Transcript NGC 3370 Spiral Galaxy - University of Kentucky

Supernova in a distant galaxy
• Many radioactive isotopes are created in
the explosion. They are unstable and
decay down to stable nuclei.
• In supernova explosions, emission lines in
spectra can show these isotopes. Many of
which only live for a few hours or a few
days. This shows that the isotopes are
created in the explosion.
• It is also interesting that the half-life of
isotopes can be determined from these
observations.
The decay rate of radioactive isotopes are
used to measure the age of many things.
• When a radioactive isotope decays in a
rock, it produces a new element (called a
daughter isotope.)
• The decay rate tells us how fast a given
sample of an isotope will decay
• If we know the decay rate and the amount
of the isotope that has decayed in the
rock.
• It is possible to tell how old the rock is.
• We can figure out the decay rate of any
given isotope by taking a sample of the
isotope and measuring how fast it decays.
• One assumption: The decay rate for a
given isotope has always been the same,
over the entire lifetime of the rock.
Why does the decay rates of isotopes in
supernova allow astronomers to show that these
rates remain constant?
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1. If the rate is the same
during the violence of a
supernova it must be the
same everywhere
2. Supernova that are one
billion light years away
exploded 1 billion years
ago.
3. It doesn’t, it only shows
what the decay rates are
for a supernova.
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NGC 4526, distance ~ 200 million light years.
• Elements like Gold, Silver, Platinum, are
very rare, because they only form in the
hour or so during the supernova explosion.
• When the shock wave of material collides
with the molecular clouds, it sets off star
formation AND also seeds the cloud with
new elements.
• The result is that when new stars form,
they have more heavy elements than the
previous generation of stars.
The neutron star spins very
rapidly.
• Stars rotate and before the core collapse
the core of the star was rotating as well.
• Why might we expect the neutron star to
be rotating extremely fast?
Why might we expect that neutron stars spin
very rapidly?
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1. The supernova
explosion will spin
them like a top.
2. Angular momentum
is conserved
3. Neutrons spin rapidly
so a star made of
neutrons should also
spin rapidly
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• Conservation of angular momentum tells
us that as the radius shrinks the velocity
increases. L = mvr
• Where L is angular momentum
• If the radius of the core was to shrink from
1 x 105 km down to 10 km then the radius
would be 10,000 times smaller.
• The new velocity would have to be 10,000
times faster.
Pulsars– spinning neutron stars
• Neutron stars have very strong magnetic
fields. They can redirect material near the
surface of the neutron star, out along the
magnetic poles. (bi-polar outflow again)
• When the material hits other atoms it
produces radio signals that beam out
along the magnetic poles.
• As the neutron star spins, the beam of
light hits the earth and we see a pulse.
The light-house effect
• The pulsar is similar to a light house. As
the beam of light passes by us we get a
very large signal. When the beam moves
away the signal dies out.
• Some pulsars give a burst of light every
second. This means the neutron star is
spinning once every second.
• The fastest neutron stars spin 1000 times
every second.
Crab Nebula – exploded in 1054 AD.
Actually it is 6500 light years away so it exploded
in 5500 BC.
Pulse signal from Crab pulsar.
Spins once every 0.0331 seconds or about
30 times every second
As the neutron star spins the beams of light
are sometimes directed at the Earth.
Where does the energy come from in
the Crab nebula?
• The crab nebula has a total luminosity that is
100,000 times the luminosity of the Sun
• The only real source of energy is the rotating
pulsar at the center of the nebula.
• If the nebula is radiating this much energy, the
pulsar must be losing the same amount of
energy every second.
• Observations of the Crab pulsar shows it is
slowing down. The amount of rotational energy
lost is exactly the same as the amount of energy
being radiated by the nebula.
• As the rotation of pulsars decreases and
as material falling into the pulsar
decreases the pulsations begin to
disappear.
• The neutron star is still there, but the
pulsation die away.
• Neutron stars only live in the pulsar phase
for a couple hundred thousand years
(~200,000 years)
• Where would you expect the majority of
the pulsars to be found in our Galaxy?
Where in the Galaxy would you expect to
find most of the pulsars.
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1. Near the spiral arms
2. Spread uniformly
throughout the disk
of the Galaxy
3. Outside of the
Galaxy because they
would be shot there
from the supernova
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Pulsars trace out the spiral arms of our
Galaxy
The
Sun’s
position
Center of
the
Galaxy
• Pulsars come from high mass stars that
live very short lives. They die near the
spiral arms where they form.
• Pulsars effect only last a few hundred
thousand years.
• So the pulsars are also found near spiral
arms.
Stars that have core masses greater than
about 3-4 solar masses do not become
neutron stars.
• At this extreme mass, the repulsive force
between neutrons can not hold up against the
inward force of gravity.
• The core continues to collapse, with no other
repulsive left to stop the collapse.
• The core continues to collapse forever and
becomes a black hole.
• The core becomes infinitely small and is called a
singularity.
• For a stellar mass black hole there is a region
near the singularity where gravity is so strong that
even light can’t escape. The outer most portion of
this region is called the event horizon.
• Once an object enters the event horizon it is
forever lost to our universe.
Artists renditions of Black Holes
• Virtually all black holes that have been
discovered where found because of the
light that is given off by material falling into
the black hole. The material in the
accretion disk and the bipolar outflows.
• It is extremely difficult to find a black hole
when it is not accreting material, because
the black hole gives off no light of its own.
Artists renditions of Black Holes
What does it mean that light can not escape
a black hole.
• For an object to be lifted off the surface of the
Earth and escape the Earth completely, it must
be traveling about 7 miles/second.
• For something to leave the surface of the Sun, it
must be traveling about 400 miles/second.
• To leave the Event Horizon of a Black Hole you
must travel 186,000 miles/second.
• The size of the event horizon for a black hole
with 5-10 solar masses is about R = 10 km. Or
about 6-7 miles. That is about the size of a
neutron star. Beyond this distance the escape
velocity is smaller than the speed of light.
Here is a little problem to think about
• We know Newton’s law of gravity:
F = Gm1m2/r2 where m1 and m2 are the
masses of two objects and r is the distance
between them.
Here is a little thought experiment. What if the Sun
were to collapse and form a black hole right
now? Let’s suppose that all of the mass of the
Sun falls into the black hole. So the mass
doesn’t change at all. What would happen to the
Earth as it orbits this newly formed black hole?
The Earth and the Sun
r
What would happen to the Earth if the Sun
collapsed to from a Black Hole?
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1. The Earth would be
sucked into the black
hole
2. The Earth would be
shot out into
interstellar space.
3. Nothing. The Earth
would continue to
orbit like before.
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Nothing! The Earth would keep
orbiting like before.
Old
surface of
Sun
r
• It is only in close to the Black Hole where
gravity becomes extremely strong.
• The escape velocity of an object at the old
surface of the Sun (dashed circle) would
still be 400 miles/second.
• The difference is that the mass is all
concentrated at the center and you can
get closer to the mass now.
• Inside the dashed circle the gravity will
continue to increase until you finally reach
the Event Horizon where the escape
velocity becomes 186,000 miles/second.
Here’s why.
• Imagine there was a hole at the center of
the Earth. If you were able to travel down
and be inside the hole at the center of the
Earth, what would it be like?
What would gravity be like if you were in a
hole at the center of the Earth?
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1. Extremely strong
because the distance
to the center would
be zero
2. You would be
weightless
3. Extremely strong
because the mass of
the Earth would be
pulling from all sides
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There would be no net gravity. You
would float weightlessly.
Gravity
What if you only went half way to the
center?
On this side
of the line
there isn’t
as much
mass, but it
is closer to
you.
On this side
of the line
there is more
mass but it is
farther away.
Gravity
Mass
interior to
your
position
• The mass that is exterior to your radius
exactly cancel out. Only the mass interior
to your radius matters. And there is less
and less mass interior to you as you get
closer to the center.
• When you finally reach the center the net
gravity is zero.
Gravity is strongest at the surface of the Earth.
Same thing would happen if you traveled to the
center of the Sun.
• NOTE: THIS DOESN’T MEAN THAT THE PRESSURE
INSIDE THE SUN IS LOW. IT ONLY MEANS THAT IF
THERE WERE A TUNNEL TO THE CENTER OF THE
SUN THE GRAVITY WOULD DROP TO ZERO!
• But with the Black Hole you can get closer to the surface
and not have overlying layers cancelling out.
If the radius shrinks then the surface is much
closer to the center of mass.
New radius
Much high
gravity at
surface
So a black hole is NOT an interstellar
vacuum cleaner
• Black holes are usually seen in binary
systems, where the material from the one
star is being transferred to the black hole
• As the material spirals in (accretion disk)
the hot gas glows and indicates a black
hole is present.
• The mass of the black hole can be
measured using Kepler’s 3rd Law.
• But PLEASE note. The black hole doesn’t
do anything differently to the companion
star, that a normal star of the same mass
would do. Mass is transferred for two
reasons:
• 1) The star and black hole are in a close
orbit, and the star that made the black hole
already was stealing gas from the
companion.
• 2) The companion evolves into a giant or
supergiant star, and the surface gets close
to the black hole.
So a black hole is NOT an interstellar
vacuum cleaner
It is now time to find out what is really
going on.
• To really understand a black hole we have
to abandon Newton. Newton’s Laws work
fine under normal conditions, but for things
like black holes and the Big Bang,
Newton’s Laws fail.
• We can only really describe these extreme
events the Theory of Relativity. This was
developed by Albert Einstein from 1905 to
1915.