Introduction to The Universe on the Grand Scale
With the naked eye we can see the moon, most of the planets of the solar system, nearby stars, and even some neighboring galaxies. With the most powerful telescopes, we can see remote galaxies in the far reaches of the universe.
The most important object in the heavens—to us—is the one we cannot see at night, the sun. The sun is a star of average size and brightness. It is the brightest object in the sky only because it is so close to Earth.
Orbiting the sun are the nine planets of our solar system: Mercury, Venus, Earth, Mars, Jupiter, Uranus, Neptune, and Pluto. The planets shine only because they reflect light from the sun. Most of the planets, including Earth, are themselves orbited by smaller bodies called moons. There are more than 40 of these natural satellites in the solar system, with the Earth's own moon ranking sixth in size.
In the grand scheme of things, the planets and their moons are a bit of an accident, having formed from the debris of the material that came together to form the sun about 4.6 billion years ago. Astronomers think it is likely that many stars have planets.
Light Bulbs of the Universe
Stars are the light bulbs of the universe. They come in many sizes and brightnesses, but all of them are basically hot balls of gas that are powered by nuclear reactions at their cores.
Those reactions, in which a star consumes hydrogen to produce helium and energy, proceed at different rates, depending on the mass of the star. (Mass is the amount of matter something contains.) The most massive stars are also the brightest. That is because in these very large stars, the matter at the core of the star is squeezed together by gravity (which increases with mass) to a very high density, causing the nuclear reactions to occur at a more rapid rate. But the glory of such stars is short-lived. Giant blue-white stars typically exhaust their nuclear fuel in the relatively brief time of 10 million years or so.
The least massive stars, on the other hand, use their fuel extremely sparingly. Small orange or reddish stars may live for hundreds of billions of years. The sun, a yellow star of average mass, has been shining with its present radiance for about 4.5 billion years and will continue to do so for another 5 billion years before running out of fuel.
An estimated 80 to 90 percent of all visible stars are in the prime of life and are known as main sequence stars. Stars that are past their prime include three types of exotic stars—white dwarfs, neutron stars, and black holes.
White dwarfs are small, dense stars that slowly cool and fade from view, like the dying embers in a fireplace. Average sized stars like the sun end their days by first swelling into what are called red giant stars and then shrinking to become white dwarfs.
The Aftermath of Supernovae
But if a star has more than about eight times as much mass as the sun, it dies a spectacular death. When the fuel of one of these large stars is used up, the star collapses and then blows itself apart in a titanic explosion known as a supernova. In most cases, gravity compresses the remaining core of a supernova to form a neutron star, an incredibly dense ball of tightly packed subatomic particles called neutrons. Some rapidly spinning neutron stars emit powerful beams of radio waves, resembling the rotating beams of a lighthouse. These beams are detected on Earth as pulses of radio waves. Thus, the stars are known as pulsars.
If the remnant of a supernova exceeds about three times the mass of the sun, its compression by gravity cannot be stopped. All the matter in the core is crushed together until it becomes a black hole, a body whose gravitational field is so powerful that not even light can escape its grasp. A black hole cannot be seen, but it can be detected by its effect on surrounding matter. As matter falls into a black hole, it releases huge quantities of energy into space.
The Milky Way and Other Galaxies
There are more than 100 billion galaxies in the universe. In only the nearest galaxies can astronomers make out individual stars. Most galaxies are so far away that their stars can be seen only as a haze of light.
Like stars, galaxies come in a variety of sizes and types. Although a typical galaxy contains about 100 billion stars, some have far less, and the smallest—called dwarf galaxies—have fewer than 1 million stars. The Milky Way is a relatively large galaxy, with about 200 billion stars.
Some galaxies have an irregular appearance, but most fall into one of two main categories. Many are elliptical (round or oval shaped). Even more numerous are the spirals, which account for 70 to 80 percent of known galaxies.
Spiral galaxies are among the most beautiful objects in the heavens. Seen face on, they resemble great pinwheels in space, and from the side they look much like luminous flying saucers. The Milky Way and the neighboring Andromeda galaxy are both spirals. Our solar system is located in one of the spiral arms of the Milky Way, about halfway between the Galaxy's center and its outer fringes.
The most spectacular galaxies are quasars, which, despite being thousands of times smaller in size than large galaxies like the Milky Way, are hundreds of times brighter. Quasars are the most distant objects we can see in the universe, yet, because of their radiance, they appear in telescopes as bright, starlike objects. Most astronomers believe that quasars are powered by gigantic black holes at their centers. Quasars are thought to be a stage that many galaxies went through early in their evolution. The Milky Way, which seems to have a huge black hole at its center, may once have been a quasar.
Galaxy Clusters, Superclusters, and Voids
Most galaxies have been pulled together by gravity into groups. The Milky Way and Andromeda, together with about 20 smaller galaxies, make up a collection of galaxies called the Local Group. Although most galaxies are found in small groups, about 10 percent of the galaxies in the universe are part of larger systems called clusters. Galaxy clusters may contain anywhere from hundreds of galaxies to thousands of galaxies. Astronomers have identified about 10,000 clusters. The Milky Way is located on the outskirts of a cluster called the Virgo Cluster, which consists of about 1,000 galaxies.
Clusters, in turn, are organized into even larger collections called superclusters. Both the Local Group and the Virgo Cluster are members of the Virgo Supercluster.
There are also vast regions of space, known as voids, that are nearly empty of galaxies. Astronomers' understanding of these areas is somewhat fuzzy. Voids could simply be regions of space that were left empty when gravity drew galaxies together to form clusters and superclusters. Or they could be expanses of space that were swept clean of matter by shock waves from supernovae or that never contained much matter to begin with. It is even possible that voids are not really empty. They may be teeming with galaxies that are just too faint to be seen.
The Universe's Vast Distances
The distances between galaxies—and even between stars—are unimaginably huge. Astronomers measure cosmic distances not in kilometers but in light-years. One light-year, the distance light travels in one year at a speed of about 300,000 kilometers (186,000 miles) per second, is about 9.5 trillion kilometers (5.9 trillion miles). The nearest star outside the solar system, Proxima Centauri, is 4.3 light-years away. The farthest observable galaxies and quasars are up to 15 billion light-years from us–about the distance that light has had time to travel since the big bang. Although astronomers cannot see any farther than that, they are confident that the universe does not end there.
Because it takes time for light to travel across the vast distances in space, when we view faraway celestial objects, we are seeing the light that left them many years ago. Thus, by looking deep into space, we are also looking back in time. When astronomers view the surface of the sun, for example, they are seeing it as it existed 8.3 minutes earlier, because it takes sunlight that amount of time to travel the 150 million kilometers (93 million miles) to Earth.
The farther things are from us in space, the more removed they are in time. When we turn our telescopes on Proxima Centauri, we see it as it appeared 4.3 years ago, and in viewing the more distant Andromeda galaxy we are looking 2 million years into the past. Light from the most remote galaxies and quasars took 15 billion years to reach us, giving us a glimpse of the universe in its youth.
Mind-boggling Sizes
The size of celestial structures is just as mind-boggling as the distances between them. Consider our own Galaxy. The starry spiral of the Milky Way spans a distance of about 100,000 light-years. If we could construct a scale model of the Milky Way the size of Chicago, the sun would be a speck visible only with a magnifying glass. The entire solar system would be no wider than a grain of salt. The largest structure in the universe yet identified is known as the Great Wall. This is a chain of galaxy clusters about 550 million light-years long, 200 million light-years wide, and 10 million light-years thick.
The Mysterious Great Attractor
Perhaps the most mysterious object that we have yet come across in the universe is one discovered in the late 1980's by astronomers at the Carnegie Institution of Washington, D.C., Cambridge University in England, and several other U.S. and British institutions. They noted that the Milky Way and thousands of other galaxies within a radius of about 100 million light-years were moving in the direction of two large constellations (groups of stars) named Hydra and Centaurus.
The researchers concluded that the galaxies were being drawn in that direction by a large concentration of mass about 150 million light-years away. They dubbed the massive object the Great Attractor. By their estimate, the Great Attractor contains more than 10,000 times the mass of the Milky Way, causing it to exert an enormous gravitational pull on everything in its vicinity.
Another Mystery: Dark Matter
Astronomers have observed other gravitational effects in the universe that are even more mysterious than the Great Attractor. In the 1930's, the Swiss-American astronomer Fritz Zwicky calculated that galaxies in large clusters were moving so fast that the gravity provided by their visible stars was insufficient to hold the galaxies together. His work provided the first hint that there is much more matter in the universe than meets the eye. Researchers now estimate that this unseen matter—called dark matter because it emits no light—makes up at least 90 percent, and perhaps as much as 99 percent, of the universe's mass.
But what is dark matter? Astronomers have not yet been able to find the answer to that question. Dark matter could simply be stars of very low mass that are too faint to see, or it could be dead stars–neutron stars or black holes. Or dark matter could be something far more exotic, such as an unknown form of matter left over from the earliest moments of the universe.
Mapping the Universe
While some astronomers contemplate this invisible portion of the universe, others are concentrating on mapping the parts we can see. This is a difficult task, and one that is far from complete. Our present understanding of the visible universe's “geography” is similar to Europeans' knowledge of the Earth's geography in the early 1500's, when world maps contained large areas of unknown territory.
Astronomers map the universe by charting the positions of galaxies as they appear across the “dome” of the sky and recording the positions in catalogs. Astronomers in the 1930's through the 1980's mapped the positions of more than 5 million galaxies.
Although the catalogs of galaxy positions are useful, they have one big drawback: They provide only a two dimensional view of the universe, while the universe is three dimensional. To appreciate the shortcomings of such maps, imagine being given the view of a 100-story building, looking upward from the basement, with the floor plans of all 100 floors overlying one another on a single sheet of paper. Such a map would be completely incomprehensible.
Only with a three dimensional view that separates the layout of each floor from those of all the others would you be able to discern the overall design of the building. So it is with mapping the universe.
A Valuable Mapping Tool—the Red Shift
Fortunately, astronomers have a tool at their disposal to produce a three-dimensional map of the universe. It is the red shift, the fact that light emitted by an object moving away from an observer is shifted toward the red end of the spectrum.
The red shift has played an important role in astronomy throughout the 1900's. In 1929, U.S. astronomer Edwin Hubble, after learning that the light from other galaxies is shifted toward the red end of the spectrum, concluded that the universe is expanding. Analyzing the red shift of galaxies, Hubble noted further that there is a consistent, proportional relationship between a galaxy's distance and the speed of its movement away from the Milky Way. The farther away a galaxy is, he discovered, the faster it is receding from us. (The Milky Way is also moving, of course. An observer in another galaxy would see our galaxy receding at high speed.)
The relationship between a galaxy's velocity and its distance, which became known as Hubble's law, made it possible to figure the distance to any galaxy, no matter how far away, just by measuring its red shift. For example, a galaxy with a red shift indicating a velocity of 30,000 kilometers [18,600 miles] per second is about 2 billion light years away. A galaxy receding at twice that speed is twice as far away.
This discovery has been invaluable to astronomers, but analyzing the light from distant galaxies to obtain their red shifts is time consuming work. Therefore, making three-dimensional maps of the universe–or red shift surveys, as they are usually called—has proceeded at a much slower pace than earlier mapping efforts. By 1993, astronomers had measured the red shifts of about 40,000 galaxies.
The largest red shift survey has been carried out by a research team led by astronomers Margaret Geller and John Huchra at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. By 1993, they had mapped the volume of space within about 500 million light-years of the Milky Way—only about 3 percent of the distance we can see.
Also in 1993, scientists at several U.S. institutions—the University of Chicago; the Fermilab accelerator laboratory in Batavia, Ill.; Johns Hopkins University in Baltimore; Princeton University in Princeton, N.J.; and the Institute for Advanced Study, also in Princeton—were working on an ambitious project to map the positions of about 1 million galaxies. Known as the Sloan Digital Sky Survey, the project will map 25 percent of the sky to a depth of 2.5 billion light-years from the Milky Way. The survey, which is expected to last 10 years, will be carried out with a specially designed 2.5-meter (100-inch) telescope being constructed in New Mexico. The telescope will be linked to an array of electronic recording instruments that will measure 600 red shifts at once.
Determining the Structure of the Universe
The researchers involved with this latest mapping effort hope their survey will be large enough to yield a representative sample of the universe. Astronomers have learned that on a very large scale, the universe is pretty much the same wherever you look. The New Mexico survey, which will be based on a sizable volume of space, should therefore contain enough information to enable astronomers to infer the structure of the entire universe.
This mapping effort can be likened to the voyages of discovery undertaken in the 1700's by the British naval explorer Captain James Cook. In three voyages in the 1760's and 1770's, Cook investigated and charted large areas of the Pacific Ocean, most of which had been unknown territory to Europeans.
Cook and his crews were the first Europeans to see Antarctica, New Zealand, and the Hawaiian Islands and to sail along the east coast of Australia. With Cook's expeditions, mapmakers could at last show the Earth as it really was. Although many blanks remained to be filled in, world maps would now contain all of the planet's major land masses, each shown its proper size and in its correct location. The Earth had become a more familiar place. So, too, as we map the galaxies, will we gain a clearer picture of how the universe looks on the grand scale.