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 5

High-Mass Stars (M > 9 MSun) massive stars zig-zag in HR Diagram using new fuels:

fuel used upcore of star shrinks, heats upnew nuclear fuel ignites until last fuel is used upSUPERGIANTS6H, He, C, Ne, O, Si, FeA 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/second12

The Crab Nebula pictures taken in 1973 and 2001CORPSE13Historical 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

16You are Star StuffDying stars give important chemicals back:supernova explosions (main source of Fe)planetary nebulas (source of C, N, O)

POLLUTION OF GAS CLOUDSSTAR FORMATIONLOW-MASS STAR DEATHHIGH-MASS STAR DEATH17What'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 Stars18Neutron 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)

21PulsarsThe 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

22M > 3 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?Nothing23High-Mass Evolution

Pulsar and Supernova remnants in the MilkyWay

The purple dots represent the known pulsarsThe dark red dots represent the known supernova remnantsInterludeBefore 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

26RelativityEinstein'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.27RelativityFor 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 mass28General RelativityThe General Theory of Relativity is based on the "Principle of Equivalence"

That is, Inertial Mass = Gravitational Mass

29General 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

30Newton 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 move31The 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)32"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?33The 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"

34Tests of General RelativityPrecession of the Perihelion of MercuryBending of StarlightBinary PulsarsGravitational Redshift35Precession 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.

36Bending of StarlightWhile photons do not have mass, they do have mass-energy, therefore the curvature of space-time should cause them to curve about a massive objectDuring a total solar eclipse a star was observed next to the Sun, however, the actual position of the star was behind the SunThe path the starlight took followed the curving 'landscape' The predicted deflection and matching measurement was 1.75"37Bending of Starlight

The starlight just follows the shortest path38Gravitational LensingAn object located behind a massive compact object will have multiple images formed

Einsteins Crossan Einstein ringgalaxy directly behind a galaxy39Gravitational RedshiftGeneral relativity also predicts that photons, since they must use energy to "climb out of the gravitational well" formed by the curved space-time will exhibit this energy loss by shifting their wavelength toward the red end of the spectrum.

Again, this can be measured experimentally and agrees with the prediction to within 2x10-4ObjectGravitational RedshiftEarth10-9Sun10-6White dwarf10-4Neutron Star10-3Black holeLARGE: Proportional to mass/radius40The Ultimate RedshiftIn 1783, John Mitchell, an English clergyman and amateur astronomer, determined the escape velocity for several objects: He calculated that to escape Earth's gravitational pull an object must accelerate to 25/1000 the speed of light (about 11 km/sec). He then further postulated that to escape the Sun's gravitational pull and object must accelerate to 1/500 the speed of light (618 km/sec). Intrigued, he wrote that if the sun's mass was increased by a factor of 500, the escape velocity would equal that of the speed of light. In a letter to a colleague he wrote, "all light emitted from such a body would be made to return toward it by its own proper gravity."41Complete Gravitational CollapseIf the core undergoes complete gravitational collapse, space and time warp.The gravity gets so strong that not even light can escape.This is a Black Hole.42There is a border within which nothing can escape, the Event Horizon. Outside of the event horizon, it is just a mass --- but inside!

Complete Gravitational Collapse43Black HoleA black hole is NOT a cosmic vacuum cleaner!

It is NOT some colossal drain into which all the Universe is flowing!

Beyond its event horizon, it acts like any other mass. You could safely orbit as long as you don't get inside the event horizon.

Once inside, however, there is no escape44Black Holes

45Vacuum FluctuationsLets pause for a moment and instead of thinking about the large-scale universe we consider the smallest scales possible.In classical physics, the vacuum is totally empty; it is the absence of everythingIn quantum physics, the vacuum is a seething hotbed of activity. The vacuum is filled with virtual photons continually creating/destroying pairs of particles. This pair creation/annihilation is known as the Vacuum Fluctuation.46Vacuum FluctuationsVacuum fluctuations can be pictured as:e-e+A virtual photon creates an electron/positron pair, which immediately annihilate each other to become a virtual photon.This has been measured in the laboratory as the Casimir Effect47Hawking RadiationWhat does vacuum fluctuations and other quantum weirdness' have to do with Black holes?Suppose the virtual pair was produced just outside the event horizon of a black hole.One member of the pair could fall in while the other escape.Conservation of mass-energy then requires the black hole to shrink a bit

The radiation from this is named after its discoverer, Stephen Hawking.

A black hole will evaporate in a time proportional to M348WormholesGeneral relativity also predicts the existence of connections between folds of the Universe.This could permit time-travel and therefore paradoxes. Hawking feels that quantum theory will prohibit wormholes and avoid the paradoxes.

49Gravitational WavesRipples in the curvature of space-timeThe observational evidence is their emission by binary pulsars. The first studied was the PSR1913+16 which is formed by two neutron stars, Hulse and Taylor were able to measure its orbital parameters and found that the two bodies are spiraling one into the other as they lose energy by emission of gravitational waves. These measurements are in excellent agreement with the prediction of General Relativity50