Only consider the first ascent for this quiz

Quiz #7

• On the H-R diagram, a lower mass star that is evolving off the main sequence will become redder in color and have a tremendous increase in luminosity. Write out the equation for luminosity in terms of surface temperature and radius. Then discuss which parameter is primarily controlling the luminosity as the star evolves.

The Death of High Mass Stars

• When a high mass star runs out of hydrogen in its core, the core begins to shrink. The outside of the star expands and the star moves right on the H-R diagram.

• The temperature is cooling and the radius is growing, but the luminosity is virtually constant.

• Since L = σT 4 (4 πR 2 ); T 4 the same rate as R 2 must be changing at • The star becomes a supergiant (luminosity class I star)

The Death of High Mass Stars

• As the star tracks to the right for the first time the inert helium core is contracting and hydrogen shell burning is occurring.

• At the farthest right, helium core burning begins, converting helium into carbon. And still hydrogen shell burning.

• The star begins to move to the left on the H-R diagram.

The Death of High Mass Stars

• When the helium runs out in the core, the core begins to contract again, there is helium shell burning into carbon, and hydrogen shell burning into helium.

• The star moves right again, toward cooler temperatures and larger radii.

The Death of High Mass Stars

• Finally the carbon core is hot enough to fuse carbon into oxygen and nitrogen.

• The star moves back to the left on the H-R diagram. There is a core changing carbon into oxygen and nitrogen, a shell changing helium into carbon, and a shell changing hydrogen into helium.

A rule of thumb.

• Every time a high mass star moves to the right (cooler temp) on the H-R diagram, the core is inert, but contracting.

• Every time a high mass star moves to the left, the core is fusing one element into another.

• Throughout all of this there is shell burning going on.

Final stage.

• • • • • • The core of the high mass star fuses: hydrogen into helium helium into carbon carbon into oxygen and nitrogen oxygen and nitrogen into sulfur and silicon And finally silicon into IRON.

• At last the core is iron. This is where everything stops with a bang!

The final core and shells of a high mass star

Fusing Iron does not release energy.

Think of it like a stairway • A ball at the top of a stairway has potential energy and can release it to make kinetic energy. This can continue all the way down to the floor. • Once on the floor the ball can be made to go back up the stairs but energy is not released. It has to be provided by someone in order to move back up again.

Nuclear Fusion

• So, Iron (Fe) can fused into other elements. But just like the ball at the bottom of the stairs, this process doesn’t release energy. It actually takes energy from the system to make other elements.

• When the star’s core is completely Fe, it can be fused into other elements, but in so doing, energy is not released. Instead the reaction robs the surrounding core of kinetic energy. Making the core cool down. This drops the pressure in the core and gravity wins the fight.

• In chemistry this type of reaction would be called endothermic. The reaction doesn’t provide energy it uses energy from its surroundings causing the surroundings to cool.

• With no pressure to fight against gravity the core collapses. NOT contracts, but collapses. Like a house of cards.

• The gravity is too great for the electrons to hold up the core like what happens in white dwarfs.

• If the core mass is greater than 1.4 solar masses, the electrons are forced into the protons to make neutrons.

• The neutrons become wave-like and hold up against gravity. If the core is less than 3-4 solar masses.

Remember all the way back to the start of the course.

• The atom is mostly empty space. The nucleus is tiny compared to the size of the electron shells.

• If all the space inside your atoms were removed, so that the nuclei were “touching” each other, you would be the size of a

pin head.

If the Nucleus is a grape on the 50 yard line in Commonwealth Stadium, then the electrons are tiny gnats flying in the cheap seats

• A second atom would be the size of another stadium, right next to the Commomwealth Stadium. It also has a grape for a nucleus. When the electrons can no longer hold the nuclei apart, all the space between the nuclei disappears, until the nuclei are in “contact”. In other words, the neutrons stop the contraction.

• The core becomes an enormously big nucleus • The result is a core with radius ~ 6 miles and a mass > 1.4 solar masses.

• The density of a neutron star is so high that one table spoon full of neutron star material would weigh as much as an entire mountain.

• Remember, a similar amount of white dwarf material would weigh as much as a car.

Neutron star

• The core is now a neutron star, if the core mass is less than ~ 3 solar masses. If more a black hole forms.

• Gravity is so strong that the collapse occurs at nearly the speed of light, and the material above the core follows the collapse down at similar speeds.

Layers above core follow it in at speeds close to the speed of light. At the center they run into the densest object in the universe.

What will happen when the layer hits the neutron core?

1. It will stop and become a layer on the surface of the neutron star 2. It will penetrate inside the neutron star and allow fusion in the core to start again 3. It will rebound off the core and head outward in the star

Layers bounce off the neutron star and head back out at nearly the speed of light. There they run into other layers on their way toward the center.

The collision forms a shock wave that moves out through the star

The shock wave blows the star apart in about 2 hours.

• The result is a supernova explosion.

Supernova 1987a as it looked in 1994 Shock wave

It will take hundreds of years for the shock wave to reach the molecular clouds around it, but when it does it will set off star formation.

Also, in the explosion, the elements heavier than iron are created. • Neutrons are from the core collide with nuclei and build up the heavy elements.

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.

30

Why does the decay rates of isotopes in supernova allow astronomers to show that these rates remain constant?

60

1. If it is the rate is the same in 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

2

for a supernova.

3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 11 33% 12 1 13 14 33% 15 2 16 17 18 33% 19 3 20

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?

• 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 10 5 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 neutrons stars spin 1000 times every second.

Crab Nebula – exploded in 1054 AD.

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.