On the morning of 5 August 1988, the readers of the daily, Libération, read on page 19 a headline which was intriguing, to say the least: ‘They see planets everywhere’. This was intriguing and somewhat enigmatic. You had to read the introduction to better understand: ‘Two astronomers, a Canadian and an American, claimed the day before yesterday to have discovered new solar systems. Shivers. And doubts.’

Baltimore is in the American state of Maryland and is home to the Space Telescope Institute, the control centre of the brand new Hubble Space Telescope. In August 1988, the town welcomed the twentieth General Assembly of the International Astronomical Union for several days. Nearly 2000 astronomers were expected from 54 different countries. This was the moment that two teams of researchers chose to announce their discoveries. On the one hand, there were the Canadians Bruce Campbell and Gordon Walker, from the University of Victoria (British Columbia), and on the other, a team led by David Latham, the American from the Harvard–Smithsonian Center for Astrophysics.

Since the beginning of the 1980s, Campbell and Walker had followed about twenty nearby stars, looking for substellar mass companions: brown dwarfs, of course, but also hopefully giant planets. Their quest seemed to have succeeded. Nine of their stars showed behaviour that could well have been due to such companions. According to the two researchers, it was very unlikely that the objects were brown dwarfs, because, they argued, if they were brown dwarfs, then they would have been detected by astrometrical techniques.

The Universe is a zoo inhabited by exotic creatures. The celestial menagerie reveals the creativity of physical forces: forces that astrophysicists untiringly try to explain by theory, experiment and observation.

In the 1930s, researchers like Lev Landau, Robert Oppenheimer, George Volkoff, Fritz Zwicky and Walter Baade, having gone through the calculations, became convinced of the theoretical existence of a star never hitherto observed. It was an extremely dense star, the core of which was just an aggregate of neutrons. Does it really exist? This question was asked for nearly thirty years until thanks to the observations of a young Irishwoman it was possible to confirm the theory. But what that theory could never have predicted was that one day a Polish researcher, employed by an American university and working on a radio telescope in Puerto Rico, would discover the first exoplanets around one of these dizzying stars.

PULSAR, YOU SAID A PULSAR?

At the age of eleven Susan Jocelyn Bell failed the entrance exam that would have enabled her to attend a state grammar school. However, her father, an architect who was curious about everything and astronomy in particular, instead sent her to a private school, where she thrived. Perhaps she owed her success there to her physics teacher, whose enthusiasm for the subject was matched by an ability to explain it. Whatever the reason, Susan developed a passion for her chosen subject that was to lead to one of the major astronomical discoveries of the twentieth century.

We saw in the previous chapters that cosmic fauna is incredibly diverse. But you don't need to be on first name terms with objects as exotic as black holes or pulsars to see this. Even the ‘normal’ star family has too many children to easily keep track of. Some way had to be found to classify this stellar family according to some sensible scheme. Every star is today identified by its colour (or spectral type) and by its luminosity (or absolute magnitude). The different spectral types have each been given a name, in fact a letter of the alphabet, and the sequence is now: OBAFGKM. The O stars are those whose surfaces are much hotter than any of the others. Some of them are well beyond 30000°C. Our Sun, at 5700°C, is in the G class. The M class consists of the coldest stars, with mean surface temperatures of 2600 °C. Whatever their peculiarities and differences, all stars have, however, something in common: the thermonuclear fusion reactions of hydrogen that take place in their cores and which make them members of the main sequence, the club of normal stars.

There are so many stars undergoing nuclear combustion that it seems almost as ordinary as walking the dog. But those on Earth who try to control fusion, which is more powerful and less polluting than the fission used in nuclear power plants, know that it's a very difficult process to tame.

If our stone age ancestors had left us with stellar maps carved into rocks or painted on cave walls, we would have noticed striking differences with today's maps. Several tens of thousands of years ago, the constellations didn't look quite the same. Since despite what people thought up until the eighteenth century, the sky is everchanging. Stars move. They travel. And while this movement is often tiny, or even totally negligible over the scale of a human lifetime, it exists. This is a stroke of luck for astronomers, who found it to be the way to write some of the most beautiful pages of nineteenth and twentieth century astronomy, pages that go by the names of stars like 70 Ophiuchus, 61 Cygnus, Barnard, Epsilon Eridanus or Lalande 21185.

These were the true beginnings of the experimental hunt for exoplanets. The going was tough, with an extraordinary degree of groping in the dark, surprises and failures. In fact, none of the claimed planets of the time were confirmed. Why were there so many setbacks? Probably because the detection methods of the time were stretching limits. A tiny instrumental error was enough to see planets where really there was nothing. Dozens of years went by in a vain scrutiny of the stars in the hope of seeing a possible wobble that would betray the existence of a planet.

If the quest for exoplanets excites us so much, it's because it holds in promise the hope of maybe one day finding life elsewhere, life that was born in the light of another sun. It makes you dizzy just to think about it. What a shock it would be for humanity to discover that we are not alone in this Universe!

At the dawn of the third millenium, we're accustomed to talking about the vastness of the Universe. Infinity is almost ordinary. The latest generation of telescopes delivers images of the furthest jewels in the Universe to us. We're on first name terms with primordial galaxies, the first to have formed after the Big Bang. Bit by bit, we're putting together the history of the Cosmos. It's a tough job, but it can be done thanks to the progress in science since the beginning of the twentieth century and to the genius of people like Georges Lemaître, Alexander Friedman and Edwin Hubble, who showed that the Universe is not static, that it's expanding like a soufflé. The consequences of this discovery are nearly as infinite as the Universe itself. Because if it's getting bigger, then it must have been smaller when younger, it even had to have been born, from a ‘singularity’, as the experts say.

History's time line swept through 1963 with a breathtaking pace. The community of nations was about to welcome the birth of its newest member, Kenya, which that year attained independence from Great Britain. The Vietnamese military, meanwhile, was in the process of overthrowing the regime of Ngo Dinh Diem, deepening the US involvement in Southeast Asia and setting the stage for a decade of discordant relations among the superpowers. Ironically, this was also the year in which the first test ban agreement between the USA and the Soviet Union was ratified, concluding a nervous endeavor to ease growing nuclear tensions. For the individuals in society, the issue of women's rights resurfaced, promoted by Betty Friedan's just-released book Feminine Mystique. And while readers were being exposed to the idea of a modern woman discarding her traditional role, humanity as a whole was gaining some leverage over nature with the discovery of a vaccine against the measles. Many remember 1963 for the tragic assassination of President John F. Kennedy.

This tessellation of historical markers stirring the world in 1963 formed quite a backdrop for two minor events that would lead, over time, to the eventual uncloaking of the most powerful objects in the universe. At Mount Palomar Observatory, Maarten Schmidt was pondering over the nature of a starlike object with truly anomalous characteristics, while Roy Kerr, at the University of Texas, was making a breakthrough discovery of a solution to Albert Einstein's (1879–1955) general relativistic field equations.

Other than the spectacle of an obscured event horizon quivering before a bright sheet of background light, the most spectacular blackhole phenomenon astronomers can witness from the remoteness of Earth is a relativistic jet of plasma piercing the darkness of intergalactic space. Among the most dizzying cosmic displays in nature, these funnels of energetic particles probe the medium surrounding roughly one in 20 known supermassive black holes. A prominent jet was evident on the very first quasar photograph (of 3C 273), and glows even more brilliantly as a high-energy ray of light in modern Chandra images (see Fig. 1.2). For the most part, however, black-hole jets manifest themselves in a “parallel” universe – indeed, their ghostly apparitions pre-empted the discovery of supermassive black holes by several decades, though without any portent of what they would later reveal. And once again, astronomers can thank the telephone company for facilitating one of the most amazing advances in the history of science, on a par with the discovery – six decades later – of the cosmic microwave background radiation through the commercialization of space.

Not long after a demonstration that the substance of light behaves like a series of waves undulating through time and space, Guglielmo Marconi (1874–1937) successfully initiated transatlantic communications in 1901 using wireless radio.

Though some Hubble images of distant galaxies feature destructive collisions that could trigger quasar activity, others show that many normal, undisturbed aggregates of stars are oblivious to the cosmic thunder within their midst. This is an indication that a variety of mechanisms – some quite subtle – may be responsible for igniting a quasar. Whatever the formative process is, however, these supermassive objects seem to have spared their hosts from any obvious damage, so their prodigious outpouring of matter and radiation may be a shortlived phenomenon. Still, this observation is not sufficient to guide astronomers toward the identification of a coherent, single pattern of quasar birth and growth.

For years, astrophysicists concerned with the nature of supermassive black holes have been asking themselves a cosmological “chicken and the egg” question: “Which came first, the gargantuan pit of closed spacetime, or the lively panorama of gilded stars and glowing gas that we call a galaxy?”

Prior to a remarkable recent discovery that now seems to have answered this question for the majority of cases, the evidence in favor of black holes appearing first was anchored by the telling observation that the number of quasars peaked 10 billion years ago, early in the universe's existence. The light from galaxies, on the other hand, originated much later – after the cosmos had aged another 2 to 4 billion years.

Settling on the banks of the Tiber river, the Latini would establish a city in the seventh century BC that later came to dominate much of the civilized world. They used the word gravis to denote heavy or serious, and the corresponding noun gravitas for heaviness and weight. Our modern word gravity, and its more precise derivative gravitation, trace their roots to this early usage, which itself is linked to a yet older root that includes the Sanskrit guru (for weighty or venerable), among others. The ancients were evidently quite aware of this ever-present property of matter – that it should have an unwavering attraction toward the Earth – though up to the time of Galileo and Newton, gravity simply remained a name for the phenomenon, without any explanation or even an adequate description.

THE INEXORABLE FORCE OF GRAVITY

Toward the end of the seventeenth century, attempts to account for the behavior of objects changing their motion in response to external influences were primarily concerned with the nature of forces that one could easily identify. In the story of Goliath's slaying, for example, the stone was dispatched toward his forehead after David released the sling. Prior to that moment, the diminutive combatant was able to restrain the motion of the stone with a force applied by his hand mediated through the string. Newton argued that the Earth must itself be exerting an attractive force on matter since everything falls down in the same direction.

Supermassive black holes are certainly the most powerful objects in the universe, yet even this attribution may not adequately convey the severity with which they stress their surroundings. Yes, their force of attraction is inexorable, but more than this, it is – as far as we can tell – infinitely unassailable once matter approaches so close that even something moving at the speed of light cannot break free. The radius at which this happens is known as the black hole's event horizon, for nothing within it can communicate with the universe outside. Thus, we have no way of directly seeing such an object. Instead, its presence may be deduced on the basis of the shadow it casts before a bright screen, such as a dense cluster of stars. To have any hope of carrying out such an observation, however, we must be close enough to the highly concentrated mass to actually resolve the dark depression among the myriad other details likely to be present in its environment.

We become aware of a supermassive black hole primarily because of the incomparable cosmic power it exudes. For example, the image of 3C 273 in Fig. 1.2 attests to its nature as one of the brightest beacons in the visible universe. Yet it should be black, drawing everything into a catastrophic fall toward oblivion, releasing nothing – particles or light – to breach its cloak of secrecy.

Our view of the night sky is a panoply of stars choreographed to the galaxy's spiral melody. A deep exploration of the universe beyond our immediate neighborhood would therefore not be possible were it not for the occasional chance alignment of interstices among these swarming points of light. For ten consecutive days in December of 1995, the Hubble Space Telescope peered through just such a clearing, and produced our deepest ever view of the universe, graced with thousands of galaxies bursting into life at the dawn of time.

THE HUBBLE DEEP FIELD

Called the “Hubble Deep Field” (see Fig. 6.1), this image contains not only classical spiral and elliptical galaxies, but also boasts a rich variety of other galaxy shapes and colors that hint at the influences governing the evolution of the early universe. Some of these objects may have condensed within 1 billion years of the Big Bang.

Covering a speck of sky only one-thirtieth the diameter of the full moon, the view of the Hubble Deep Field (one quarter of which is shown here) is so narrow that just a few foreground stars in our galaxy are visible. Most of the objects contained within it are instead so distant that our eyes would have to be four billion times more sensitive in order for us to see them without the aid of a telescope.