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.

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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33% 33%33%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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33% 33%33%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

Center of the Galaxy

The Sun’s position

• 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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33% 33%33%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.

r

Old surface of Sun

• 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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33% 33%33%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?

Gravity

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.

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 radiusMuch 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.