Transcript: A fundamental prediction of General Relativity is the fact that time slows down in strong gravitational fields.  The ultimate test of this idea would be to observe someone falling into a black hole carrying a clock.  In theory, the clock would slow down and come to a complete halt as they reached the event horizon.  We can’t do that experiment, but physicists have done other experiments in weaker situations of gravity and seen the effect of time slowing down.  Atomic clocks do run slower on the Earth’s surface than they do in a plane five miles up where the gravity is slightly weaker, and a light beam passing by the limb of the Sun slows down by a measurable 250 millionths of a second.

Gravity and Time

Transcript: Any change in a gravitational field or gravitational configuration causes ripples in space time to be emitted.  These disturbances which travel at the speed of light are called gravity waves or gravitational radiation.  Pulsars slow down slightly in their periods, and this corresponds to the conversion of rotation energy into gravitational wave energy.  It’s a very subtle effect and can only be measured because pulsars are such excellent clocks.  In 1993 this indirect prediction of General Relativity, the emission of gravity waves, was confirmed by Joe Taylor and Russell Hulse who subsequently won the Nobel Prize in physics.

Gravity Waves

Transcript: If you throw an object up into the air it will eventually slow down and fall back to Earth.  The object is losing kinetic energy by trying to climb out through the gravitational field of the Earth.  Photons also lose energy as they climb out of the pit of gravity.  This effect is called the gravitational redshift.  It’s a very subtle effect.  For the Sun the gravitational redshift is only 0.0002 percent, but it has been measured.  In a situation of more intense gravitational field, the gravitational redshift is proportionally larger, and of course at the event horizon of a black hole, the gravitational redshift is infinite.  The infinite redshifting or loss of energy corresponds to the trapping of radiation at the event horizon.

Gravitational Redshift

Transcript: A light beam is deflected slightly by gravity.  According to E = mc2 any photon has energy, and so it has equivalent mass.  Therefore, a mass acting on a photon will bend it.  A better way to think of it is that the mass causes a distortion of space, so the photon is following the distortion of space and time.  In 1919, an eclipse expedition showed that light bent around the limb of the Sun exactly by the amount predicted by General Relativity, an amount larger than the deflection predicted under Newton’s theory of gravity.  Most physicists stayed up all night in eager anticipation of the result, but Einstein himself went to bed.  He was supremely confident in the correctness of his theory.

Gravitational Deflection of Light

Relativity

Transcript: Collapsed stellar objects offer the best chance to test Einstein’s Theory of General Relativity.  General Relativity is based on the idea that acceleration due to gravity is not distinguishable from acceleration due to any other force.  The consequence of this idea is that gravity distorts both space and time.  For example, if someone were to fall into a black hole, as seen from afar, they would take an infinite amount of time to reach the event horizon, their clock slowing down asymptotically as they reach the event horizon.  For the person falling in, however, it would take only a finite time, and they would see no difference in their clocks.  Mass tells light how to move as well because light has an equivalent mass by E = mc2, and so gravity deflects light.  These subtle interactions are hard to detect in a situation of normal masses like planets and stars like the Sun, but when stars collapse and densities rise by factors of millions, as happens in the late stages of stellar evolution, the effects of General Relativity can become measurable.

Tests of General Relativity

Transcript: The exotic nature of black holes encourages speculation.  Could anyone survive a journey into a black hole?  Probably not.  For a black hole about the mass of the Sun or slightly larger, the tidal forces, say, on a human body or a spacecraft passing through the event horizon would be such as to rip the person or the spacecraft apart.  So it’s unlikely a real person could ever survive passage into a black hole.  What’s inside the event horizon?  The laws of physics give no answer to this, and nobody knows.  Even if someone could survive passage into a black hole, they would have passed the information barrier of the event horizon and could not get word out of what was inside.  Black holes are pinched off regions of space-time which leads to the speculation that they might connect to other regions of the universe via wormholes.  This is pure speculation; there is currently no way of proving the idea that regions inside a black hole connect to other parts of the universe.

Speculation About Black Holes

Transcript: If black holes are totally dark, how can they possibly be detected?  A black hole that was isolated in space would indeed by very difficult to detect, but if it’s in a binary system it is possible.  Astronomers look for binary systems where one member of the pair is massive enough to have left a core three times the mass of the Sun or larger.  In a binary system mass is pulled from the companion onto the black hole and accelerated.  The gas forms a disk around the black hole called an accretion disk, and the hot accretion disk emits ultraviolet and especially soft x-ray emission.  So to find black hole candidates astronomers start with x-ray surveys and look for intense x-ray emitting stars.  They then look for evidence of a binary system and especially a dark companion with a mass of three times the mass of the Sun or greater.  About a dozen excellent systems are known where it’s very likely that a dark companion black hole exists.  Are black holes perfectly proven?  No, but if astronomers were asked for the likelihood that black holes exist most would say its 90 or 95 percent proven.

Detecting Black Holes

Transcript: Contrary to common belief, black holes are not vacuum cleaners that suck up everything in sight.  If the Sun were instantly replaced by a black hole of the same mass, life on Earth would of course die because there would be no energy coming from the star, but the orbit of the Earth would continue essentially uninterrupted.  A parsec away from a black hole the mass of the Sun, the escape velocity is a mere 94 meters per second.  Forty AU away from such a black hole, the escape velocity is four kilometers per second.  At the Earth’s orbit distance, one AU, the escape velocity is forty-two kilometers per second.  At a distance of the Earth radius from the black hole, 6,300 kilometers, the escape velocity rises to 6,500 kilometers per second.  But all the extreme effects of General Relativity on time and space occur relatively close to the event horizon.  Within 1.5 Schwarzschild radii, 4.5 kilometers, the distance called the photon sphere, half the light is trapped, and at the event horizon, 1 Schwarzschild radius or 3 kilometers, all light and radiation is trapped.

Environment of a Black Hole

Transcript: Black holes have few observable properties; after all, there’s nothing to see.  Mass is a fundamental property, and it can be measured in principle by the gravitational interactions of a black hole with a nearby star.  Angular momentum is another property.  As with neutron stars, black holes have collapsed by a large factor from a normal stellar state, so they must be rotating very rapidly.  A black hole also has a measurable surface area at its event horizon, and it has an amount of electric charge.  We cannot say what a black hole is made of because information of the material that went in to make it is lost as soon as that material passes the event horizon.  The idea of information being lost is connected to the fact that the entropy of black holes is very large, about 100 million times larger than the equivalent entropy of a star like the Sun.

Properties of Black Holes

Transcript: The radius corresponding to the event horizon is called the Schwarzschild radius after the first theorist who solved Einstein’s equations of General Relativity for the situation of a collapsed object.  Mathematically, the Schwarzschild radius is given by twice the gravitational constant times the mass of the star divided by the velocity of light squared.  This is a fairly simple relationship which means that the Schwarzschild radius scales proportionally to the mass of the star.  If the Sun were turned into a black hole it would have to be compressed to three kilometers in radius, the size of a small town, but a more realistic situation is a star with a core mass of three times the mass of the Sun where the Schwarzschild radius is nine kilometers.  In the theory of General Relativity any star that ends its life with a core mass of three solar masses or larger must become a black hole because no known force can prevent its collapse to within the event horizon.

Schwarzchild Radius

16. Stars 3

Transcript: Evolved massive stars have sufficient pressure and gravity that the temperatures in their cores can cause heavy element creation beyond carbon.  Consider the progress of a set of stars; one of 4, one of 6, one of 8, one of 10, and one of 12 solar masses.  In the 4 to 6 solar mass stars, in their helium rich cores carbon can be produced by the triple alpha process.  In the 8 solar mass stars a set of reactions beyond carbon can continue because the temperatures are about 500 million Kelvin.  Thus, carbon can form oxygen, neon, and even magnesium in a set of reactions that add helium nuclei to the carbon nucleus.  In the heaviest stars, 10 to 12 solar masses, the temperatures exceed a billion degrees Kelvin, and reactions such as two carbon nuclei combining to make a magnesium nucleus, two oxygen nuclei combining to make a sulfur nucleus, and two silicon nuclei combining to make a nickel-56 nucleus, which rapidly decays to cobalt and then iron, can all occur.  Iron is the end of the chain of heavy element production in massive evolved stars.

High Mass Evolved Stars

Transcript: The event horizon is the imaginary surface of a black hole, the region from within which no object, no particle, no radiation, no wave, nothing can escape.  Any star that collapses to this point essentially disappears from the universe, betraying its presence only by the force of gravity.  The event horizon is not a physical surface or barrier.  It is a mathematical surface and essentially acts as an information membrane.  Information can flow into the event horizon but not out.  Some theorists have speculated that a law of physics might potentially be violated by this because information is only passing in one direction and so can be lost from the universe.  The properties of black holes within the event horizon are mysterious and are not even predicted by the theory of relativity.

Event Horizon

Transcript: Black holes can only properly be understood in terms of Einstein’s Theory of General Relativity, but speculation about their existence first occurred over 200 years ago.  In 1784 the Reverend John Mitchell, an English amateur astronomer, knew that the escape velocity from an astronomical object increased with density and gravitational field, and he speculated that a sufficiently dense object could have an escape velocity larger than the speed of light.  Since light and all other electromagnetic radiation travels at 300,000 kilometers per second, an object with escape velocity larger than this would be dark, a black hole.  Black holes are the densest form of matter we can ever know or understand, even denser than a neutron star.  If the Sun were shrunk to a radius of three kilometers it would be a black hole.

Black Holes

Transcript: In 1967 Jocelyn Bell, a graduate student working at a radio telescope in Cambridge, England, noticed an unexpected source of radio emission that pulsed every one and a third seconds.  Through careful detective work she and her coworkers were able to rule out artificial sources for the radio waves and proved that they came from a celestial source.  Radio pulsing stars were unexpected and unanticipated.  If the pulse was due to rotation, the size of the star must be less than five thousand kilometers, making them much smaller than normal stars.  For a while she and her group jokingly referred to the objects as LGM 1, 2, 3, and so on, where LGM stood for Little Green Men.  They were not alien signals, nor were they artificial.  Pulsars were the long sought after neutron stars.  Antony Hewish and Martin Ryle, heads of the labs at Cambridge, received the Nobel Prize for this discovery.  Controversially, Jocelyn Bell who actually made the discovery did not share in the Nobel Prize.

Pulsars

Transcript: In 1934 American astronomers Walter Baade and Fritz Zwicky speculated that the result of a supernova explosion might be a formation of what they called a neutron star.  If the burned out core of a massive star is more than 1.4 times the mass of the Sun, degeneracy pressure of the electrons is not sufficient to support the core against further gravitational collapse.  The collapse occurs.  Electrons and protons coalesce to form neutrons.  This is a reversal of the normal neutron decay process to produce electrons and protons.  In this form of the material the neutrons with no electrical charge are packed close like eggs in a crate, and the density rises to a phenomenal 1017 kilograms per meter cubed.  Pure neutron material is the density of an atomic nucleus, but this is an entire star.  A thimble full of this material brought back to Earth would weight 100 million tons.

Neutron Star

Transcript: Supernovae are key players in the cycle of star birth and death.  Supernovae recycle elements into the medium between stars and so provide vital ingredients for planet building and for life itself.  Supernovae can also trigger the collapse of a gas cloud and so generate the birth of a new generation of stars.  What’s left behind after the explosion?  It’s possible that nothing is left behind in some cases, that the entire star is disrupted in the detonation.  However, it is also possible to form collapsed objects that are among the most bizarre beasts in astronomy.

Cycle of Star Birth and Death

Transcript: On February 23, 1987, Oscar du Halde stepped outside his telescope to check the sky conditions at the Las Companas Observatory in Chile.  He saw a new star near 30 Doradus nebula in the Large Magellanic Cloud, a small galaxy near the Milky Way.  Homo sapiens were just developing on the plains of Africa a hundred and seventy thousand years ago when the blast wave from a dying star started out.  The star was a blue super giant, twenty times the mass of the Sun, and as it exploded its iron plasma core collapsed from the size of Mars down to a size of about a hundred kilometers.  The temperature exceeded 30 billion Kelvin, and the iron nuclei fragmented, and the explosion released a blast of neutrinos.  A hundred and seventy thousand years passed; neutrinos and light traveled through space.  The neutrinos arrived first, 10 billion passing through the body of every person on Earth.  Delicate sensors detected the neutrinos in Japan and the United States.  The light arrived a few hours later, and telescopes on Earth, on the Mir space station, and on satellite observatories focused their eyes on the dying star.

Supernova 1987 A

Transcript: A supernova is the violently explosive death of a star with the corresponding release of radiant energy, a flood of neutrinos, and the creation and ejection of heavy elements into space, the most spectacular phenomena in astronomy.  Supernovae can occur in two basically different ways.  In one way an isolated massive star has a core which is beyond the Chandrasekhar limit, and it collapses further to cause an explosion and leave a stellar remnant.  A star with an initial mass of about six times the mass of the Sun will leave a core about 1.1 solar masses.  A star about eight times the mass of the Sun will leave a core of 1.4 solar masses, the Chandrasekhar limit, and so stars more massive than eight times the mass of the Sun as they begin their lives will die as supernovae called Type I Supernovae.  The second type of supernovae, Type II Supernovae, occur in binary systems where a more massive star is in orbit around a white dwarf just below the Chandrasekhar limit.  As mass is transferred in the binary system from the more massive star onto the white dwarf, and as the white dwarf exceeds the Chandrasekhar limit, it explodes in a well regulated way.  So the luminosity of Type II Supernovae is a well determined number.

Supernova

Transcript: In 1844 German astronomer Friedrich Bessel studied the motions of Sirius, the brightest star in the sky, and found that it was being tugged by an invisible companion.  The companion was eventually detected in 1915 in the glare of Sirius, and it was far too dim to have properties that placed it on the main sequence.  It was hotter and much less luminous than the Sun.  In terms of its properties, its surface must not be much larger than the surface area of the Earth.  This entirely new type of star was called a white dwarf.

White Dwarfs

Transcript: The heaviest elements are fantastically rare compared to hydrogen and helium because they can only be produced in massive stars which are far less numerous than stars like the Sun.  But what does it mean to say that gold is a trillion times less abundant than hydrogen?  Imagine the analogy of a deck of cards.  One in twelve atoms is a helium atom.  So the deck would have its aces as helium atoms, and all the other cards would be hydrogen atoms.  Oxygen has an abundance of one part in 1,500 relative to hydrogen, so that means we would have to search twenty-eight decks of cards before we found one oxygen atom.  Gold is far less abundant; we would have to search ten billion decks of cards to find a single gold atom.  It’s a testament to the incredible power of the technique of spectroscopy that astronomers can routinely detect elements that have a cosmic abundance of about one part in 1012.

The Heaviest Elements

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