The “Universe” is what we call everything: every planet, every star, and every distant galaxy. It is vast beyond human comprehension, yet that has not stopped us trying to make sense of it all. Throughout history we have peered at it, measured it and studied it in the hope of one day understanding it. We have made some significant steps; but, lest we become complacent, the Universe always has a new surprise to throw at us, a new challenge to test our imagination
The urge to understand the Universe developed early in human history. Babylonian stone tablets dating back to 3000–3500BC have been found that record the variable length of day throughout the year; the Chinese have written records of eclipses since about 2000BC. Around the world are the remains of prehistoric structures that display striking astronomical alignments. The oldest of these is the 5200-year-old tomb at Newgrange, Ireland. At dawn on the winter solstice, the shortest day of the year, the rising Sun shines its beam through a passageway onto the floor of the inner chamber.
On Easter Island in the Pacific, seven of the hundreds of enigmatic statues face in the direction where the Sun sets on the equinox, when night and day have equal length. It has been suggested that the great Cambodian Temple Angkor Wat is aligned so that the Sun rises over the eastern gate on midsummer’s day. The pyramids of Egypt are also thought to show alignment with the stars. While none of these constitute an observatory in a scientific sense, they clearly demonstrate that the builders had an understanding of the motion of the Sun and stars.
The earliest astronomical observations were almost certainly used by ancient humans to set the calendar. The phases of the Moon defined the passing of a month, and the passage of the Sun through the sky defined both the length of a day and of a year. As the year progresses, the Sun rises and sets from different points on the horizon. Stonehenge, the well-preserved stone circle on Salisbury Plain in England, has a well-known solar alignment. On midsummer’s day, the Sun rises over an offset monolith known as the Heel Stone. Initially Stonehenge was thought to be a temple to the Sun god, but some researchers have found other alignments between the stones and the Moon and suggested that it could have been a prehistoric observatory, perhaps used primarily for the prediction of eclipses.
The Greek word kosmos means “orderly arrangement,” and from it we derive today’s words cosmos and cosmology. Cosmology is the branch of astronomy that goes about answering that most fundamental of questions, “What is the Universe?,” by studying the way the Universe behaves, how it began, and how it will all eventually end.
Cosmology as a true science only really started in 1916, when Albert Einstein published his General Theory of Relativity (see Was Einstein Right?). Before this, astronomers lacked the mathematical framework in which to describe the behavior of the whole Universe, and so pre-20th-century cosmology tended to be an uneasy mixture of speculation and religious sensibility. Ancient cosmology especially was usually inspired by religion and the assumption that Heaven was located somewhere above our heads, in space.
The Egyptians based their cosmology on the human reproductive cycle. They believed that the sky goddess Nut gave birth to the Sun god Ra every year and that the changing altitude of the Sun with the seasons was its gestation in Nut’s star-studded body. The Sun was said to be reborn every winter solstice, and returned inside Nut through her mouth at the spring equinox. In this way, Ra continually recreated himself, making the Universe an eternal, self-sustaining entity.
Early civilizations told stories inspired by the patterns of stars in the night sky. They imagined the lines joining stars to form pictures of familiar or mythical characters. In Mesopotamia (modern-day Iraq) archaeologists have unearthed stone tablets and clay ledgers dating back to 1300BC, which detail many such “constellations” including the 12 signs of the zodiac. These zodiacal constellations were given special significance because they inhabited regions of the sky through which the Sun passed, and they were subsequently adopted by the Greeks—the Assyrian Hired Man and the Swallow became Aries and Pisces, for example, and the Goatfish and the Great Twins became Capricorn and Gemini. In Ancient Greece wandering minstrels would drift from village to village, recounting the star myths in exchange for food and lodgings. At the same time, philosophers would come up with their own fanciful tales to explain the nature of the Universe. One of the earliest was the philosopher Thales, in the sixth century BC. He put forward the idea that space was filled with water, in which the Earth floats, that earthquakes were caused by waves in this water, and the stars moved because they were caught in gentler currents.
“Astronomy compels the soul to look upward and leads us from this world to another.”
PLATO 4TH CENTURY BC GREEK PHILOSOPHER
The Greek astronomer Claudius Ptolemy, who lived in the first century AD, compiled a list of 48 constellations, but since not all the sky could be seen from Greece, the regions around the South Pole remained uncharted until intrepid astronomers ventured far from Europe during the 16th and 17th centuries in order to chart the southern stars. Other new constellations were also proposed to fill gaps in Ptolemy’s classical sky map. Inevitably this led to arguments as astronomers disagreed. In England, Edmond Halley proposed a constellation called Robur Carolinum (Charles’ Oak), after the tree in which Charles II had hidden from the Roundheads following the battle of Worcester. While the King was delighted with the honor, some of Halley’s fellow astronomers were not keen, and quietly discarded the constellation from their maps.
Much later, in 1922, things were finally put on a firm footing when the International Astronomical Union ratified 88 constellations with defined boundaries, mostly based on the Greek model. These were not the only aspects of Greek astronomy to pass into modern usage. There was one Ancient Greek in particular who was not prepared to tell stories about the stars, or speculate about them; he realized that the first step on the way to true understanding was to measure them. That man was Hipparchus and he defined a system of classification for the stars still in use today.
Even to a casual observer of the night sky, it is obvious that some stars are brighter than others. More than two millennia ago, Hipparchus meticulously compiled a catalog of 850 stars, recording each star’s position and ranking its brightness. He had no equipment to measure the brightness; he simply made his estimates by eye. The brightest stars he called “first magnitude,” the faintest he termed “sixth magnitude,” and the rest he ranked in the categories between. Amazingly, astronomers still use this seemingly crude magnitude system today, although modern measuring devices have extended Hipparchus’s original six classifications. At the top end of the scale, the very brightest stars are now given negative numbers; at the other end, the stars that can only be seen with the aid of a telescope are assigned magnitudes with numbers often much higher than six. From the surface of the Earth, the best telescopes can detect stars of between 24th and 27th magnitude, but in orbit, above the distorting effects of the Earth’s atmosphere, the Hubble Space Telescope can detect 30th magnitude stars. Each magnitude category is about two and half times brighter than the previous one, so 30th magnitude is about 3.5 billion (3500 million) times fainter than the naked eye can see.
But in measuring these perceived brightnesses, we are forgetting that the brightness of a luminous object is affected by its distance from the observer, as well by as the actual amount of light it gives out. Thus, a nearby dim star may well appear brighter than a highly luminous star far away. This behavior is governed by what is known as an “inverse square law,” which means that if the distance doubles, the intensity of the light drops to a quarter; treble the distance and the intensity drops to one ninth of its original value. To acknowledge this, magnitudes measured without any correction for distance are known as “apparent” magnitudes. The “absolute” magnitude is the brightness value that has been corrected for distance. The red star Betelgeuse—widely known because it can be pronounced “beetle juice”—has an apparent magnitude of 0.58, but leaps up to –5.14 on the absolute scale. It is a truly bright star indeed but comparatively far away. On the other hand, because it is so close, the Sun has an enormous apparent magnitude of –26.7, the brightest object in the sky. However, when corrected for its proximity, its absolute magnitude is just 4.8. In other words, our Sun, for all its glory and importance in driving life on Earth, is nothing but a thoroughly average star.
The Ancient Greek astronomers, with their dedication to detailed observations and recordings, have left us a rich legacy of knowledge about the stars. The nature of five particular stars, however, eluded them. They called them planetes, meaning “wanderers,” because of their movement across the sky from one night to the next, unlike all other stars which remained “fixed.” From their Greek name you might correctly deduce that these planetes are in fact planets—our nearest five planets, Mercury, Venus, Mars, Jupiter and Saturn, which can be seen with the naked eye. The Greeks could have no concept that these were worlds in their own right, and imagined them to be gods, or at the very least emissaries of the gods, whose influence affected the fortunes of individuals on Earth.
Two of these wandering planets, Mercury and Venus, follow orbits between Earth and the Sun. So, viewed from Earth they stay close to the Sun and are only ever seen in the twilight sky. Mars, Jupiter and Saturn orbit the Sun further out than the Earth and can be clearly seen making their slow paths through the night sky. Many early astronomers became dedicated to tracking the motion of all the wandering planets so that their future positions could be predicted. This task was seen as important because when planets drew close to one another, their influences were thought to combine and magnify. Thus, conjunctions, as they were known, were significant events that needed to be predicted in order to cast horoscopes.
With the advent of the Christian era, the opinion was widely adopted that the motion of the Heavens would always remain mysterious because the sky was God’s domain and mankind’s puny intellect could never understand His omnipotent will. This perspective began to change in the first decades of the 17th century when Johannes Kepler distilled the movement of the planets into three mathematical laws of planetary motion (see Why Do the Planets Stay in Orbit?). This proved that the Universe could not only be measured, but understood.
At the same time, in Italy, Galileo Galilei was making discoveries that sparked our fascination with the wider Universe. In 1609, he raised his telescope and pointed it at the misty band of light that stretches across the night sky, known as the Milky Way. Through his basic telescope, tiny by today’s standards, Galileo could see that the Milky Way was composed of a multitude of faint stars. This was a revelation to all, because it had been believed that the entire Universe contained only what could be seen with the naked eye. Now, however, Galileo had shown that there was far more that lay beyond unaided vision. This realization was the start of the centuries-long fascination, with each generation of astronomers developing larger and larger telescopes to see fainter and fainter objects, which continues to this very day. The largest optical telescopes in use now are fully 10 meters across, some 500 times larger than Galileo’s original telescope.
Today we know that the Sun is one star in a giant collection known as the Galaxy, which contains at least 100 billion stars arranged in a spiral pattern in a flat disk and orbiting a bulbous hub of even more stars. From our position in one of the spiral arms, we see the disk as a haze of myriad stars—the Milky Way. The center of the Galaxy is toward the south, in the constellation of Sagittarius. If you could observe from a dark site in the southern hemisphere, you might be able to see the Milky Way widen into the vast star clouds of the Galaxy’s central bulge.
The thickness of the Milky Way’s stellar disk is estimated to be about 1000 light years, one light year being simply the distance that light travels in a year. According to laboratory measurements, light travels through a vacuum at approximately 300,000 kilometers every second (186 thousand miles per second), so in a year it travels about 9.5 trillion kilometers (5.9 trillion miles). This is the distance of one light year; using this unit means we can keep the mind-bogglingly large numbers a bit more manageable. In the galactic disk, the density of stars is about one star every four light years or so, but in the central heart of the Galaxy, some 25–30,000 light years away from the Sun, stars are densely packed and create an elongated bulge about 27,000 light years in diameter and 10,000 light years high.
OUR HOME, THE MILKY WAY GALAXY
In the Sun’s immediate neighborhood within the disk, there are 33 stars. By “neighborhood,” astronomers mean stars closer than 12.5 light years. The majority of our neighbors are smaller, dimmer stars than our own Sun. Known as red dwarfs (see How Old Is the Universe?), these celestial minnows make up the largest population of stars in the Universe. Just two stars in our neighborhood are similar in size to the Sun, and only one is greater: Procyon in the constellation Canis Minor is estimated to be twice the Sun’s diameter and to contain one and a half times the Sun’s mass.
“The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.”
GALILEO GALILEI 17TH CENTURY ASTRONOMER
Near the center of the galactic bulge, the density of stars is 500 times greater than in our neighborhood. If the Sun and its family of planets were suddenly placed in the center of the Galaxy, there would be other stars, possibly with their own planetary systems, just ten times further away than Pluto. In reality, within our solar neighborhood the nearest star is more than 5000 times further away than Pluto. At the very center of the Galaxy, astronomers believe that the density of matter is so great that a black hole exists (see What Are Black Holes?).
As vast as it seems, the Galaxy is not the whole Universe; in the grand scheme of things, it is little more than a small island in an expansive ocean and there are innumerable other islands. Each one is a separate galaxy in its own right, containing anything from a few million to a trillion (a million million) stars. Galaxies come in three basic types: spirals, barred-spirals and ellipticals. The spiral galaxies are particularly beautiful, with their sweeping arms of bright young stars surrounding a central bulge of older stars. The barred-spiral galaxies, of which our own Galaxy is an example, are similar but with a pronounced elongation that connects the central bulge to the spiral arms. The ellipticals are totally different in appearance; they can be much larger than the spiral or barred-spiral galaxies and anything from cigar-shaped to perfectly spherical. There are some oddballs as well, known as the irregular galaxies, some of which may once have been spiral galaxies, and others that are truly disordered.
TYPES OF GALAXIES
Among the largest and brightest galaxies, the spiral and barred-spirals account for about three quarters of the population. But there are also huge numbers of small elliptical and irregular galaxies spread across the Universe, referred to as “dwarf galaxies.” When these are taken into account, the ratios reverse because tiny spirals are rare.
Although many galaxies seen are isolated, some of them are drawn together in clumps, attracted by one another’s pull of gravity. At the smallest end of this behavior, a collection of less than 50 galaxies is simply known as a group. Our Galaxy is part of the Local Group, which contains one other large galaxy—a spiral called the Andromeda Galaxy—and around 30 smaller galaxies. What we call “clusters” of galaxies are essentially large groups containing more than 50 galaxies and in some cases more than a thousand. The nearest clusters to the Local Group are the Virgo Cluster containing about 1300 galaxies, the Coma Cluster with over 1000 and the Hercules Cluster that has around 100 members.
The various groups and clusters themselves gather into even greater collectives, appropriately named “superclusters.” These vast collections are strung out through the Universe along giant “sheets” or “walls” known as “filaments.” They appear to surround gigantic spaces containing scarcely any galaxies at all. If such voids can be thought of as celestial “objects,” then they are the largest objects in the Universe. A helpful way of visualizing the distribution of galaxies is to imagine the foam in a bubble bath—the galaxies are distributed on the thin soap film that surrounds the bubbles.
“Equipped with his five senses, Man explores the Universe around him and calls the adventure Science.”
EDWIN HUBBLE 20TH CENTURY COSMOLOGIST
As for the number of galaxies that populate the Universe, with each passing year, the estimate goes up. Back in 1999, astronomers used Hubble Space Telescope observations to estimate that there were 125 billion galaxies. Not long afterward, a new camera was installed on the Hubble and revealed many more, forcing the estimate to be doubled. Supercomputer estimates now suggest that there may be 500 billion (500,000 million) galaxies spread throughout the Universe.
To investigate the origin of these galaxies and thus uncover the evolution of the Universe, cosmologists exploit the fact that light does not travel instantaneously across space. As fast as the speed of light is by our everyday standards—light could circle Earth’s equator seven times in a second—it still takes many years to traverse the vast tracts of space between celestial objects. If a star is 100 light years away, its light takes 100 years to cross space to reach us and, as a consequence of this, we see it not as it is today but as it looked 100 years ago when the light began its journey. Cosmologists call this the “look-back time.” It is like an archaeologist digging down through successive rock strata to uncover older and older fossils; the further an astronomer looks into space, the more ancient the celestial objects that appear in the eyepiece. With current telescope technology we can see celestial objects as they looked billions of years ago, and trace the way they developed into those we see around us today. Welcome to the world of the cosmologist.