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Stellar Evolution after the Main Sequence High Mass Stars

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Stellar Evolution after the Main Sequence

Stellar Evolutionafter the Main SequenceHigh Mass Stars1The Path to the Main Sequence O B A F G K M1.110.0110010002After the Main SequenceAs the star ages (at a much faster rate), the process begins in the same manner as for a low-mass star.H He which forms an inert coreAfter the He core becomes substantial, then things begin to happen differently.The star heats and compresses faster, the He doesn't get a chance to form the electron gas and so there is no He flash.Instead, the He reaches the 100 million K needed to begin He C3More NucleosyntheisThe Carbon core in turn becomes substantial, but if the star is massive enough, it begins to react turning Carbon into Neon and OxygenOnce the Oxygen core begins to become substantial gravity again begins compressing and heating it until it achieves temperatures sufficient to change Oxygen into Silicon4A Many-layered StarHHeCNeSiFeOThe sequence of contraction, heating, ignition continues until we have a set of shells:H HeHe CC NeNe OO SiSi Fe 5A day later!You can see that the Silicon Iron stage takes place in a single day. It's here that there is A Serious Problem for our massive star.Iron occupies a rather special place for the elements. Iron is at the top of the "Binding Energy Curve". This means creating all of the elements by nuclear fusion has released energy. This energy in the form of radiation and therefore heat has balanced the force of gravity. However, in order to create elements above Iron (26Fe) we have to ADD ENERGY. This means that iron is the heaviest element we can create which will give off energy to balance against gravity.

It takes about a day for the iron core to reach 1.4 Solar masses. When this (Chanrasekhar's Limit) is exceeded, the electron pressure cannot withstand gravity any longer.The Core Collapses!7A day later!You can see that the Silicon Iron stage takes place in a single day. It's here that there is A Serious Problem for our massive star.Iron occupies a rather special place for the elements. Iron is at the top of the "Binding Energy Curve". This means creating all of the elements by nuclear fusion has released energy. This energy in the form of radiation and therefore heat has balanced the force of gravity. 12C4He8Be16O56Fe1HBinding energy per nucleonAtomic weightfusionfission8Iron Core CollapseHowever, in order to create elements above Iron (26Fe) we have to ADD ENERGY. This means that iron is the heaviest element we can create which will give off energy to balance against gravity.

It takes about a day for the iron core to reach 1.4 Solar masses. When this (Chandrasekhar's Limit) is exceeded, the electron pressure cannot withstand gravity any longer.

The Core Collapses!

Danger, Will Robinson!!StepCore Temp (K)Time (years)H-burning40 million70 millionHe-burning200 million500 thousandC-burning600 million600Ne-burning1.2 billion1O-burning1.5 billion1/2Si-burning2.7 billion1 dayFe Core Collapse5.4 billion0.2 secondsCore 'Bounce'23 billion1 millionth second"Boom"1 billion10 seconds10AftermathThe result is a Type II supernovaIt is up to 100 billion times more luminous than the SunThe light rapidly rises to maximum brightness then gradually decreases over several weeks to monthsThis happens in a galaxy similar to the Milkyway about once every fifty years on the average.

11Supernova

The image here and on the last slide is that of the Crab Nebula (M1).

It is about 6300 lightyears away, but was so bright that it could be seen during the day when its appearance was recorded by Chinese astronomers in 1054 AD

At this time it is about 6 lightyears in diameter and still spreading out. The average rate is about 30,000 miles/second12High-Mass Evolution

Historical SupernovaDate (AD)ConstellationApparentMagnitudeDistancekpcObservers185Centaurus-62.5China369Cassiopeia-310China1006Lupus-53.3Asia, Europe,Arabia1054Taurus (Crab)-52China,North America,Arabia1572Cassiopeia-45Europe(Tycho, et al)1604Ophiuchus-26Europe(Kepler, et al)1987LMC+350The world14More supernovae

These exploded in 2001What about the rest of the elements?If iron is the heaviest element a star can create, how is gold, silver, uranium, and the rest of the periodic table formed?The answer is in those brief seconds of the Supernova explosion when there is more than enough energy available.You are made up of StarStuff the results of the death of a massive star

16What's LeftAfter the massive star implodes (followed by the supernova explosion) the inner part of the star remains.If the mass of this inner core is less than about 4 solar masses then it becomes stable.What's left is about the size of Manhattan Island (with up to 4 times the mass of the sun compressed into it)The immense gravity is balanced by degenerate neutron pressure. When the protons and electrons were forced too close they were transformed into neutrons which are capable of withstanding more pressure than the electron gas holding apart the white dwarf.These stars are now Neutron Stars17Neutron StarsStellar core squeezed together to neutronsSupported by neutron degeneracy pressureAstonishingly small size and large densityNeutron star Mt. Everest

A sugar cube of neutron starAll of humanityA cubic centimeter of neutron star weighs as much as all of humanityNeutron StarsOn the surface of the EarthLGMsA young graduate student, Jocelyn Bell, was using a radio telescope and found that there was a strange signal. The first thought was this was a radio beacon from LGMs(that is Little Green Men)

20PulsarsThe source instead is a rapidly rotating neutron starIts radio signal similar to the light beam from a lighthouseAs the beam sweeps by you get a pulse

21M > 4 MsunWhat if the remainder from the supernova has more than 3-4 solar masses?Then the neutron pressure cannot withstand the force of gravity and the core collapses.What can withstand these pressures and bring the star's core back into balance?Nothing22InterludeBefore we can discuss the region of space near the Black Hole, we first have to deal with the nature of time and space.

In 1905, Albert Einstein realized that Newton's view of the universe was not quite correct.

In Newton's Universe, space had 3 dimensions where objects were located.

They moved from point to point in time according to some absolute, or universal clock which was independent of space.

In Einstein's Universe, space and time are linked; time is another dimension and objects are located and move in Spacetime

23RelativityEinstein's 1905 joining of space and time is known as the Special Theory of Relativity. Another way of looking at this is that for Newton, there is some absolute frame of reference, at rest, from which everything can be measured. For Einstein, there is no such reference - all things have the same status; everything must be measured relative to each other

It is 'Special' in the sense that it is 'limited' It does not deal with non-uniform motion.24RelativityFor 10 years, Einstein worked to extend his ideas to non-uniform motion. The result was 1915's General Theory of Relativity

Recall that some time ago we discussed Newton's laws and wrote down:F = m aINERTIAL massandF = G m M/r2GRAVITATIONAL mass25General RelativityThe General Theory of Relativity is based on the "Principle of Equivalence"

That is, Inertial Mass = Gravitational Mass

26General RelativityEssentially, this means you cannot tell the difference between accelerating or being in a gravitational field.

Suppose you were enclosed in a windowless box (an elevator cage, for example). You could be out in space being pushed by a rocket or sitting on earth there would be no way to determine which is the truth

27Newton versus EinsteinThe Tao of Newton:Mass tells gravity how to exert a forceForce tells mass how to move The Tao of Einstein:Mass-energy tells space-time how to curveCurved space-time tells mass-energy how to move28The Tao of NewtonConsider a small mass passing near a larger one:The masses create a force according to the law:F = GmM/r2As they get closer, the force increases between the massesThe masses accelerate according to F = m a, causing them to move (the smaller mass curves about the larger)29"Houston, There's a problem"How does the force communicate across the distance separating the masses?

According to Newton, it acts instantaneously so that for each 'update' of positions, the force changes and can act on the masses immediately.But, according to Special Relativity, nothing can move faster than the speed of light so nothing is instantaneous

"What we have here is a failure to communicate"

So how does it work?30The Tao of EinsteinConsider the same small mass passing near the same larger one:The large mass causes space-time to curve about it similar to the effect of a heavy ball placed on a thin rubber sheet.In deep space, away from any other masses, space-time is "flat" and the small mass moves in a straight line.The small mass simply follows the curve of space-time, altering its path and ending up swinging around the large one. Not because of any instantaneous forces, but simply following the "landscape"

31Tests of General RelativityPrecession of the Perihelion of MercuryBending of StarlightBinary PulsarsGravitational Redshift32Precession of the Perihelion of MercuryInstead of Mercury's orbit being stable and retracing its path, it precesses. Some of this can be explained by Newton's theory, but there is still an error of 42.98"0.04"/century left unexplained. General Relativity predicts the precession to be 42.98"/century.

33Bending of StarlightWhile photons do not have mass, they do have mass-energy, therefore the curvature of sp

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