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...

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...

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...

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...

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...

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...

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...

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...

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...

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. ...

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. ...

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...

Transcript: Pulsars make excellent clocks. The collapse of the star by a factor of a million increases the spin rate by the same factor so that the star spins a number of times per second. A typical pulsar might have a frequency of ten Hertz in its spin rate, and its rotation period will typically slow by about a thirty-millionth of a second per year. Pulsars are the most accurate time keeping devices known. Even so, the spin rate is slowing down which corresponds to release of energy in...

Transcript: Over 2,000 pulsars are now known. Pulsars are found by large radio telescopes that can sensitively search through the Milky Way galaxy for these rare stellar remnants. The telescopes also tune through various frequencies to detect all the different periods of a pulsing neutron star. The periods range from around a millisecond to a few seconds. Imagine that there are stars that rotate hundreds or even a thousand times in a second. Pulsars are just the subset of neutron stars...

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...

Transcript: Neutron stars are truly remarkable objects. Think of something with the mass of the Sun, normally one and a half million kilometers across, compressed down to the size of a small asteroid, about twenty kilometers across. Conservation of angular momentum dictates that when a star collapses to this small size its rotation speed will increase. The surfaces of neutron stars are probably rotating at ten to twenty percent of the speed of light. Magnetic fields normally thread stars,...

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...

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...

Transcript: Supernova 1987 A was the first time a dying star had been visible to the naked eye in nearly four centuries, but it was in another galaxy. What would it be like to have a ring-side seat for the death of a star? In a sense, we don’t want to know the answer. Spica is a massive star at a distance of eighty parsecs or about 260 lightyears, and it’s the only one in the nearby universe that might one day explode as a supernova. If a supernova did go off within fifteen parsecs or...

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...

Transcript: Supernovae are rare because they represent the death stage of rare massive stars. On average, one occurs every fifty years in an entire galaxy. We might expect one in a human lifetime in the Milky Way, but a supernova might not be visible if it lies behind the dusty plane of the Milky Way. Ancient Chinese astronomers called them guest stars, and there’s good evidence that the star of Bethlehem was in fact a supernova. Perhaps the most famous supernova is the explosion that...

Transcript: For a few days after a supernova explosion, the dying star rivals in brightness the entire Milky Way galaxy. The expanding gas cloud moves outward at a speed of ten thousand kilometers per second or over twenty million miles per hour. The light curve of Type I Supernova has a characteristic exponential decay that’s powered initially by the decay of radioactive nickel-56 with a half-life of 55 days and subsequently by the decay of radioactive cobalt-56 with a half-life of 78...

Transcript: The advance evolutionary stages of a massive star represent a crescendo of nuclear activity. After millions of years of creating helium from hydrogen by the fusion process, each of the late stages of fusion take less than a thousand years, the creation of carbon, neon, and oxygen. The creation of iron from silicon and sulfur takes only a few days, and then with iron, the most stable element, there is no more energy support and the core collapses. The core collapses at about 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...

Transcript: White dwarfs cannot have a larger mass than about 1.4 times the mass of the Sun because above this mass the white dwarf structure becomes unstable; gravity overcomes the degeneracy pressure of the electrons and further collapse occurs. This is called the Chandrasekhar Limit, and it applies to the core mass not the initial mass. Remember that much of a massive star’s envelope is ejected into space on its way to the stellar graveyard. The Sun will loose forty percent of its mass...