Transcript The Death of Massive Stars

Stellar Evolution and Death
Lecture 11: Interstellar Matter and Stellar Evolution
HR
Star Birth
• Times spent as protostars:
• M-class stars may remain protostars for hundreds of
millions of years.
• G stars (like the Sun) spend about 30 million years in
the protostar phase.
• Massive O- and B-type stars may spend only 100,000
years as protostars before joining the main sequence.
• Evolutionary track is the path on the H-R diagram
taken by the star (and its precursor cocoon and Pre Main
Evol - Sun
protostar) as its luminosity and color change.
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PreMain
All stars
Lecture 11: Interstellar Matter and Stellar Evolution
Star Birth
• Upper limit of Star’s Mass: Astronomers
calculate that a star with a mass greater than
100 solar masses will emit radiation so intense
that it will prevent more material from falling
into the star, thereby limiting the star’s size.
• Lower limit of Star’s Mass: Protostars with
masses of less than 0.08 solar masses do not
have enough internal pressure to ignite
hydrogen fusion.
– What about those stars whose masses are
between this and Jupiter?
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Lecture 11: Interstellar Matter and Stellar Evolution
Star
conditions
Stellar Maturity
Stellar Nuclear Fusion
• Stars of low mass like the Sun (<1.5 M) use the
proton-proton chain to generate energy.
• Stars of mass greater than 1.5 M have higher
core temperatures that allow the CNO cycle to
fuse of hydrogen into helium (4H  He).
– The CNO cycle is more efficient at the higher core
temperatures of these stars
• This series of reactions involves hydrogen with
carbon, nitrogen, and oxygen as catalysts.
• Hydrostatic equilibrium (pressure balances
gravity) maintains fusion at a uniform rate.
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Lecture 11: Interstellar Matter and Stellar Evolution
Stellar Maturity
Towards Star Death
• Until their lives end on the main sequence,
the main difference between the evolution of
stars of various masses is the amount of
time they spend as protostars and main
sequence stars.
• Stars can be grouped by mass as low-mass
or high-mass depending on their eventual
end state.
•  STAR’S LIFETIME ON MAIN
SEQUENCE DEPENDS THE STAR’S
INITIAL MASS
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Lecture 11: Interstellar Matter and Stellar Evolution
Very-Low-Mass Stars
Very-Low-Mass Stars
• In stars with a mass of less than about 0.4
solar masses, convection occurs throughout
most or all of the volume of the star.
• Hydrogen from throughout the star is cycled
through the core, and the entire star runs
low on hydrogen at the same time.
• A very-low-mass star will take 20+ billion
years to completely burn its hydrogen.
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Very
Low
Mass
Conv
Lecture 11: Interstellar Matter and Stellar Evolution
Very-Low-Mass Stars
• Ultimately, very-low-mass stars will (should?)
become white dwarfs through gravitational
shrinkage.
• The hypothetical lifetime of a very-low-mass
star is more than the assumed age of the
universe.
• Consequently, white dwarfs currently
observed must have originated in a different
manner.
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
Flowchart
Low-Mass Stars
• Low-mass stars include stars with masses To Red
Giant
between 0.4 and 6 solar masses (includes Sun).
• The core shrinks as hydrogen is depleted.
• Heat from contraction of the core then heats a
shell surrounding the core to temperatures that
permit fusion of hydrogen to begin.
• These two sources of energy (gravitational HRdia
and nuclear) cause the outer portions of the
star to expand and cool.
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expand
Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
• Consequently, the star moves to the right on the
H-R diagram and upward (due to increasing
luminosity) becoming a red giant.
• A red giant can have a lower surface temperature
(less radiation per square meter) but a higher
luminosity because its diameter will expand 200
times or more.
14-14C
• As a red giant evolves and hydrogen burning
takes place in outer layers of the star, the helium
“ashes” are dumped back onto a degenerate
core, raising the temperature of the core.
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
Electron Degeneracy
• The core of a red giant will not continue to Electron
degeneracy
contract indefinitely because of electron
degeneracy.
• Electron degeneracy is a quantum state of a
gas in which its electrons are packed as
densely as nature permits.
• The temperature of such a high-density gas is
not dependent on the pressure as it is in a
“normal” gas.
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
• When the degenerate core temperature reaches
100 million K, helium nuclei begin to combine
3-a
through the triple alpha process forming carbon.
• The initial fusion of helium proceeds in a runaway
process called the helium flash, expanding the
core, returning the core to a non-degenerate state,
HR
and shrinking the star to a yellow giant.
• Following the helium flash, the center of the star
forms three layers - an inner degenerate carbon
core, a layer of helium that fuses to carbon in a
conventional manner, and an outer hydrogenRed
fusing shell.
Giant
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
Yellow Giants and Pulsating Stars
• Many yellow giants (whether an aging highmass or low-mass star) swell and shrink
rhythmically: they pulsate.
• These pulsating yellow giants are located in the
instability strip of the H-R diagram.
• High-mass pulsating giants are Cepheid
variables (periods of about 1-70 days).
• Low-mass pulsating giants are RR Lyrae
variables (periods of about 12 hours).
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
• The cause for the pulsation is a special situation
where the yellow giant’s atmosphere can trap
some of its radiated energy.
• This heats the atmosphere which then expands
to a point that the star’s trapped radiation can
escape.
• This causes the atmosphere to cool, shrink to its
original size, and start the process all over again.
• The regular pulsation process of variable stars
has led to the period-luminosity relation: higher
average luminosity leads to longer periods.
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
Post Helium Flash
• After the helium flash, a yellow giant then
expands again into a red giant.
• Stars more massive than 2 solar masses do
not experience a helium flash, but will simply
expand through the yellow giant stage to its
one and only red giant stage.
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Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
Mass Loss In Stars
• The solar wind carries away about 10–14 of the
Sun’s mass each year. Over the course of 10
billion years, the Sun will lose only 0.01% of its
mass this way.
• In red giant stars, it is thought that core
instabilities and pulsations are responsible for
the large mass loss.
• A typical red giant loses 10–7 solar masses a
year and hence can last at most 10 million
years.
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PN
drawing
Lecture 11: Interstellar Matter and Stellar Evolution
Low-Mass Stars
M57
Planetary Nebulae
• A Planetary nebula is a spherical shell of gasM57IR
that is expelled by a red giant near the end of
its life.
NGC6543
• The material in the shell glows because UV
radiation from the central hot star causes it to
fluoresce.
Others
• Pulsations and/or stellar winds are thought to
cause planetary nebulae.
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Planetary
HR-PN
Lecture 11b: Stellar Remnants
White Dwarfs
• White dwarfs are the cores of red giants that
remain after the outer parts of the original
stars have blown away.
• Electron degeneracy supports the star
against gravity. Nuclear fusion no longer
degen
occurs.
• White dwarfs have observed surface
temperatures between 4,000 K and 85,000 K.
Their masses range from perhaps 0.02 solar
masses up to 1.4 solar masses.
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size
Lecture 11b: Stellar Remnants
White Dwarfs
• When a gas gets extremely dense the
electrons have trouble moving to different
energy levels
• When a gas gets into this state, it is
called a degenerate gas
– The gas resists further compression, since degen
only a certain number of electrons can be at
a given energy level
– Also, pressure of a degenerate gas does not
depend on the temperature (unlike an
normal, ideal
gas).
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Lecture 11b: Stellar Remnants
White Dwarfs
• A typical white dwarf has 0.8 solar mass, a
10,000 km diameter (3/4 of Earth’s), and a
teaspoon of white dwarf material would
weigh 2 tons.
• Astronomers estimate that 10% of all stars
are white dwarfs.
• A black dwarf is the theorized “final” state
of a star with a main sequence mass less
than about 8 solar masses, in which all of its
energy sources have been depleted so that
it emits no radiation.
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size
Lecture 11b: Stellar Remnants
White Dwarfs
• A binary system of a white dwarf and a newly
formed red giant will result in the formation of
an accretion disk around the white dwarf.
• Hydrogen build-up on the white dwarf can
ignite an explosive fusion reaction blowing off a
gas shell that causes the white dwarf to
brighten by 10 mags in a few days - the
Accretion
brightening is called a nova.
or
• The explosion does not disrupt the binary
system. Infalling H ignition can recur with
periods ranging from months to thousands of board
years.
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Lecture 11b: Stellar Remnants
White Dwarfs
• If you add heat to this degenerate gas it
does not expand (unlike gas in this room)
• As you add more mass to the white
dwarf, it gets smaller!
Chand
• Theory predicts that white dwarfs radius
will go to zero when the mass of the white
dwarf becomes 1.4 Solar masses .
– This is the Chandrasekhar limit
– Most isolated stars lose enough mass to
avoid this
Lecture 11b: Stellar Remnants
Type I Supernovae
• If accretion brings the white dwarf mass above the
Chandrasekhar limit, electron degeneracy can no
longer support the star, and it collapses.
SN
• The collapse raises the core temperature and
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runaway carbon-fusion begins, which ultimately
leads to the star’s explosion.
• Such an exploding white dwarf is called a Type I
supernova.
• While a nova may reach an absolute magnitude of
–8 (about 150,000 Suns), a Type I supernova attains
an absolute magnitude of –19 (5 billion Suns).
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Lecture 11b: Stellar Remnants
Massive Stars
Massive (High-Mass) Stars
• Massive stars (> 6 solar masses) will expand
beyond the red giant stage to become
Red
supergiants.
supergiant
• Typical supergiants have luminosities a million
times that of the Sun and absolute magnitudes
of –10.
• The greater core temperatures and pressures,
produce heavier elements, such as neon,
silicon, and iron.
• The creation of these elements is known as
nucleosynthesis.
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Lecture 11b: Stellar Remnants
Massive Stars
• Fusion of heavier and heavier
elements continues until the
creation of iron (Fe) occurs
• Iron is the most tightly bound atom
– To fuse iron requires energy, so this is
not a natural thing to do
– Without fusion the core will collapse
(again gravity will win)
Lecture 11b: Stellar Remnants
Type II Supernovae
• Type II supernovae begin with the conversion
of silicon to iron. The fusing of silicon to iron in
a supergiant star will take only a few days.
• Because the iron fusion reaction absorbs more
energy than it releases, the core shrinks, heats
up, but produces no new more massive
elements.
• At the Chandrasekhar limit, the core collapses
violently. Protons and electrons combine and
form neutrons. After reaching its minimum size,
the core rebounds, colliding violently with
SN
infalling material.
A14.5
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Lecture 11b: Stellar Remnants
Type II Supernovae
• This collision between the infalling
material and the rebounding core
produces two effects:
• 1. Enough energy is produced to fuse
iron into heavier elements.
• 2. Shock waves are sent outward that
throw off the outer layers of the
supergiant. These shock waves may be
further heated by neutrinos escaping the
collapsed core.
How?
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Demo
Lecture 11b: Stellar Remnants
Supernovae in General
SN
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Property
Type I
Type II
Spectrum
No hydrogen
lines
Prominent
hydrogen lines
Peak absolute
magnitude
Light curve
–19
–17
Sharp peak
Broader peak
Expansion rate
10,000 km/s
5,000 km/s
Mass ejected
0.5 solar masses 5 solar masses
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Supernova in M81
April 23, 1992
April 1, 1993
Lecture 11b: Stellar Remnants
The Neutron Star
• Neutron star is the remainder of a 6-12 solar
mass star that has collapsed to the point at
which it is supported by neutron degeneracy.
• The diameter of a typical neutron star is only
0.2% of the diameter of a white dwarf (about
20 km) and the neutron star is a billion times
more dense.
• Neutron stars have masses between 1.4 and 3
solar masses.
• Temperature = 10,000,000 K
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Lecture 11b: Stellar Remnants
The Neutron Star
Observation - The Discovery of Pulsars
• In 1967, Jocelyn Bell discovered an
unknown source of rapidly pulsating radio
waves.
• Subsequent discoveries of similar sources
gave rise to the name pulsar.
• A pulsar is a celestial object of small
angular size that emits pulses of radio
waves with a regular period between
about 0.03 and 5 seconds.
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pulsar
Lecture 11b: Stellar Remnants
The Neutron Star
• Objects that emit pulsing signals with a
duration of 0.001 sec cannot have a diameter
any greater than 0.001 light-secs, which is a
lighthouse
few hundred kms.
• Such a small size ruled out white dwarfs
(Earth-sized objects), leaving the
hypothesized neutron star as the explanation
for pulsars.
• The lighthouse model is a theory that
explains pulsar behavior as being due to a
spinning neutron star whose radiation beam
we see as it sweeps by.
15-12
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Lecture 11b: Stellar Remnants
The Neutron Star
• The beam is created by charged electrons
spiraling in the magnetic poles of the neutron
star leading to the emission of synchrotron
radiation (nonthermal radiation).
• The high spin rate of a neutron star is obtained
from the original star’s spin as a result of
angular momentum conservation.
Ang
• Neutron stars may undergo “glitches” in their Mom
rotation rates.
• Neutron stars may be members of x-ray
binaries (caused by infall of material from other star).
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Lecture 11b: Stellar Remnants
The Neutron Star
• More than 1000 pulsars have been discovered,
most with periods between 0.1 and 4 seconds.
• The Crab pulsar spins so rapidly because it
formed so recently. Over time it will lose
rotational energy, slow down, and emit less
energy.
• The Crab pulsar is slowing down because of
the “drag” of the electrons propelled out into
the nebula surrounding the pulsar.
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Crab Nebula
Supernova
remnant–
the Crab
Nebula
Pulsar in
the middle
Lecture 11b: Stellar Remnants
Very-Massive Stars
Very-Massive Stars
• Very-massive stars differ primarily from
massive stars in what happens to them when
their core is compressed to a density greater
than electron degeneracy can support.
• In a massive star, the resulting supernova
leaves a neutron star. In a supermassive
star, the core collapses into a black hole.
• Neutron degeneracy cannot support a
neutron star whose mass is greater than
about 3 solar masses.
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Lecture 11b: Stellar Remnants
Black Holes
• The Schwarzschild radius is the radius
of a spherical region in space within
which no light can escape:
RS = 3M (RS in km; M in solar masses)
• The size of the Schwarzschild radius
depends on the mass within the sphere.
• A black hole is a spherical volume of space with
a radius given by the Schwarzschild formula
above and with an escape velocity that exceeds
the speed of light.
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board
Lecture 11b: Stellar Remnants
Warp space
Photon
deflects
Black Holes
• The event horizon is the spherical surface of
radius RS around a black hole from which
nothing can escape.
• Inside a black hole, an object will eventually be
subjected to extreme tidal forces pulled apart.
• The final destination of an object (or so it is
thought) in a black hole is to be crushed out of
existence at a central singularity.
• Spinning and charged black holes are more
complex.
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Lecture 11b: Stellar Remnants
Black Holes
Detecting Black Holes
• If a black hole has a close binary companion,
material may be pulled from the companion to
form an accretion disk around the black hole.
• The accretion disk radiates x-rays and gamma
rays as the gas is heated to very high
temperatures as it approaches the event
horizon.
• From their x-ray emissions, Cygnus X-1 and
AO620-00 are two very good black hole
candidates.
binary
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Lecture 11b: Stellar Remnants
Black Holes
Black Holes Forever?
• Jacob Bekenstein discovered that the
black holes can be assigned a
temperature.
• Soon thereafter in 1974, Stephen Hawking
showed that this temperature meant that a
black hole emits thermal radiation through
a quantum/gravity energy exchange.
• Hawking radiation means black holes
radiate away, although a one-solar-mass
black hole will take 1067 years to do so.
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Deep
Sky
Objects
Star Quiz
practice
#1
Review – the fates of stars
What happens after all the H is used up in the core?
• Very Low-mass stars (0.4 Msun or less): Star cease fusing
material in the core after all the Hydrogen is used up
• Low-mass stars: Hydrogen shell burning, eventually leads
to Helium flash in core, planetary nebula phase, leaving a
carbon-oxygen white dwarf (if in a close binary, accretion
may create nova or type I supernova)
• Medium-Low-mass stars: same as Low-mass stars except
Helium burning is steady
• High-mass stars: keep burning heavier atoms in core and in
shell, until iron is left in core
– Core collapse results in supernovae (Type II) explosion
– Neutron star forms in the core
Fates
flowchart
• Very High-mass stars: same as High-mass stars except that
a black hole forms in core
The End
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