in the inner parts of the

accretion disk can exceed a million Kelvin--causing the gas to emit X rays.

– Also, the matter falling inward from the disk can cause a violent explosion if it accumulates on a white dwarf.

Accretion Disks

The Deaths of StarsNovae

• You have learned that ‘nova’ refers to a new star that appears in the sky for a while and then fades away.

• Modern astronomers know that a nova is not a new star but an old star flaring up. – After a nova fades, astronomers can photograph the

spectrum of the remaining faint point of light. – Invariably, they find a short-period spectroscopic

binary containing a normal star and a white dwarf. – A nova is evidently an explosion that involves a white

dwarf.

The Deaths of Stars

• Observational evidence reveals how nova explosions occur.

• As the explosion begins, spectra show blueshifted absorption lines.– This informs you that the gas is dense and

coming toward you at a few thousand kilometers per second.

Novae

The Deaths of Stars

• After a few days, the spectral lines change to emission lines.– This informs you that the gas has thinned.

• However, the blueshifts remain.– This shows that a cloud of debris has been

ejected into space.

Novae

The Deaths of Stars

• Nova explosions occur when mass transfers from a normal star through the inner Lagrangian point into an accretion disk around the white dwarf.– As the matter loses its angular momentum in the

accretion disk, it settles inward onto the surface of the white dwarf and forms a layer of unused nuclear fuel—mostly hydrogen.

– As the layer deepens, it becomes denser and hotter until the hydrogen fuses in a sudden explosion that blows the surface off of the white dwarf.

Novae

The Deaths of Stars

• Although the expanding cloud of debris contains less than 0.0001 solar mass, it is hot and its expanding surface area makes it very luminous.– Nova explosions can be 100,000 times more

luminous than the sun.• As the debris cloud expands, cools, and

thins over a period of weeks and months, the nova fades from view.

Novae

The Deaths of Stars

• The explosion of its surface hardly disturbs the white dwarf and its companion star. – Mass transfer quickly resumes, and a new layer

of fuel begins to accumulate.– How fast the fuel builds up depends on the rate

of mass transfer.

Novae

The Deaths of StarsNovae

• Accordingly, you can expect novae to repeat each time an explosive layer accumulates.– Many novae take thousands of years to build an

explosive layer, but some take only decades.

The Deaths of Stars

• The binary star Algol contains a lower-mass giant star and a more massive main-sequence star. – As the two stars must have formed

together, they must be the same age. – Then, the more massive star should

have evolved first and left the main sequence.

– However, in binary systems such as Algol, the lower-mass star has left the main sequence first, or so it seems.


The Deaths of Stars

• You can understand this paradox if you add mass transfer to your theory of stellar evolution. – The first star to leave the main sequence and become

a giant star must be the more massive star.– If the stars are close together, however, the star that is

initially more massive can fill its Roche lobe and transfer mass to its companion.


The Deaths of Stars

– The companion can grow more massive, and the giant can grow less massive.

– The lower-mass giant star you see in the system may have originally been more massive and may have evolved away from the main sequence first—leaving its companion behind to increase in mass as it decreased.


The Deaths of Stars

• Low- and medium-mass stars die relatively quietly as they exhaust their hydrogen and helium and then drive away their surface layers to form planetary nebulae.

The Deaths of Massive Stars

The Deaths of StarsThe Deaths of Massive Stars

• In contrast, massive stars live spectacular lives and destroy themselves in violent explosions.

The Deaths of StarsNuclear Fusion in Massive Stars

• Stars on the upper main sequence have too much mass to die as white dwarfs.

• However, their evolution begins in much the same way as it does in their lower-mass cousins. – They consume the hydrogen in their cores and ignite

hydrogen shells.– As a result, they expand and become giants or, for the

most massive stars, supergiants. – Their cores contract and fuse helium first in the core

and then in a shell—producing a carbon–oxygen core.

The Deaths of Stars

• Unlike medium-mass stars, the massive stars can get hot enough to begin carbon fusion at a temperature of about 1 billion Kelvin.

• Carbon fusion produces more oxygen and neon.– However, as soon as the carbon is exhausted in

the core, the core contracts and carbon ignites in a shell.

Nuclear Fusion in Massive Stars

The Deaths of Stars

• The pattern of core ignition and shell ignition continues with fuel after fuel, and the star develops a layered structure.– There is a hydrogen-fusion shell above a helium-fusion

shell above a carbon-fusion shell, and so on.– After carbon fuses,

oxygen, neon, and magnesium fuse to make silicon and sulfur.

– Then, the silicon fuses to make iron.


The Deaths of Stars

• The fusion of these nuclear fuels goes faster and faster as the massive star evolves.

• You have learned that massive stars must consume their fuels rapidly to support their great weight.

• However, other factors also cause the heavier fuels like carbon, oxygen, and silicon to fuse at increasing speed.


The Deaths of Stars

• For one thing, the amount of energy released per fusion reaction decreases as the mass of the fusing atom increases.– To support its weight, a star must fuse oxygen

much faster than it fused hydrogen.


The Deaths of Stars

• Also, there are fewer nuclei in the core of the star by the time heavy nuclei begin to fuse. – Four hydrogens make a helium nucleus, and

three heliums make a carbon.– So, there are 12 times fewer nuclei of carbon

available for fusion than there were hydrogen.


The Deaths of Stars

• Thus, the heavy elements are used up and fusion goes very quickly in massive stars.– Hydrogen fusion can last 7 million years in a 25-solar-

mass star, but that same star will fuse its oxygen in 6 months and its silicon in a day.


The Deaths of StarsSupernova Explosions of Massive Stars

• Theoretical models of evolving stars combined with nuclear physics allow astronomers to describe what happens inside a massive star when the last nuclear fuels are exhausted.– It begins with iron nuclei and ends in cosmic

violence.

The Deaths of Stars

• Silicon fusion produces iron—the most tightly bound of all atomic nuclei. – Nuclear fusion releases

energy by combining less tightly bound nuclei into a more tightly bound nucleus.

– However, iron is the limit.

Supernova Explosions of Massive Stars

The Deaths of Stars

– Once the gas in the core of the star has been converted to iron, there are no nuclear reactions that can combine iron nuclei and release energy.

– Thus, the iron core is a dead end in the evolution of a massive star.


The Deaths of Stars

• As a star develops an iron core, energy production declines, and the core contracts.

• For nuclei less massive than iron, such contraction heats the gas and ignites new fusion fuels.


The Deaths of Stars

• Nuclear reactions involving iron tend to remove energy from the core in two ways.

• First, the iron nuclei begin capturing electrons and breaking into smaller nuclei.– The degenerate electrons helped support the core.– So, the loss of the electrons allows the core to contract

even faster.


The Deaths of Stars

• Second, temperatures are so high that the average photon is a high-energy gamma ray.– These gamma rays are absorbed by iron and

other atomic nuclei, which break into smaller fragments.


The Deaths of Stars

• The removal of the gamma rays allows the core to contract even faster.

• With the loss of the electrons and the gamma rays, the core of the star collapses inward in less than a tenth of a second.


The Deaths of Stars

• The collapse happens so rapidly that the most powerful computers are unable to predict the details.

• Thus, models of supernova explosions contain many approximations. – Nevertheless, they predict that the collapsing core of

the star must quickly become a neutron star or a black hole, while the envelope of the star is blasted apart in a supernova explosion.


The Deaths of Stars

• Although the core of the star cannot generate energy by nuclear fusion, it can draw on the tremendous energy stored in its gravitational field.

• As the core contracts, the temperature shoots up, though not by enough to stop the collapse.


The Deaths of Stars

• Similarly, as the inner core falls inward, a shock wave—a ‘traffic jam’—develops and begins to move outward.

• The shock wave moves outward through the star aided by two additional sources of energy.


The Deaths of Stars

• First, when the iron nuclei in the core are disrupted, they produce a flood of neutrinos. – For a short time, the core produces more energy per

second than all the stars in all the visible galaxies in the universe.

– 99 percent of that energy is in the form of neutrinos. – This flood of neutrinos carries large amounts of

energy out of the core—allowing the core to collapse even further—and helps heat the gas outside the core and accelerate the outward-bound shock wave.


The Deaths of Stars

• Also, the torrent of energy flowing out of the core triggers tremendous turbulence, and intensely hot gas rushes outward from the interior.


The Deaths of Stars

• Again, this rising hot gas carries energy out into the envelope and helps drive the shock wave outward.– Within a few hours,

the shock wave bursts outward through the surface of the star and blasts it apart.


The Deaths of Stars

• The supernova seen from Earth is the brightening of the star as its distended envelope is blasted outward by the shock wave.

• As months pass, the cloud of gas expands, thins, and fades.

• The rate at which it fades informs astronomers more about the death throes of the star.


The Deaths of Stars

• Essentially all the iron in the core of the star is destroyed when the core collapses.

• However, the violence in the outer layers can produce densities and temperatures high enough to trigger nuclear fusion reactions that produce as much as half a solar mass of radioactive nickel-56. – The nickel gradually decays to form radioactive

cobalt, which decays to form normal iron.


The Deaths of Stars

• The rate at which the supernova fades matches the rate at which these radioactive elements decay.

• Thus, the destruction of iron in the core of the star is matched by the production of iron through nuclear reactions in the expanding outer layers.


The Deaths of Stars

• The presence of nuclear fusion in the outer layers of the supernova testifies to the violence of the explosion. – A typical supernova is equivalent to the explosion of

1028 megatons of TNT—about 3 million solar masses of high explosive.

– Of course, the explosion is entirely silent.


The Deaths of Stars

• So, collapsing massive stars can trigger violent supernova explosions.

• There is, however, more than one kind of supernova.


The Deaths of StarsTypes of Supernovae

• In studying supernovae in other galaxies, astronomers have noticed that there are a number of different types.

• Type II supernovae have spectra containing hydrogen lines and appear to be produced by the collapse and explosion of a massive star by the process you have just learned about.

The Deaths of Stars

• Type I supernovae have no hydrogen lines in their spectra.

• Astronomers have found at least two ways a supernova could occur and lack hydrogen:– Type Ia supernovae– Type Ib supernovae

Types of Supernovae

The Deaths of Stars

• A type Ia supernova is believed to occur when a white dwarf gaining mass in a binary star system exceeds the Chandrasekhar limit and collapses.– The collapse of a white dwarf is different from the

collapse of a massive star because the core of the white dwarf contains usable fuel.

Types of Supernovae

The Deaths of Stars

• As the collapse begins, the temperature and density shoot up, and the carbon–oxygen core begins to fuse in violent nuclear reactions.

• In a flicker of a stellar lifetime, the carbon–oxygen interior is entirely consumed, and the outermost layers are blasted away in a violent explosion that—at its brightest—is about six times more luminous than a type II supernova.

Types of Supernovae

The Deaths of Stars

• The white dwarf is entirely destroyed.– No neutron star or black hole is left behind.– No hydrogen lines are seen in the spectrum

of a type Ia supernova explosion because white dwarfs contain very little hydrogen.

Types of Supernovae

The Deaths of Stars

• The less common type Ib supernova is believed to occur when a massive star in a binary system loses its hydrogen-rich outer layers to its companion star.– The remains of the massive star could develop

an iron core and collapse—producing a supernova explosion that lacked hydrogen lines in its spectrum.

Types of Supernovae

The Deaths of Stars

• A type Ib supernova is just a type II supernova in which the massive star has lost its atmosphere. – Hydrogen lines appear in the spectra of type II

supernovae explosions because those massive stars have not lost their atmospheres.

Types of Supernovae

The Deaths of Stars

• To summarize, a type II supernova is caused by the collapse of a massive star.

• A type Ia supernova is caused by the collapse of a white dwarf.

• A type Ib supernova is caused by the collapse of a massive star that has lost its outermost envelope of hydrogen.– These different types of supernovae can be

recognized from their spectra.

Types of Supernovae

The Deaths of StarsObservations of Supernovae

• In AD 1054, Chinese astronomers saw a ‘guest star’ appear in the constellation known in the West as Taurus the Bull.– The star quickly became so bright that it was

visible in the daytime.– After a month, it slowly faded, taking almost two

years to vanish from sight.

The Deaths of Stars

• When modern astronomers turned their telescopes to the location of the ‘guest star,’ they found a peculiar nebula, now known as the Crab Nebula—for its many-legged shape.

Observations of Supernovae

The Deaths of Stars

• In fact, the legs of the Crab Nebula are filaments of gas that are moving away from the site of the explosion at about 1,400 km/s.


The Deaths of Stars

• Comparing the radius of the nebula, 1.35 pc, with its velocity of expansion reveals that the nebula began expanding nine or ten centuries ago—just when the ‘guest star’ made its visit. – The Crab Nebula is

clearly the remains of the supernova seen in AD 1054.


The Deaths of Stars

• The blue glow of the nebula is produced by synchrotron radiation.


The Deaths of Stars

• This form of electromagnetic radiation is produced by rapidly moving electrons spiraling through magnetic fields.– It is common in the nebula produced by

supernovae.


The Deaths of Stars

• In the case of the Crab Nebula, the electrons travel so fast they emit visual wavelengths.

• In most such nebulae, though, the electrons move slower and the synchrotron radiation is at radio wavelengths.


The Deaths of Stars

• Supernovae are rare.• Only a few have been seen with the

naked eye in recorded history. – Arab astronomers saw one in AD 1006.– The Chinese saw one in AD 1054. – European astronomers observed two—one in AD

1572 (Tycho’s supernova) and one in AD 1604 (Kepler’s supernova).

– In addition, the guest stars of AD 185, 386, 393, and 1181 may have been supernovae.


The Deaths of Stars

• Then, in the early morning hours of February 24, 1987, astronomers around the world were startled by the discovery of a naked-eye supernova still growing brighter in the southern sky.


The Deaths of Stars

• The supernova—known officially as SN1987A—is only 53,000 pc away in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way.


The Deaths of Stars

• This first naked-eye supernova in 383 years has given astronomers a ringside seat for the most spectacular event in stellar evolution.


The Deaths of Stars

• Neutrinos are so difficult to detect that the 19 neutrinos actually detected mean that some 1017 neutrinos must have passed through the detectors in those 15 seconds.

• Furthermore, the neutrinos were arriving from the direction of the supernova.– Thus, astronomers conclude that the burst of

neutrinos was released when the iron core collapsed, and the supernova was seen hours later when the shock wave blasted the star’s surface into space.


The Deaths of Stars

• Most supernovae are seen in distant galaxies, and careful observations allow astronomers to compare types. – Type Ia supernovae—caused by the collapse of white

dwarfs—are more luminous at maximum brightness.– They decline rapidly at first and then more slowly.– Type II supernovae—produced by the collapse of

massive stars—are not as bright at maximum.– They decline in a more irregular way.


The Deaths of Stars

• Astronomers believe that most type II supernovae are caused by the collapse of red supergiants.


The Deaths of Stars

• Although supernova explosions fade to obscurity in a year or two, expanding shells of gas mark the sites of the explosion.

• The gas, originally expelled at 10,000 to 20,000 km/s, may carry away a fifth of the mass of the exploding star.


The Deaths of Stars

• The collision of that expanding gas with the surrounding interstellar medium can sweep up even more gas and excite it to produce a supernova remnant—the nebulous remains of a supernova explosion.


The Deaths of Stars

• Some supernova remnants, such as Cassiopeia A, show evidence of jets of matter rushing outward in opposite directions.


The Deaths of Stars

• These may have been ejected as the rotating star collapsed and, conserving angular momentum, spun up to very high speeds.– The first matter blown outward

could have emerged as jets from the poles of the spinning star.

– Astronomers are only now understanding such explosions.


The Deaths of Stars

• Supernova remnants look quite delicate and do not survive very long—a few tens of thousands of years—before they gradually mix with the interstellar medium and vanish.


The Deaths of Stars

• The Crab Nebula is a young remnant—only about 950 years old and about 8.8 ly in diameter.

• Older remnants can be larger.


The Deaths of Stars

• Some supernova remnants are visible only at radio and X-ray wavelengths. – They have become too tenuous to emit detectable light.– However, the collision of the expanding hot gas with the

interstellar medium can generate radio and X-ray radiation.


The Deaths of Stars

• A type II supernova occurs when a massive star reaches the end of its usable fuel and develops an iron core. – The iron is the final ash produced by nuclear

fusion.– Also, it cannot fuse because iron is the most

tightly bound nucleus.


The Deaths of Stars

• When energy generation begins to fall, the star contracts.

• As iron can’t ignite, there is no new energy source to stop the contraction. – In seconds, the core of the star falls inward and a

shock wave moves outward. – Aided by a flood of neutrinos and sudden turbulence,

the shock wave blasts the star apart.– Seen from Earth, it brightens as its surface gases

expand into space.


astronomy: horizons 10th edition

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