Transcript Stellar Structure - McMurry University

Lecture 22
RIP
The Death of Stars
Announcements

Tonight is the last regular Lab. A
signup sheet will be posted next to the
door for the make-up lab next week.
Please indicate which labs you are
missing so that I can decide how to do
the make-up.
The Main Sequence
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-3
-1
On the HR diagram,
the sun starts here.
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
Early Red Giant
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-3
-1
1
3
By the time the sun first
becomes a red giant, it
is now here on the
diagram (in the region
for giants).
5
7
9
40,000 20,000 10,000 5,000 2,500
Just Before The Helium
Flash
By the time the
sun reaches the
helium flash, it is
here on the
diagram.
-5
-3
-1
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Red Giant Branch
-5
This path stars
follow as they
become red
giants is often
called the giant
branch of the
HR diagram.
-3
-1
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
A Helium-Burning Star
-5
After the helium
flash, the sun
becomes,
smaller, warmer,
and dimmer than
before.
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-1
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Horizontal Branch
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-3
-1
Once a solar-type star
begins helium burning,
it ends up somewhere
along this horizontal
line on the HR diagram.
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Horizontal Branch
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-3
For this reason, heliumburning solar-type stars
are called horizontal
branch stars.
-1
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Horizontal Branch

The core helium burning phase is
sometimes called “the second main
sequence” because of similarities to
the hydrogen burning phase:
– Energy is again produced in the core (but
using a different fuel).
– Pressure-Temperature thermostat is very
effective again: star’s size/temperature
stays very stable.
The End Of The Reprieve

Important Differences:
– Helium burning doesn’t last as long.
– Helium fusion is not as efficient as
hydrogen fusion: produces less energy
per kg of nuclear fuel.
– Sun is still 40 times brighter than today.
– Starts to run out of helium in only about
250 million years.
The Second Ascension
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
As helium fuel runs out:
– Carbon core starts
shrinking.
– Helium burning begins
in shell around carbon
core.
– Hydrogen burning
begins in shell around
helium shell.
The star is swelling into a
red giant again! Called the
second ascension.
About How Big Will Our
Sun Get?

This phase is the largest and brightest
our sun will ever get.
Here’s original size for
comparison.
L = 4,800
T = 3,000 K
R = 260
The Death Of Earth?
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
During this phase,
the sun will swallow
the Earth.
Probably won’t
make it out to Mars.
The Second Giant Branch
-5
There are two
giant branches
on the HR
diagram, side by
side.
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-1
1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
The Second Giant Branch
-5
-3
The second one
is called the
asymptotic
giant branch.
So stars in their
second
ascension are
often called
AGB stars.
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1
3
5
7
9
40,000 20,000 10,000 5,000 2,500
AGB Giants
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Very large, luminous,
and red.
– R > 200
– L = 5,000-10,000
– T ~ 3,000 K


Energy source is
helium and hydrogen
shell fusion.
Star has inert C, N, O
core.
AGB Giants

AGB Giants experience
significant mass loss.
– Gravity too low to hold
onto distended outer
layers.
– Dust forms in cool outer
layers; “reflects” core
light, helping to push
outer layers out into
space.
– Lose up to 1 solar mass
every 100,000 years.
Mass Loss In Giant Stars


Giant stars have
strong stellar winds
and weak surface
gravity.
During the giant
phase, these winds
carry off a large
percentage of the
star’s mass.
By The End Of The Giant
Phase…

Up to half or more
of the star’s gasses
can end up as a
nebula around the
giant star.
The Planetary Nebula

Forms in two stages:
1. Early in AGB stage,
mass loss occurs in the
form of a slow cool
wind. Forms an
expanding shell of gas
around the star.
2. After expulsion of
outer layers, core is
exposed to space.
The Planetary Nebula


Hot, fast stellar
wind from core
slams into cool
expanding shell.
Gas glows by
emission. Result is
a planetary nebula.
Planetary Nebulae


Mass loss not
necessarily
symmetric:
Cold shell may be
less dense at poles.
– Easier for hot wind
to get through the
poles.
– Results in an
asymmetric nebula
(like an hourglass).
Planetary Nebulae

Planetary Nebulae
are very short lived:
– Expansion of nebula
rapidly cools gases.
– Emission fades,
nebula becomes too
dim to observe after
a few 10,000’s of
years.
The Final Collapse


Core finishes
consuming all nuclear
fuel.
Gravity wins!
– Core collapses until
electron degeneracy
prevents further
contraction.
– What’s left of star is
now about the size of
Earth, but very, very
hot: a white dwarf star.
White Dwarf Stars


No nuclear fusion. Star is
“dead.”
Electron degeneracy
(From Quantum
Mechanics) provides the
pressure that prevents
gravity from collapsing
the star.
– Pauli Exclusion Principle:


No two electrons can be in
the same place at the
same time, doing the
same thing.
Electrons can exert a
powerful outward pressure
to keep from getting too
close together!
White Dwarf Stars
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Heat is left over from
energy released during
gravitational collapse.
Star starts out very
hot: 100,000 K!
No way to replace heat
radiated into space.
Star slowly cools down
over billions of years.
End stage is black
dwarf – but none have
formed yet!
The Structure Of A White
Dwarf



Mostly a sphere of C,
N, and O that is
completely electron
degenerate.
Atmosphere of
hydrogen and helium.
Carbon center may
crystallize to form a
giant diamond!
Daily Quiz 22 – Question 1
What prevents gravity from shrinking a white
dwarf to a smaller size?
A.
B.
C.
D.
Helium core fusion.
Helium shell fusion.
Hydrogen core fusion
Degenerate electrons (electromagnetic
force).
White Dwarf Sizes


Higher mass results in smaller, denser
white dwarf.
Upper mass limit of 1.44 solar masses.
– Called the Chandrasekar limit.
– Above this mass, gravity overcomes
electron degeneracy.
– The white dwarf collapses!
White Dwarf Sizes
Novae!

Occur in binary
systems.
– One star is “normal”
(often a giant or
supergiant).
– Other star is a white
dwarf.
Novae!
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Companion star loses
mass to the white dwarf.
Forms an accretion disk
that deposits hydrogen
onto the dwarf’s surface.
Hydrogen crushed to
degeneracy.
Pressure and
temperature increase as
more hydrogen is added.
“Kindling point” is
reached, and …
Novae!


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Surface of dwarf is
consumed in a
thermonuclear explosion!
Light output jumps to
10,000’s to 100,000’s of
times normal!
Hydrogen layer is ejected
from white dwarf.
White dwarf is not
“damaged”
– Process begins again.
– Most nova recur!
And Now: Supernovae!
A much bigger class of stellar
explosion is called a Supernova
Supernovae have two
types:

Supernovae classed by spectrum:
– Type I
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Spectrum shows no hydrogen lines.
Some Type I SN’s just as bright as Type II: called Type
Ib.
Remaining Type I SN’s soar to 4 billion times solar
luminosity, then fade quickly: called Type Ia.
– Type II
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Spectrum shows hydrogen lines.
Caused by core collapse in massive star. Hydrogen
lines from exploding outer layers of star.
Type Ia Supernova
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Some supernova are
exploding white
dwarfs.
How do you blow up
a white dwarf?
Start with a star
system similar to
setup for a nova:
– White dwarf drawing
material from
companion star.
Blowing Up White Dwarfs
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
BUT white dwarf is
very close to
Chandrasekar Limit
(1.44 solar masses).
Matter “stolen” from
companion star drives
mass above
Chandrasekar Limit
before a nova can
occur.
Blowing Up White Dwarfs
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
White dwarf collapses. Internal
temperature reaches kindling point for
Carbon before dwarf reaches neutron
degeneracy.
Gas still electron degenerate – no
pressure/temperature thermostat:
– Runaway fusion – all carbon fused all at once!
– Resulting thermonuclear explosion totally
blasts the white dwarf apart! Result is a Type
Ia Supernova!
Daily Quiz 22 – Question 2
What can happen to the white dwarf in a close binary
system when it accretes matter from the companion
giant star?
A.
B.
C.
D.
The white dwarf can become a main sequence star
once again.
The white dwarf can ignite the new matter and flare
up as a nova.
The white dwarf can accrete too much matter and
detonate as a supernova type Ia.
Either the white dwarf can ignite the new matter and
flare up as a nova, or the white dwarf can accrete too
much matter and detonate as a supernova type Ia.
And Now: Type Ib and II
Supernovae!
The Times Listed Are For An
M=25 Star
The Supergiants

Core runs low on H
fuel. Collapses and
ignites He.
– He burning creates
C, N, and O.
– Ignites H to He
burning shell around
core.
– Star’s luminosity
increases. Swells in
size.
Countdown to Disaster

After 7 million
years:
– H to He fusion in
core ends.
– He to C, N, O fusion
in core begins.
– H to He burning
shell forms.
– Star becomes
supergiant.
Countdown to Disaster

500,000 years later:
– He in core exhausted.
– Core collapses, heats up to 800 million K.
– C, N, O burning begins, producing Ne and
Mg.

600 years later:
– Core C, N, O supply used up.
– Core collapses, heats up to 1.5 billion K.
– Ne and Mg burning begins, producing Si.
Countdown to Disaster

Six months later:
– Core supply of Ne and Mg used up.
– Core collapses, heats to 3 billion K.
– Si fusion begins, producing Fe.

Now there’s a problem! Remember, we
can’t fuse iron into heavier elements and
make energy!
Countdown to Disaster

One day after Silicon fusion begins:
– Si is running low in the core.
– Heat/Pressure from Si fusion cannot
support Fe core.
– Fe core begins to collapse. Core heats
up.
– Fe cannot be fused into heavy elements
(and still release energy)!
Countdown to Disaster

Only milliseconds to go:
– Temperature in Fe core soars above 100 billion
K!
– Two nuclear reactions can occur at this
temperature:
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
Neutronization – protons and electrons react to form
neutrons.
Photodisintegration – photons hit Fe nuclei and shatter
them into He nuclei!
Countdown to Disaster

Both reactions require energy! Core
rapidly cools down!
– Loss of heat/pressure speeds up collapse!
– Result is a catastrophic, runaway collapse
of the Fe core!
The Fuse is Lit!

500 km Fe core
collapses to 10 km
across.
– Reaches same density
as nuclear matter.
– Core collapse stops
abruptly as core
becomes unimaginably
rigid.
– Outer layers of star slam
into now rigid core at
extreme speeds.
– Shockwave forms,
rocketing outward
through the star!
KABOOM!

One hour later:
– Shockwave erupts
through surface of
star.
– Everything but
collapsed core
blasted into space:
star dies in a
spectacular
explosion!
The Supernova

Star-destroying
explosion called a
supernova.
– Light output exceeds
600 million solar.
– Extreme heat/energy in
shockwave results in
nuclear fusion in outer
layers of star.
– Fusion reactions in
supernova create
elements heavier than
Fe.
Type Ib Supernovae
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
Similar to how a type II
supernova happens, but
without hydrogen lines in the
spectra.
How to “get rid” of hydrogen
lines?
– Eject hydrogen-rich outer
layers before core collapse.
– Example: Wolf-Rayet stars.
 >40 solar masses.
 Extremely unstable:
violent stellar wind
eventually ejects outer
layers of star.
 After core collapse and
supernova, very little
hydrogen is left in star to
create spectral lines.
Type Ib Supernovae
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
Similar to how a type II
supernova happens, but
without hydrogen lines in the
spectra.
How to “get rid” of hydrogen
lines?
– Could also strip outer layers
by being part of a binary
system.
Daily Quiz 22 – Question 3
Why can't massive stars generate energy from
iron fusion?
A. The temperature at their centers never
gets high enough.
B. The density at their centers is too low.
C. Iron fusion consumes energy.
D. Not enough iron is present.
Observations of Supernovae
Supernovae can easily be seen in distant galaxies.
Local Supernovae and Life on Earth
Nearby supernovae (< 50 light years) could kill many life forms
on Earth through gamma radiation and high-energy particles.
At this time, no star
capable of producing a
supernova is < 50 ly away.
Most massive star
known (~ 100 solar
masses) is ~ 25,000 ly
from Earth.
Supernova Remnants
X-rays
The Crab Nebula:
Remnant of a
supernova
observed in 1054
Cassiopeia A
Optical
The Cygnus Loop
The Veil Nebula
The Remnant of SN
1987A
Most recent nearby SN
was in February 1987.
Ring due to SN ejecta
catching up with preSN stellar wind; also
observable in X-rays.
Daily Quiz 22 – Question 4
Which type of star eventually develops several
concentric zones of active shell fusion?
A.
B.
C.
D.
Low mass stars.
Medium mass stars.
High mass stars.
White dwarfs.
The Neutron Star
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Formed from the
collapsing iron core
of a massive star.
Core collapses until
neutron
degenerate.
Often 1-2 solar
masses squeezed
into a ball 20 km
across!
The Neutron Star


A neutron star has
an outer crust (2
km thick) made
from super-dense
iron.
Inside is an ocean
of superfluid
neutrons that form
whirlpools under
the surface of the
star.
The Neutron Star

What are they like?
– Extremely hot:
1 million K
– Rotate very fast:
conservation of
angular momentum.
– Extremely powerful
magnetic fields.
– Extreme surface
gravity.
The Neutron Star


Very powerful
magnetic field.
Believed to create
beams of
electromagnetic
radiation.
Pulsars
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As a neutron star rotates, the beams
sweep through space.
Pulsars

When a beam sweeps over the Earth,
we see a flash of light.
Pulsars
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Since the neutron star rotates so quickly,
the flashes (“pulses”) of light happen many
times a second.
When observed with telescopes, these
rapidly flashing (“pulsing”) objects were
originally called pulsars.
Pulsars are just neutron stars that are
easy to observe because the pulsing makes
them stand out.
The Neutron Star Mass
Limit
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Like white dwarfs, neutron stars have
a mass limit.
Believed to be 2.5-3.0 solar masses
(not known for sure)
If a neutron star is over this limit,
nothing can stop its collapse. But
what does it become?
Forming a Black Hole
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
ANY object that shrinks
enough will develop
surface gravity high
enough to prevent
everything from
escaping.
One example is a
collapsing neutron star:
if it collapses enough
its surface gravity will
get intense enough to
form a black hole.
Anatomy of a Black Hole
Event Horizon
Singularity
Ergosphere
Anatomy of a Black Hole

The Event Horizon
– The point of no return – once you enter,
you can never leave.
– Inside all paths lead to the singularity.

The Ergosphere
– Space itself getting dragged around the
black hole.
– Nothing can stay stationary within.
– Once you enter, half your mass must go
into the event horizon so the other half
can escape.
Anatomy of a Black Hole

Why don’t black holes suck in
everything in the Universe?
– Only dangerous if you are very close
Black Holes Are Simple
Objects

No Hair Theorem – Black holes have
no hair.
– “Hair” represents “details” – a black hole
is described by only three quantities:
mass, electric charge, and rotation.

The Law of Cosmic Censorship – There
can be no naked singularities.
– Weird, universe-destroying things happen
there! They must be “shielded” by an
event horizon.
The Schwarzschild Radius
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
The size of a black hole’s event
horizon is related to its mass:
R = (3 km) M (in solar masses)
So a 20 solar mass black hole has a
Schwarzschild Radius of 60 km.
As a black hole eats, it gets bigger!
The Accretion Disk

Although we can’t
directly observe the
black hole, we can
see the X-rays
created by
superheated gas
flowing into the
hole in an
accretion disk.
Observing Black Holes
No light can escape a black hole
=> Black holes can not be observed directly.
If an invisible
compact object is part
of a binary, we can
estimate its mass from
the orbital period and
radial velocity.
Mass > 3 Msun
=> Black hole!
Cygnus X-1
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
The first X-ray source
discovered in Cygnus
was found to be a
very compact object
(more than 3 solar
masses) in orbit
around the blue
supergiant star HDE
226868.
First example of an
X-ray source believed
to be a black hole.
And Others…
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But it was certainly
not the last.
Many others have
been found.
AND the best
evidence for real
black holes lurks at
the centers of
galaxies!
The Galactic Nucleus
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
The most
mysterious part of
the galaxy.
The very center is a
powerful radio
source called
Sagittarius A*.
Next Time

Read Units 70 and 74