16. Stars 3

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 the form of gravity waves. Not all pulsars keep excellent continuous time. Some pulsars show abrupt small changes in their spin rate called glitches. These are due to seismic variations in the neutron star crust itself. The corresponding changes in the magnetic field affect the rotation rate.

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 with strong radio emission that’s beamed towards the Earth. In 1968 the theorist Thomas Gold showed how charged particles trapped in magnetic fields on the surface of a spinning neutron star could produce focused radio waves. Think of a pulsar as a little lighthouse with a beam of radio emission emerging from a hot spot on the surface. As the pulsar spins, the beam of radio waves passes across the surface of the Earth, and we see a pulsar. But since many neutron stars either don’t have hot spots or have pulses of radio emission that do not cross the Earth, we do not see all the neutron stars; we only see the subset with radio pulses pointed at our direction.

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.

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, and when a star collapses those magnetic fields will be squeezed and strengthened. Astronomers speculate that the surface magnetic field of neutron stars can exceed a trillion Gauss. This is a field-strength a million times greater than any magnetic field produced on the Earth. You can think of a neutron star as a gigantic atomic nucleus with an atomic number of 1057. For three decades theorists had predicted neutron stars, but astronomers could do no thing to detect them and could only speculate that they existed.

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.

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.

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 about fifty lightyears of the Earth it would produce enough high energy radiations to destroy life on Earth or at the very least alter DNA and so disrupt the entire chain of life. However, the odds of that happening are very small. There seem to be no stars set to become supernovae within ten or fifteen parsecs of the Earth. However, the odds are still strong that within the human lifetime there will be a supernova somewhere in the Milky Way that will become visible to the naked eye at night and possibly even during the day.

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.

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 gave rise to the Crab Nebula. At the time it exploded it was visible in broad daylight for twenty-three days in July 1054 and at night for another six months afterwards. The Crab Nebula was recorded in Chinese, Japanese, and Islamic documents, and in Native American rock art. There have been fourteen supernovae in recorded human history, and in a sense we’re long overdue because the last one was nearly 400 years ago.

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 days. Years later a colossal expanding nebula is seen in the sky; it’s called a supernova remnant.

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 quarter the speed of light. The density rises almost instantaneously by a factor of a million, and a volume the size of the Earth is squeezed down to a size of about fifty kilometers. This all takes place in only a few seconds. Protons and electrons are forced to coalesce producing neutrons and a flood of neutrinos that flee the scene and emit 1047 watts. The luminosity of a supernova in the instant of the core collapse and just after exceeds the luminosity of the entire universe.

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.

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 as a red giant, and about 5 billion years from now it will end its life as a white dwarf of about 0.6 solar masses. And since most stars are like the Sun or less massive than the Sun, most stars will end their lives as white dwarfs.

Transcript: The physical state inside a white dwarf is extraordinary. The density is in the range of 108 to 1011 kilograms per cubic meter. A teaspoon of white dwarf material brought back to the Earth would weigh as much as an elephant. At these densities and temperatures electrons move nearly at the speed of light, and matter no longer behaves as a perfect gas. In fact, it’s a new state of matter called degenerate matter. The atomic nuclei essentially form a lattice with the relativistic electrons moving between them. It’s as if the star were a huge diamond given that so many of the heavy nuclei are carbon atoms. The quantum theory says that no two particles can have exactly the same set of properties, and this limitation acts as a pressure, called degeneracy pressure, that keeps the matter supported. And this is the reason that the star does not further collapse. White dwarfs are truly extraordinary items essentially with the density of an atomic nuclei but with an atomic number of 1057.

Transcript: The theory of white dwarfs was first worked out by Subrahmanyan Chandrasekhar. Born in India, Chandra, as he was universally known, began thinking about white dwarfs on a long boat voyage to college in England. He experienced prejudice many times in his life, but nothing was more hurtful to him than the scorn that senior astronomers poured on his idea of how white dwarfs worked. But he persevered, and he was proved correct. He was a brilliant theorist, the father to a whole generation of astrophysicists, and the editor of the most premier journal in astronomy for decades. In 1983 he was awarded the Nobel Prize in physics.

Transcript: For stars that begin their lives in the range of a tenth times the mass of the Sun to a few times the mass of the Sun the temperature in the core will never be sufficient to create elements beyond carbon in the periodic table. Eventually the fuel supply is exhausted, and without pressure support the core of these stars must collapse and produce the state of an exceptionally dense star called a white dwarf. The temperature of white dwarfs ranges mostly from 10 to 20 thousand Kelvin, although they can be found as hot as 100 thousand Kelvin. This large amount of energy storage in a small area means that the cooling times of white dwarfs are exceptionally long. The oldest white dwarfs in the universe have only cooled to about 4,000 Kelvin and have luminosities less than a ten thousandth of the Sun’s luminosity, taking 10 billion years to do so. Trillions of years from now these ember stars will still be slowly cooling.

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.

Transcript: Every star in the night sky will one day exhaust its nuclear fuel and die. A battle rages within the heart of every star between the inward force of gravity and the outward pressure caused by energy release from nuclear reactions. This is a battle that gravity will always win in the end, and the mode of a star’s death depends, as its life does, on its mass. Low mass stars will die by having their material entombed in a cooling ember, and high mass stars will die in a last pyrotechnic flourish to create and spit heavy elements out into the space between stars and leave a collapsed stellar remnant.

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.

Transcript: The cosmic abundance of different elements in the periodic table is a fundamental property of the universe that astronomers have successfully explained. The most abundant elements, hydrogen and helium, three quarters hydrogen and one quarter helium by mass, can only be explained when we understand the big bang. Helium was created in the first few minutes of the universe itself. Almost all the heavier elements come from nucleosynthesis within stars. The features of a graph of cosmic abundance versus atomic number show that after hydrogen and helium there’s a deep hole in abundance. Boron and beryllium are extremely rare elements because the decay prohibits them from being built up rapidly. There’s then a peak, carbon, nitrogen, and oxygen, the life elements, at abundances of 10-4 relative to hydrogen, then a slow decline, elements like magnesium and silicon at an abundance of 10-5 relative to hydrogen, and the pattern of elements show a sawtooth pattern every two in atomic number representing the addition of a helium nucleus in the way that the elements are built up. Then follows the iron peak, iron the most stable element, followed by a rapid fall down to abundances of only 10-9 or 10-10, silver and tin are at abundances of 10-11 , and then slow decline to the heaviest elements of all; gold has an abundance of 10-12 relative to hydrogen.

Transcript: The iron at the center of a massive evolved star is not the familiar metal that we are used to, even though the density is very high. The temperature is also high, a billion Kelvin or more, so the iron is a high temperature gas or plasma. Iron is the most stable element, so energy is consumed to make elements more massive than iron. Thus, the heart of a massive star is an iron tomb. There are, however, two ways in which elements heavier than iron can be produced. Helium capture does not work, but neutron capture does. And so in a process called the S process, S for slow, neutrons can gradually be added to heavy element nuclei to build elements in the periodic table all the way up to bismuth. The second way is when the energy is derived from the core collapse of a dying star, a supernova. In this rapid process enough energy is deposited in an explosion to create heavy elements up to radium, uranium, and plutonium by what’s called the R process, R for rapid.

Transcript: In the most massive evolved stars, the star has an onion skin layering where heavier and heavier elements are concentrated in layers closer and closer to the core because the temperature and pressure continue to increase moving towards the center of the star. For example, in a fifteen solar mass star the outermost layers will be filled with a cosmic mixture of three-quarters hydrogen, one-quarter helium by mass. The inner four solar masses will be helium at a temperature of about 60 million Kelvin. Inside that is a layer of carbon at 200 million Kelvin. The inner two solar masses will be oxygen and silicon, successively at a temperature of about a billion Kelvin, and at the center is one solar mass of iron at a temperature of 6 billion Kelvin. Not only is there a layering but there’s also a speed involved in the timescale of producing heavy elements. Heavier and heavier elements are produced on faster and faster timescales late in the star’s life.

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.

Transcript: Producing heavy elements by the fusion process requires extreme temperatures. This is because of the electrical force of repulsion that operates between protons. Forcing two protons to combine is four times easier than forcing two helium nuclei to combine, and so on up the fusion chain higher and higher temperatures are required to make heavy elements. In low mass stars, less than about one and a half times the mass of the Sun, the temperature in the core never exceeds the temperature required to produce elements heavier than carbon, and so the fusion process stops with carbon. That’s essential because carbon is a life element, and when it reemerges in the interstellar medium it can seed the formation of planets and eventually life. But for most stars in the universe carbon is the end point of the fusion chain.

Transcript: Where did the heavy elements come from? They were not present at the big bang, the birth of the universe. Stars are the cauldrons that have produced the calcium in our bones, the nitrogen and oxygen in the air we breathe, the metals in the cars we drive, and the silicon in our computers. The story of the creation of heavy elements was first worked out in the 1950s by four young astrophysicists working at the California Institute of Technology. They were the husband and wife team of Geoffrey and Margaret Burbidge, Fred Hoyle, an iconoclast astrophysicist from the north part of England, and Willie Fowler, an American. All four became famous; the Burbidges were both observatory directors, Fred Hoyle was knighted, and Fowler won a Nobel Prize. Their masterwork was a long paper which produced a perfect explanation of the origin of the heavy elements in the advance stages of stellar evolution.