In the winter of 1610, the Italian scientist Galileo Galilei, using a telescope he had designed, saw the night skies as they had never been seen before. He observed the face of the Moon, identified sunspots, and puzzled over the changing illumination of Venus over a period of weeks. He saw moons of Jupiter that vanished and reappeared periodically. He saw that the Milky Way is not merely a whitish band across the sky but consists of a vast number of stars, far more than the few thousand visible to the naked eye. Though human beings had been studying the sky for centuries, Galileo was the first to observe its elements at high magnification—and the conclusions he drew from those observations would change the way human beings understand both the universe and our place in it.
Astronomy is unique among the sciences because most of the objects it studies are not directly accessible; huge distances separate Earth from even nearby astronomical objects. With the exception of bodies in our own solar system, all the objects are too far away for direct sampling or spacecraft reconnaissance (techniques in use for only a few decades). Instead, information about distant objects is gleaned from the collection, analysis, and interpretation of electromagnetic radiation—light, X-rays, and other forms of energy given off by all objects in the universe. Collection of data is done through the use of a variety of telescopes far more powerful and sophisticated than the device used by Galileo some 400 years ago.
No single astronomical instrument has changed the way we perceive the universe more than the Hubble Space Telescope, launched into space by the United States in 1990. Named after the great 20th-century astronomer Edwin Hubble, the telescope sent back images that were like nothing ever seen. Since it observes from a low orbit in space, the Hubble telescope captures images undistorted by Earth’s atmosphere and is able to record more clearly very faint amounts of light from the farthest and darkest parts of the universe. More than 6,000 articles based on data from the Hubble have been published, including a more accurate
dating of the age of the universe (14 billion years), information on how galaxies form, the discovery of numerous extrasolar planets, as well as new theories about the nature of energy and gravity. In 2013, the Hubble will be replaced by the even more powerful James Webb Telescope, which will be sent deeper into space.
♦The Universe The contents of the universe range in size from individual gas atoms and dust grains to enormous clusters and superclusters of galaxies. As Galileo began to observe, these contents are in a constant state of flux in relation to one another: planets orbit stars, stars collapse or explode, galaxies expand and contract.
The Sun and all other stars are massive bodies of gas that undergo fusion reactions in their core. As a result of these reactions, stars emit visible light as well as electromagnetic radiation at other wavelengths. Temperature, mass, and luminosity are properties of all stars: the brightest stars are also the highest in both mass and temperature. Small stars are called dwarfs—the brightest are blue dwarfs, the dimmest are red dwarfs. Brown dwarfs are bodies too small to be classified as stars, but too large to be planets, at between 13 and 80 times the mass of Jupiter. They glow dimly as a result of energy released by gravitational contraction.
Almost half the stars in the visible universe are actually part of a binary system—pairs of stars that orbit each other. Astronomers can sometimes see both stars, but more commonly they identify binaries by observing the influence of the dimmer star’s gravitational pull on the other. Sometimes material from one star in a binary collides with the other, causing a phenomenon called a nova, in which the luminosity of the star system dramatically increases.
A supernova occurs when old stars explode, resulting in a phenomenon many times brighter than an ordinary nova. In fact, Chinese astronomers in A.D. 1054 reported a supernova that was visible in the daytime. The remnants of these explosions are usually termed nebulas, interstellar clouds of dust, hydrogen gas, and plasma that emit light as a result of energy from stars ionizing the gas. Some patches of dust reflect the light of nearby stars and seem to glow. The most striking nebulas consist of glowing gas surrounded by opaque dust or vice versa, which gives the nebula a definite shape.
At the center of many nebulas is a neutron star, sometimes called a pulsar. A neutron star is one that has collapsed in a violent explosion so powerful that the gravity compacting the star’s atomic matter is stronger than the nuclear forces that normally keep individual subatomic particles from touching. All the star’s neutrons and protons can touch, forming the equivalent of a giant atomic nucleus. The star is electrically neutral because of the charge of the collapsed electrons. Such a star may be only a dozen miles in diameter but may have a mass twice that of the Sun. Pulsars are neutron stars that emit electromagnetic signals from their magnetic poles in a direction that reaches Earth. As the star rotates, the light appears to “pulse” from the star, in an effect similar to the rotation of a lighthouse beacon.
Another possible result of a supernova explosion is the creation of a black hole. This occurs when a stellar body becomes so dense for its size that not even light can escape its powerful gravitational pull. Black holes have been observed at the center of many galaxies, including our own.
Young stars like our Sun “burn” hydrogen in a nuclear fusion process resulting in helium. When a star has consumed all the hydrogen in its core, new fusion reactions begin, changing the helium to carbon. The new reactions are much hotter than the earlier fusion of hydrogen to helium, and this added energy causes the relatively cool outer layers of hydrogen and helium to expand and turn red, creating what’s known as a red giant. When the Sun becomes a red giant in the distant future, it will expand almost to the orbit of Earth, completely engulfing Mercury and Venus, and charring Earth to a cinder.
Until 1995, our Sun was the only star known to have planets. Since then, however, hundreds of planets have been detected orbiting other stars. Many of these extrasolar planets orbit their star at very close distances, but astronomers have also found systems similar to ours, with Jovian-sized planets at large distances from their stars.
There are also objects called quasars that produce far too much energy to be stars. No one knows for sure what they are, but there is some evidence that quasars are energy torrents from black holes in the center of distant galaxies, so distant their light has not yet reached Earth.
♦Galaxies Ordinary matter is believed to form only 5 percent of the universe by mass. Galaxies—systems of many stars separated from one another by largely empty space—rotate in a way that indicates they
must be embedded in a gravitational field caused by undetectable matter (missing mass, more commonly called “dark matter”), while all galaxies are flying apart with increasing speed caused by some unknown force that opposes gravity (dark energy). There are a number of competing theories as to the nature of these entities. The missing mass may consist of slow-moving unknown subatomic particles, while dark energy might be a cosmological constant inherent to the vacuum of empty space.
Galaxies are classified into several types. A spiral galaxy has a bright, flattened disk of stars, gas, and dust with spiral arms, along with a central bulge and diffuse halo. Elliptical galaxies, on the other hand, are roughly ovular or spherical in shape, with stars orbiting the galaxy center in random directions. Elliptical galaxies contain mostly old stars, have little gas or dust, and show no evidence of ongoing star formation. Galaxies that do not fit the spiral or elliptical descriptions are classified as irregular galaxies. These tend to be smaller than the other types of galaxies and have asymmetrical shapes.
♦The Milky Way The galaxy to which our Sun and its planets belong is a spiral galaxy called the Milky Way. It is estimated to contain at least 200 billion stars as well as star-forming material and other matter. The Sun is located about halfway between the center of the galaxy’s flattened disk of stars and its edge. The disk contains most of the stars in the galaxy and nearly all of the interstellar gas and dust. The common name Milky Way dates to antiquity and refers to the white “spilled milk” appearance of the night sky when one’s vision is directed toward the galactic core.
Clusters and superclusters are groups of galaxies associated in space. There may be just a few members of a cluster or as many as thousands, but they are recognizable by the proximity of galaxies (within clusters) or clusters of galaxies (within superclusters). About 30 galaxies form the Local Group (our cluster), including the Milky Way, the Andromeda galaxy, and the Large and Small Magellanic Clouds. The Local Group is a member of a cluster of galaxies called the Virgo Cluster, which contains over 1,000 galaxies. On an even larger scale, the Virgo Cluster is just one cluster in the Virgo Supercluster, which is a collection of over 1 million galaxies.
♦The Big Bang The Big Bang is the generally accepted theory derived from the observation that as the universe ages, it is rapidly expanding outward in all directions. If the universe is expanding over time, then it must have been smaller as we travel backward in time. The origin of the universe is thought to have been an initial moment when all observable matter was compressed into a tiny space of unimaginable heat and mass, followed by an explosive expansion. Data collected through various astronomical observations support the idea that the universe began in a hotter, denser state than we currently find it. Cosmic expansion is the most direct byproduct, as space continues to stretch outward, carrying galaxies along with it and away from one another. The abundance of hydrogen and helium is naturally explained as having formed during the first extremely hot moments after the Big Bang, when the universe was hot enough to produce matter from energy and fuse hydrogen into helium. The radio glow seen in all directions, known as cosmic background radiation, is the remnant of heat from the Big Bang, weakened by the stretching of space since that time.
The speeds of galaxies combined with their distances from one another allow scientists to estimate how long they have been receding from one another—therefore estimating the time elapsed since the Big Bang. Careful measurements of the recession rate and the cosmic background radiation combine to give an age of the universe of 13.7 billion years. For comparison, the Sun and solar system formed in the Milky Way 4.5 billion years ago.
“We shall place the Sun himself at the center of the Universe,” wrote Nicolas Copernicus in his great work On the Revolutions of the Heavenly Spheres, published in 1543. This conclusion by the Polish polymath—he was an astronomer, mathematician, physician, diplomat, governor, translator, and Catholic cleric—contradicted the widely held belief that Earth was at the center of the universe. Copernicus was the first to correctly attribute the daily motions of the Sun and stars to the rotation of Earth, annual changes in the appearance of the sky to the orbital motion of Earth around the Sun, and retrograde motion to the relative speeds of planets as they orbit the Sun. His model wasn’t entirely correct,
however, because it still relied on the perfect circular orbits of planets described by Aristotle. It took 30 years of careful observations by the Danish astronomer Tycho Brahe (1546–1610), and the interpretation of Brahe’s data by the German mathematician Johannes Kepler (1571–1630), to show that planets orbit the Sun on elliptical paths, moving faster along their orbits when closer to the Sun, and that there is a precise mathematical relation between a planet’s distance from the Sun and the time it takes to complete an orbit.
Today, of course, we know that the star we call the Sun isn’t the center of the universe, but rather the center of our solar system, which comprises the Sun and all the objects that orbit it. The Sun, which provides the energy that allows life on Earth to exist, is believed to have been formed out of a molecular cloud some 5 billion years ago and is composed primarily of hydrogen and helium. The temperature of the Sun is approximately 28,280,000 degrees Fahrenheit (15,710,000 Celsius) at its core. By itself it comprises about 98.6 percent of the solar system’s mass.
♦Planets The largest objects that orbit the Sun are referred to as planets. These are grouped broadly into two categories: the terrestrial planets and the Jovian planets. The terrestrial planets are the four closest to the Sun: Mercury, Venus, Earth, and Mars. These are composed primarily of silicon-based rock and metals. The Jovian planets include Jupiter, Saturn, Uranus, and Neptune. Jupiter and Saturn are composed mostly of hydrogen gas, whereas Uranus and Neptune are composed of icy cores surrounded by hydrogen atmospheres.
Mercury As the planet closest to the Sun, Mercury has the shortest orbit. Its view from Earth is usually obscured by the Sun’s glare, but it is sometimes visible on the horizon just after sunset, when it is called the Evening Star, or just before dawn, when it is called the Morning Star. About 14 times each century, Mercury can also be seen crossing directly in front of the Sun.
It is a waterless, airless world that alternately bakes and freezes as it orbits the Sun. The high temperatures on the sunlit side mean that the planet cannot retain a substantial atmosphere. Its surface is scarred with hundreds of thousands of craters, probably formed during the early history of the solar system, when large numbers of asteroids and comets slammed into planetary surfaces. Many craters have been smoothed over
by ancient lava flows. The surface is also crisscrossed by huge cliffs, or scarps, probably formed as Mercury’s surface cooled and shrank.
Venus In Earth’s night sky, Venus is second only to the Moon in luminousity, as it passes closer to Earth than any other planet. Since it orbits between Earth and the Sun, Venus, like Mercury, can be seen either just before sunrise or just after sunset.
The Venusian atmosphere consists almost entirely of carbon dioxide. Thick clouds shroud the planet’s surface from direct view, and droplets of sulfuric acid and water have been identified in the clouds. The clouds and high level of carbon dioxide have combined to trap heat in the lower atmosphere where temperatures are hot enough to melt lead.
More than a thousand geographical features have been identified on the surface of Venus, including mountains, volcanoes, rifts, basins, and impact craters. About 10 percent of the planet’s surface is highland terrain, 70 percent rolling uplands, and 20 percent lowland plains. Volcanic activity dominates Venusian geology; the planet is covered with volcanic domes and lava channels.
Earth Third from the Sun, Earth is the only one in the solar system known to harbor life, the result of its distinct atmosphere and the presence of an ozone layer that, together with Earth’s magnetic field, blocks deadly radiation. From space, our planet appears as a bright blue-and-white sphere—blue because some 70 percent of the surface is covered by water, and white because clouds cover about half the planet’s surface.
Earth’s only natural satellite, the Moon, is over one-quarter the size of Earth in diameter. The Moon’s rotation and revolution are synchronized, meaning the same side of the satellite always faces Earth. Analysis of Moon rocks brought back to Earth by astronauts has led to the hypothesis that the Moon formed when the collision of a large protoplanet stripped material from Earth’s crust.
The Moon is airless and devoid of life. A mixture of fine powder and broken rock blankets the surface. The near side (the side seen from Earth) also has large regions of solidified lava. The surface is pockmarked with craters and larger impact basins and is broken by huge mountain ranges.
Mars The distinctive coloring of the “Red Planet” comes from iron oxide in the soil. The surface is heavily cratered, and there is extensive evidence of once-active volcanoes. Ice caps cover both poles, which advance
and recede with changes in the seasons. There are also spectacular features such as Olympus Mons (an extinct volcano three times as high as Earth’s Mt. Everest), mammoth canyons, one of which is four times deeper than the Grand Canyon and as wide as the U.S., and a gigantic basin that is larger than Alaska. The two irregularly shaped satellites of Mars are Deimos and Phobos. They are the only satellites besides the Moon that orbit a terrestrial planet; neither Mercury nor Venus have moons.
The big question of whether there is (or was) life on Mars has yet to be answered with certainty. Water once flowed there in great quantities, and the soil is similar to that found in many backyards here on Earth, so the possibility cannot be ruled out.
Jupiter The largest planet in the solar system, Jupiter has 2.5 times more mass than all the other planets of the solar system put together. It has a faint but extensive ring system and more than 60 moons, many of them quite small.
The most prominent feature of Jupiter is its colorful cloud layers. Because the planet spins so fast, its clouds tend to form bands that give the planet a striped appearance. There are numerous eddies and swirls in Jupiter’s atmosphere, most famously the Great Red Spot, a massive hurricane located in the southern hemisphere near the equator.
The interior composition of Jupiter is largely a mystery. Its density is only about one-fourth that of the terrestrial planets, indicating that it is composed of light atoms, more than 90 percent of them hydrogen. Although hydrogen normally takes the form of a gas on Earth, it exists in more exotic states at the high pressures inside Jupiter: below a gaseous cloud layer, the hydrogen is compressed into a liquid and then a liquid metal. At the very center of the planet there may be a small core of rocklike and icelike material.
Saturn The sixth planet from the Sun, Saturn is the second largest. It has a pale yellowish color and a spectacular ring system consisting of more than 1,000 rings, which are composed mostly of ice particles and a smaller amount of rocky debris and dust. Some of the rings are circular, others are elliptical, and at least two are intertwined, or “braided.” The planet has more than 30 known moons, most of them relatively small and pockmarked by meteor craters. Titan, Saturn’s largest moon, is one of the few moons in the solar system known to have an atmosphere.
Saturn’s atmosphere consists of densely compacted hydrogen, helium, and other gases. Scientists believe there is a solid core of rock about two times the size of Earth at its center.
Uranus The seventh planet in the solar system and the third of the Jovian gas giants, Uranus is a faintly greenish color, perhaps because its atmosphere contains methane. It has a system of faint rings and at least 21 moons. Uranus’s atmosphere is very cold, though scientists theorize that, as on Jupiter and Saturn, temperatures and pressures increase dramatically below the outer layer of atmosphere. At some pressure the hydrogen and helium might be sufficiently compressed to form a liquid or slushy surface “crust.” Underneath this crust is thought to be a mantle of solidified methane, ammonia, and water, and inside this mantle is a rocky core of silicon and iron.
Neptune The last of the Jovian planets, Neptune is the eighth planet in the solar system. It is a pale bluish color but has a clear atmosphere. Scientists believe Neptune has a three-layered structure similar to that of Uranus: a crust of solidified or liquid hydrogen and helium, a mantle of solidified gases and water, and a hot, rocky core.
♦The Planet That Wasn’t From 1930 through the end of the 20th century, the solar system was thought to contain nine planets, the ninth being Pluto. In recent years, however, several objects similar to Pluto were discovered in the outer solar system. This caused a rethinking of what makes a planet a planet.
In 2006, the International Astronomical Union (I.A.U.) for the first time defined the term planet. In this new definition, a planet is a celestial body that is in orbit around a star, has sufficient mass so that it assumes a nearly round shape, and has cleared its orbit of planetesimals (embryonic planets) and similar debris. Pluto meets only the first two of these criteria. It was thus removed from the rank of major planets and reclassified as a dwarf planet. The I.A.U. classifies a dwarf planet as a celestial body that is in orbit around the Sun, has sufficient mass so that it assumes a nearly round shape, has not cleared its orbit of planetesimals, and is a not a satellite of another similar object. Today, there are five recognized dwarf planets: Pluto, Eris, Ceres, Haumea, and Makemake. Ceres is the largest asteroid in the main asteroid belt between Mars and Jupiter. The other four orbit the Sun on the frozen fringes of the solar system.
♦Smaller Objects in the Solar System Asteroids are small objects with compositions similar to the terrestrial planets. They are found primarily in the asteroid belt, a band lying between the orbits of Mars and Jupiter. Smaller numbers are found in orbits that cross those of the terrestrial planets, including Earth’s orbit, and others lead or trail a planet along its journey around the Sun. More than 200,000 asteroids are known.
Comets are icy bodies, composed mostly of water ice and carbon dioxide ice. When far from the Sun, comets are essentially in deep freeze, but if a comet’s orbit carries it closer to the Sun than Jupiter, significant amounts of the ice evaporate and trail away from the main body of the comet. This forms the comet’s distinctive tail. As the ice evaporates, small, solid, grainlike particles mixed in with the ice are also released. The main body of a comet may be only a few kilometers in diameter, but upon close approach to the Sun, the tail may extend for millions of kilometers.