In its life as a red giant star the temperature and pressure in the Sun will be high enough for it to produce atoms of oxygen. There will be a small helium core at the center of the Sun, about twice the size of the Earth, where carbon and oxygen are produced for a period of about two billion years. The Sun will remain as a red giant, slowly losing matter from its outer layers into the surrounding space and turning into what is called a planetary nebula, shrinking in size until it becomes a white dwarf.
Our Sun will end its life with all its remaining matter compressed into a sphere about the size of the Earth but with a mass of about 70 percent of the original solar mass. The white dwarf is therefore an incredibly dense star; one teaspoonful of it would weigh about 5 tonnes (4.9 tons). But a white dwarf is a very dim star and for this reason comparatively few have been discovered, although one of our near neighbors has already reached this stage of its evolution. It has long been known that Sirius, the brightest star in the sky, is in fact a binary star, and the dark companion of Sirius is the nearest white dwarf star to the Sun. Thus the burning of the elements inside the stars is the process by which the elements higher up the periodic table are formed. In the case of the Sun, however, no element heavier than oxygen is created. Yet heavy elements are abundant in the Earth’s crust and elsewhere in the universe. There must therefore exist another mechanism whereby they are created.
The Death of Larger Stars
Stars with much higher masses than the Sun follow a similar course of evolution until they reach the final phases of their life, but the more massive the star the shorter is its time on the main sequence of the H–R diagram (a system of star classification based on plotting the star’s magnitude at a standard distance from Earth, against the star’s color due to its surface temperature). The period of time during which the star can create the lighter elements varies according to the mass of the star. The Sun will take about ten billion years to burn up all its hydrogen, but a more massive star, say 25 times the mass of the Sun, could exhaust all of its hydrogen in only about six million years. The larger star would take about half a million years to burn the helium to produce carbon. The carbon burns in a mere 600 years creating neon and oxygen. The neon burns in about a year and the oxygen in about six months. The silicon burns in a single day. Then there follows a very spectacular explosion as the star collapses on itself. The radiation pressure of the photons created inside the stars is the only thing that prevents them from collapsing under their own gravity. When the burning process ceases there is no pressure left to hold up the outer mantle of the star.
The star will cool rapidly by astronomical standards as the radiation pressure falls and the force of gravity takes over. In the 1930s the Indian-born American astronomer Subrahmanyan Chandrasekhar (1910–95) was able to show that if a star measured more than about 1.4 solar masses then it would have a very different future from that of the Sun. The evolution of the more massive stars is quite different from the less massive ones, and the full story took much longer to uncover. The first phase of the more massive stars is similar to that of the less massive stars. When they run out of hydrogen they are able to burn helium, and they expand to become red supergiants so large that they have a diameter about the same as the orbit of the planet Jupiter about the Sun. The gravitational field inside the larger star is immense, and the radiation pressure is not capable of supporting it. The star collapses under gravity and this drives the temperature up to more than a billion degrees kelvin. The burning of helium leaves behind carbon and oxygen, but at the incredibly high temperatures the nuclei of these elements are traveling at very great velocities—at a sizeable fraction of the speed of light. The star in turn creates nuclei of neon, silicon, phosphorus and magnesium. The time taken for all this to happen varies greatly according to the size of the star; some stages take far longer than others. The burning silicon produces iron. This is the most stable of the elements and no matter how high the temperature rises the iron will not “burn.” The nuclei of iron cannot be changed to heavier elements.
Supernovae—the Cradles of Life?
The star begins to collapse into the core. The density of the core will be about 1017 kilograms per cubic kilometer, a figure that equates to a hundred times the mass of the Earth in every cubic kilometer. This is the density of the nucleus of the atom, for in fact the star has evolved to become what we call a neutron star. As the star collapses, all the matter falls back into the hot and dense body of the star creating an inferno brighter than anything else in the sky. The result is called a supernova. Such events, once rarely seen, are now observed almost routinely in other galaxies. Supernovae were used to discover dark energy in the 1990s.
The supernovae are an important class of star, but the full realization of their value is another story that was not revealed until the 1ate 1950s and it will be told in the next chapter. It is sufficient for the moment to say that without the supernovae life on Earth or anywhere else in the universe could not exist.
Supernova Events
In the past one thousand years there have been only four sightings of exploding stars, or supernovae, in our own galaxy. The first was in 1006, and is not well documented. It was followed by another new sighting in 1054, when Chinese astronomers recorded a new star appearing in the Crab Nebula. The third was the well-documented star seen in the constellation of Cassiopeia by Tycho Brahe in 1572; it was so brilliant that it was visible in broad daylight for several weeks. The fourth supernova was seen by Kepler in 1604 and is again well documented.
Astronomers have waited patiently—and impatiently—for another supernova to appear so that it could be studied by modern methods and equipment. So far, they have had no opportunity to observe another supernova in our own galaxy. In 1987 they witnessed something almost as good—it was a supernova in our neighboring galaxy, the Large Magellanic Cloud. It was so bright it could still be seen from Earth with the naked eye.
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BEYOND THE VISIBLE SPECTRUM
With the development of the radio telescope a new and vitally important astronomical tool became available. Now objects from deep in space, hitherto unknown, could be detected, unlocking more doors and answering more questions about the universe. At the same time the discoveries made by radio astronomy posed new challenges to our understanding of the stars and planets.
Karl Jansky (1905–50) was a physics graduate who joined Bell Laboratories in New Jersey, USA, in 1928. The company was developing the use of short radio waves for a transatlantic telephone service and was discovering that spurious radio signals, otherwise known as static interference, sometimes interfered with the transmissions. Jansky’s job was to track down the source of the radio signals so that they could be eliminated. In 1931, using a rotating antenna that he had built, Jansky eventually found the source of the radiation. It seemed to originate from somewhere in the constellation of Sagittarius, in the center of the Milky Way Galaxy. Exciting and intriguing though Jansky’s discovery was, his employers refused his request to build a telescope to investigate the source of the radio waves further, since they were deemed not to be a significant problem for their planned communication system after all. Instead, Jansky was assigned to work elsewhere in the company.
It was left to another American, Grote Reber (1911–2002), a part-time astronomer living in Illinois, to explore the source of the radio waves originally located by Jansky. Inspired by Jansky’s work, Reber built a primitive radio telescope in his back yard—the first one ever made. In 1936 he detected radio emissions from the Milky Way, confirming Jansky’s earlier findings. Reber then went on to undertake a systematic survey of radio waves from the sky, and laid the groundwork for a major field of astronomical research.
Bigger Radio Telescopes
The early radio maps of the sky seem very crude compared with the optical maps of the time, because the far longer wavelength of radio means that the resolution is orders of magnitude lower. In the 1930s it was difficult to give accurate positions of objects detecte
d in the radio spectrum, and it was impossible to link the emissions with specific stars. Much more sophisticated instruments were needed. World War II (1939–45) held back the progress of radio astronomy, but the use of radar indirectly helped with the development of the science.
After the war radio astronomy developed quickly. The Jodrell Bank Telescope in Cheshire, England, was first conceived in 1951. It was designed in the form of a large metal bowl with a diameter of 76 meters (249 ft), and it was the first radio telescope of such dimensions that could be fully rotated. The telescope bowl was originally designed in the form of a wire mesh, but by the time construction began the 21 cm (8.3 in) line (named after the wavelength of the radiation) had been identified in the spectrum of hydrogen and it was realized that this would become an important wavelength in radio astronomy. The design of the bowl was therefore changed from a wire mesh to a solid reflecting surface so that the 21 cm (8.3 in) radiation could be detected. The Jodrell Bank Telescope was functioning by 1957, in time to track the signals from the Soviet satellite Sputnik I, and it played a major part in the mapping of the radio sky.
“Little Green Men” and Pulsars
In 1967 a young astrophysics research student called Jocelyn Bell (b. 1943) was working at Cambridge University where she was studying for a PhD. The university had just finished building a primitive radio telescope consisting of 2 hectares (4.5 acres) of chicken wire. Instead of rotating like the Jodrell Bank Telescope, it remained fixed and it used the Earth’s rotation to scan the skies. When the recordings of the signals received by the telescope were examined, something extraordinary was discovered. Jocelyn Bell and her colleagues noticed a regular pattern of pulses occurring—indeed, the pattern was so regular it seemed that the signal must be human-generated. The interval between the pulses was measured very accurately at 1.3373011 seconds. There was much speculation as to the cause of the pulses. The most likely origin appeared to be a nearby earthbound source, perhaps a machine from one of the locally situated electronics companies or possibly some new kind of radio-controlled agricultural equipment. The idea that the pulses originated somewhere on the Earth was soon proved to be unlikely, however, when it was discovered that the source of the pulses followed a sidereal cycle rather than a diurnal cycle. In other words, they were in phase with the rising and setting of the stars rather than with the Sun. The second most popular theory for the source was rather more imaginative than the first, and it was given the name “little green men” because it implied that the pulses were being generated from an extraterrestrial life form. When the popular press got news of the find they descended en masse to Cambridge to report on the story, hoping to print news of the first contact with an extraterrestrial civilization. The journalists received disappointingly cautious answers to their questions, however, and the little green men theory was soon abandoned.
But what did emerge eventually from the discovery was nearly as exciting for the astronomers. Attempts were made by the researchers to locate the position of the source in the sky as precisely as possible. Optical and other radio telescopes came to their aid and the source was soon located in the constellation of Taurus. Closer inspection showed it to be situated in the familiar cloud of gas called the Crab Nebula. Then, as the astronomers homed in even closer, the radiation was found to be coming from the very location of the supernova of the year 1054, the new star observed by Chinese astronomers more than nine centuries ago!
The fact that the radio pulses were associated with a supernova was an exciting find and it seemed very significant that the radio emissions should be generated not by what was seen as a “new” star but by one in the final stages of its evolution. Because of its regular radio pulses, the word “pulsar” was given to this new discovery. At that time there was no obvious mechanism linking regular radio pulses with a supernova. It was necessary to find an explanation.
Neutron Stars
As early as 1934 it had been suggested that there existed in space an amazing object called a neutron star. This was the remnant of a supernova created when a massive star became so compressed that it collapsed under its own gravity. The gravitational pressure at the center of the star became so strong that all the neutrons became fused together to create a core of nuclear fluid of incredible density and with properties that could not possibly be simulated in a terrestrial laboratory. The neutron star was an unknown object but photons and neutrons were well understood and many of the properties of the neutron star, in particular its density, could be calculated from the theory of atoms and gravitation. The more the pulsar in the Crab Nebula was studied the clearer it became that the source did indeed have the properties expected from a neutron star. The case was examined and discussed at length by the astronomical world, and the only logical conclusion seemed to be that the object was indeed a rotating neutron star. For many months afterward the supernova observed by the Chinese in 1054 became the most studied object in the night sky.
Just like the Earth, the neutron star had a magnetic axis that was inclined to its axis of rotation. As the star rotated, charged particles were accelerated by the magnetic fields to create a beam of radiation directed along the magnetic axis, and as this axis precessed the beam swept round like a searchlight describing the surface of a cone. The Earth happened to lie on the surface of that cone and consequently it received a pulse of radiation once on every rotation, in other words, every 1.3373011 seconds.
The discovery of a neutron star validated many theories. It also opened up two amazing possibilities. Both of these had been suggested some time before 1967 but both needed a neutron star to prove their case. One of these was the existence of conditions necessary for the formation of the heavy elements. The other was the existence of black holes. We will deal with heavy elements formation first, starting with an idea that originated about a decade earlier than the discovery of the pulsar in the Crab Nebula.
Solving an Elemental Conundrum
The astronomer Fred Hoyle knew that he was losing his case for the steady-state model of the universe, but in 1957 he co-published a paper, now known as the B2FH paper, which he hoped would strengthen the argument for his theory. In the debate about steady-state versus Big Bang Hoyle had pointed out a weakness in the Big Bang theory. The theory stated that in the first three minutes of the primordial fireball it was possible to synthesize the nuclei of hydrogen and helium—these acquired electrons and became atoms, and they provided the basic material needed to make the stars. The Big Bang could also have synthesized more of the lighter elements in the periodic table, but what it could not have done was create the heavier elements. It was known that the Sun was powered by the fusion of hydrogen atoms to produce helium. But the Sun also contained elements much heavier than helium. The Earth and the other planets were also built from heavy elements—a fact that had been known for centuries—and they could be refined from rocks and ores. It was also well known that the magnetic field of the Earth was due to its molten-iron core.
The question was, where did the heavier elements come from if they had not been created by the Sun? The B2FH paper described how the nuclei of the elements could be formed in the evolution of the stars. Red supergiants—in other words, very massive stars—can undertake nuclear syntheses beyond those of the Sun and they are able to create elements as heavy as carbon and oxygen. Hoyle recognized that there was a problem explaining the creation of the carbon atoms, but he was able to overcome this difficulty. A carbon nucleus contains six protons and six neutrons, so it can therefore be created from three helium nuclei. The problem is that although the collision of two particles is very common it is quite a rare coincidence for three particles to all appear at the same point at the same time. However, Hoyle was able to show that under suitable stellar conditions the triple collision is a common enough phenomenon, and also that sufficient collisions occur to create the carbon nuclei in the quantities required by observation.
Once over the “carbon hurdle” then nitrogen, oxygen and the higher elements co
uld be created with relative ease, but to create the heavier elements needed much higher temperatures and denser radiation. Hoyle reasoned that one place where these extreme conditions could be found was in the center of a supernova. As the star collapses into a neutron star temperatures become so high that the energy exists for heavier nuclei to collide, and the nuclei of elements even higher in the periodic table are created. It is a sobering thought that supernovae like the ones we observe just a few times in every millennium in our own galaxy are the source of all the atoms of the heavy elements on our planet.
It is true that atoms can be split by collisions with other particles, but in general the atom is indestructible and it can exist forever. The human body is built up from complex organic molecules, but all those molecules are themselves composed of atoms. All the heavy atoms in our body have therefore been created inside a massive star or even in some cases a supernova. The atoms in our bodies are billions of years old—minute building blocks recycled through countless generations of living organisms. Through all this time the atoms remain “as good as new.” Nor have they aged since the time they left the distant and long-forgotten supernova from where they were originally created. We all carry around inside us remnants of supernovae explosions that took place aeons ago and millions of light years away.
Elements across the Universe
Iron and the other heavy elements appear in great quantities both on Earth and in other planets of the solar system. A supernova event, like the ones we observe from time to time, seems a very slow way to form these elements, but when the years are counted in billions then it becomes clear that there have been a great many supernovae explosions throughout the history of the universe. We also need to ask some questions about how these elements have managed to cross the vast distances of space to reach our own planet and everywhere else in the galaxy. We know that it takes years to span the distance from star to star even at the speed of light, so how can all the elements cross the vast spaces between the stars to become relatively abundant around stars where planets are forming? The answer again is to be found in the vast aeons of time that have elapsed since the birth of the universe. If the atoms travel at only 1 percent of the speed of light then it takes centuries rather than years to cross the spaces between the stars. But since the birth of the universe millions of centuries have been available for this to happen, and the atoms that formed the Earth could easily have traveled the vast distances across the galaxy—especially since the huge power of supernovae blasts would have contributed to the distribution of the elements over a wide region of space.
The Story of Astronomy Page 18