The Story of Astronomy (24 page)

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.

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 B
²
FH 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 B
²
FH 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 could 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.

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 whole sequence from the birth of the stars to the creation of the planets has gone through at least two long stellar cycles. The first cycle created stars of high mass that followed the normal evolutionary pattern to die eventually as supernovae. At their death, they manufactured atoms of the heavy elements. In the second cycle of stellar evolution planets like the Earth were able to build up a solid core out of the heavy elements made available by the dying stars, and were thus provided with all the elements needed for the evolution of life.

The pulsar in the Crab Nebula helped to prove the theory of the formation of the heavy elements. There was another mystery on which it also threw some new light. After Newton, but long before Einstein, an obvious consequence of gravitation had been pointed out. The more massive a body became the stronger was its gravitational field. It
was possible, therefore, that if a star was large enough then radiation pressure would no longer be strong enough to prevent the total gravitational collapse of the star. The white dwarf star was very dense, but the neutron star was far denser. But there was an object even denser than a neutron star, an object with some astonishing properties. It was a collapsed star so dense that not even light could escape from its gravitational clutches. This dark object was given the name of a black hole. But could such a thing as a black hole exist? For many years astronomers thought such an object was physically impossible, but after the discovery of a neutron star they were forced to conclude that black holes were a possibility, and they tried to invent methods by which an object that emitted no light could be detected in the sky.

There is another way to understand the phenomenon of a black hole. If a satellite orbits the Earth at a certain speed then its velocity is sufficient to keep it in orbit for an indefinite period. However, if the satellite is accelerated sufficiently it reaches what is known as its escape velocity, which means that it is moving fast enough to escape the Earth's gravitational pull completely. In the case of the Earth the escape velocity is about 7 miles per second (11.2 km/sec), but for a satellite at the Earth's
distance from the Sun it is 26.1 miles per second (42 km/ sec). The Earth already has a speed of about 18.6 miles per second (30 km/sec) around the Sun, so the additional speed to escape from the Earth's orbit and then from the solar system is about 7.5 miles per second (12 km/sec).

Every star also has an escape velocity from a given distance, and the greater the mass of the star the higher the velocity. With an extremely massive star it is necessary to take into account the effects of relativity. But Einstein pointed out long ago that the speed of light is the highest velocity that anything can ever achieve. The question asked by the astronomers was: is it possible to have a star so massive that the escape velocity becomes equal to that of light? If so, such a star would have no appearance to us because even light would be unable to escape from its clutches. The black hole was a very appropriate name—it was black because no light could escape from it and it was a hole because anything or anybody unfortunate enough to go too near it would be drawn in with no means of escape.

The discovery of a neutron star by astronomers made it seem far more likely that black holes existed in the universe. It may be impossible for us to see a black hole because it does not emit any radiation, but it is nevertheless
possible to see the effects exerted on light and other objects by its massive gravitational field. It was not difficult for the mathematicians to work out the properties of the black hole. The space around it would be distorted by its very strong gravitational field. The bending of light predicted by Albert Einstein, for example, would be easily detectable. But when the properties of the space around the black hole were worked out it left an event horizon which is the limit of the black hole, but also an edge of the universe. The escape velocity at the event horizon equals the speed of light. To enter inside the event horizon of the black hole is to leave our universe and never to return. The distance of the event horizon can be calculated for a stationary black hole, and it is called the Schwarzschild radius.

Despite its mysterious nature, in astronomical terms, the black hole is quite a simple object. It has a measurable mass, which is usually large in terms of star masses. (There is insufficient gravity to compress all the mass if the star is less than about two or three times the mass of our Sun.) The black hole has spin, more precisely stated as angular momentum, and this means it has an axis about which it rotates and which causes it to be slightly flattened, rather like the Earth at the poles. It can also have an electrical charge.

At the time the first pulsar was discovered there were
still a few astronomers who doubted the existence of black holes. But there is now plenty of evidence to show that they do exist. Our Sun is a single star, but we now know that nearly half the stars in the sky are not alone. They have one or more companions, with the binary star being the most common arrangement. This means that there are many instances where a bright star orbits a dark star, and there are also some instances of where the dark star appears to be a black hole.

A good example of this arrangement is the X-ray source called Cygnus X-1. The visible star in the pair is a type B0 supergiant at about 8,000 light years from the Earth. It has a dark star orbiting very close to it. The star is a very strong source of X-ray radiation and the most likely explanation is that the radiation is caused by the presence of the black hole. The powerful gravity captures matter from the red supergiant and the matter orbits the black hole and it spins into a disc. Then the black hole draws the matter into itself, but in doing so the matter rises to a very high temperature and it becomes a very strong emitter of UV (ultraviolet) radiation. Some of the energy released by matter as it falls onto the black hole is converted to X-rays. The X-rays from Cygnus X-1 therefore imply that it is located very near to a black hole.

SETI, the Search for Extraterrestrial Intelligence, is an exploratory science that seeks evidence of life in the universe by looking for some evidence of its technology.

Radio waves can penetrate many parts of the galaxy where optical light cannot pass. There is a region of the radio spectrum called the “water hole” that seems to be a logical frequency at which to search for messages from extraterrestrial intelligence, since signals at this narrow bandwidth are not known to occur naturally.

The search has been set up as an international SETI project. Its Project Phoenix was the world's most sensitive and comprehensive search for extraterrestrial intelligence. It began observations in February 1995 using the Parkes Radio Telescope in New South Wales, Australia. In August 1998 Project Phoenix moved to the upgraded radio telescope at Arecibo, Puerto Rico. Unlike many previous searches, Phoenix didn't scan the whole sky, but scrutinized the vicinities of nearby, Sun-like stars. Such stars were most likely to host long-lived planets capable of supporting life. Project Phoenix observed about 800 stars, all within 200 light-years' distance.

In a joint project with UC Berkeley, SETI is building the Allen Telescope Array in California, USA. This will be a SETI-dedicated array of 360 telescopes that will equal a 100-meter (328 ft) radio telescope. The first 42 antennas
became operational in October 2007. In another SETI project, a large number of personal Internet-connected computers are used to process data. The SETI@home program is downloaded and runs when the computer is not in use, in the place of a screensaver. It downloads, analyses and then uploads the data.

There have been several false dawns, and as yet no evidence of civilizations other than our own has been found despite all our efforts.

Among the many mysterious objects in space, two in particular have fascinated scientists and non-scientists alike—even their names suggest something out of science fiction—and they may tell us more about the birth of the universe. Black holes are entities with such strong gravity that even light cannot escape from them. Quasars are bodies of indescribable energy that may exist at the very edge of the universe.