The Story of Astronomy

by Peter Aughton

  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.

  Black Holes

  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.

  No Escape

  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.

  Evidence for the Black Hole

  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

  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.

  18

  BLACK HOLES, QUASARS AND THE UNIVERSE

  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. Q
uasars are bodies of indescribable energy that may exist at the very edge of the universe.

  If we could build a spaceship to take us close to a black hole we would no doubt learn much more about these objects than we know at present. Of course, this is not a scenario within our current means of technology, and it is hard to envisage how it ever could be. Nevertheless, by studying black holes with the tools available to us we can still deduce much about these mysterious objects, even if we cannot get physically close to one of them. So, with such knowledge as we already possess, let us go on an imaginary journey to a black hole instead. We shall be visiting a single black hole—in other words, one with no orbiting companions. As we approach the black hole there will be changes in the space and time around it, although we may not be able to detect them due to relativity. So long as our spaceship keeps at a safe distance there is no immediate danger to us. We can orbit around the black hole just as we can orbit around the Earth.

  A One-way Journey

  Imagine now that we dispatch an astronaut in a space pod to make a closer approach to the black hole and to try to get inside it. To help us track his movements the astronaut sends us a pulse every second. As he approaches the event horizon, the boundary of the black hole where escape velocity is equivalent to light speed, we will notice that the interval between the pulses becomes longer and longer. This is because the space around the black hole is distorted so that time passes at a slower rate. We observe that the space pod and the astronaut are subjected to what are called tidal forces by the gravity of the black hole. This means that the part of the space pod nearer to the center of the black hole is subjected to a greater force than the part that is further away. The astronaut reaches a point where these forces impose a great stress on his pod. However, it is still not too late for him to return to the main spacecraft, provided he is still outside the event horizon, and can fire his rockets to travel at almost the speed of light. If he continues on his journey, then he and his space pod will be drawn inexorably into the black hole.

  According to theoreticians the astronaut in the space pod will perceive events quite differently from those outside, again due to relativity. He is in his own space–time zone, distorted by the strong gravitational field which makes time pass relatively more slowly. Once the pod actually enters the black hole there is no way for the astronaut to communicate his findings to those outside and there is also no way for him to get out again into the universe he has left behind.

  English mathematical physicist Roger Penrose (b. 1931) and English theoretical physicist Stephen Hawking (b. 1942) have shown that there is a singularity (a point at which density and temperature go to infinity) at the center of the black hole, but we know nothing else about the space inside it. Space and time are stretched at the boundary of the hole such that the space inside is infinite. If the black hole is very massive then, although the gravitational field is stronger, the tidal forces are weaker. We would like to think that by entering into the black hole we could enter a new universe, similar to our own with stars and galaxies and habitable planets. And as we shall see later, there are even some who believe our own universe itself could in fact be located within a black hole. Science fiction writers have suggested that the black hole could be a wormhole—a connection between two very distant parts of the universe that would provide an ideal way of crossing the great distances between the stars. However, this idea remains in the realms of science fiction and is not a view held by astronomers.

  Distant “Quasi-stellar Objects”

  In the late 1950s astronomers at Cambridge in England were putting together a catalog of all the radio sources in the sky. Sometimes it was possible to follow up the radio source with an optical sighting, but the two are not always compatible and the radio emitters are not normally bright optical stars. In 1960 the American astronomer Allan Sandage (b. 1926) used the 5-meter (200-inch) Palomar telescope to study the star known as 3C 48 in the Cambridge catalog. The star had some unusual features. Its spectrum showed strange emission lines that could not be identified from comparison with those of known elements. In 1962 another radio star called 3C 273 was discovered. It was unusual in that a long luminous jet was clearly visible in its optical image. Astronomers were puzzled by the strange emission lines that it also showed. In 1963, however, the Dutch astronomer Maarten Schmidt (b. 1929), working at the California Institute of Technology, was able to demonstrate that the unidentified lines in the spectra were in fact the well-known hydrogen lines. They had simply not been recognized due to the very high degree of redshift they were exhibiting. The objects were given the name “quasars,” an abbreviation for “quasi-stellar objects.” Furthermore, they did not originate in our own galaxy; judging from their redshift they were traveling away from us at an unprecedented speed and they were therefore an unbelievable distance away. In fact it is no exaggeration to say that they were the most distant objects ever observed in the sky. The quasar 3C 273 was estimated to be at a distance of two billion light years.

  The most distant galaxies studied before the discovery of quasars were not, after all, the end of the universe. The quasars were located in galaxies much further away, and because we were able to detect them from such a great distance it followed that they must have a very powerful source of energy. Once the first quasars had been identified, astronomers began to look for more objects with very large redshifts. They found that there were thousands of quasars visible with the most powerful telescopes. Redshifts of over 90 percent have been measured, compared with measurements of only 2 or 3 percent for the galaxies. It indicates that these quasars are many times further away. In fact, the most distant quasars are estimated to be 13 billion light years away. This means that we are observing them close to the time of the Big Bang and the origins of the universe. When the quasars came under close scrutiny it was discovered that they were located in galaxies, showing that the most distant galaxies were much further away than was originally thought.

  When quasars came to be studied more closely it was found that their brightness was not constant, but the variations from the norm were usually very short-lived. Sometimes the quasars could flare up to be as bright as 100 galaxies—another indication that they must have a very compact but powerful source of energy to be so highly visible at such a great distance. There is only one power supply that could satisfy such a voracious appetite for energy. The evidence suggests that a quasar must contain a very massive black hole at its center. The quasars are objects so powerful that they consume stars. Millions of stars must have been swallowed up to create the massive black holes that power the quasars, and when they devour a new star the energy released is so great that we see the flare of its death throes from the Earth. With the quasars we seem to have reached the very limits of the universe, but we have been wrong about this so many times that nobody can be sure. There was a time when the edge of the universe was thought to be the edge of the Earth, then it was thought to be the furthest part of the solar system, then it became the Milky Way. In the early 20th century the limit was assumed to be the most distant galaxies, now we think the quasars are at the end of the universe—but we cannot even say this with too much confidence.

  Dusty Quasar Winds

  Using the Spitzer Space Telescope infrared spectrograph instrument, scientists found a wealth of dust grains in a quasar called PG 2112+059 located at the center of a galaxy eight billion light years away. The grains, which include sapphires, rubies, peridot and periclase (naturally occurring in marble), are not normally found in galaxies without quasars, suggesting they might have been freshly formed in the quasar’s winds.

  These findings are another clue in an ongoing cosmic mystery: where did all the dust in our young universe come from? Dust is crucial for efficient star formation as it allows the giant clouds where stars are born to cool quickly and collapse into new stars. Once a star has formed, dust is also needed to produce planets. Theorists had predicted that winds from quasars growing in the centers of distant galaxies might
be a source of this dust. While the environment close to a quasar is too hot for large molecules like dust grains to survive, dust has been found in the cooler, outer regions. Astronomers now have evidence that dust is created in these outer winds.

  More than Three Dimensions

  The universe is often described as a four-dimensional space–time continuum, consisting of the well-known three spatial dimensions that we experience (length, width and depth), plus another dimension—the time dimension—that has a special significance in Einstein’s special relativity theory. Ignoring the time dimension for the moment, there is some evidence that the three spatial dimensions of the universe do not extend to infinity; the universe may be confined in a closed space in the same way that the sphere of the Earth is enclosed by its surface.