The Story of Astronomy (23 page)

In about five billion years time, when the hydrogen fuel is exhausted, the Sun will slowly expand until it becomes what we call a red giant. There are plenty of red giants visible in the sky, of which Betelgeuse in the constellation
of Orion is the best known and most widely studied. When the Sun reaches this stage it will signal the final death throes of the Earth. The Sun will expand until it is so large that it will completely swallow up the orbits of both Mercury and Venus. The temperature at the center of a red giant is high enough for the helium to burn. Once hydrogen burning ceases, the gravity will compress the core of the star, raising its temperature and density until they are high enough to start fusion of helium to form carbon and oxygen, thus providing a new source of energy to resist the inward pull of gravity.

It proved extremely difficult to work out how the carbon nuclei could be formed, and the astronomer Fred Hoyle (1915–2001) deserves much of the credit for providing the details. It requires three helium nuclei to form a nucleus of carbon, and in the process a vast amount of radiation is released as gamma rays. The triple collision between three helium nuclei is a far less likely event than the very common collisions between only two particles, but Hoyle proved that when the density and conditions were right then the carbon nuclei could be created in this way.

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.

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.

The star begins to collapse into the core. The density of the core will be about 10
¹⁷
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.

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.

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.

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 detected 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.

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.

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.