Asteroids, comets, and meteoroids

Asteroids, comets, and meteoroids: are all debris remaining from the nebula from which the solar system formed 4.6 billion years ago. Asteroids are rocky bodies up to about 600 miles (1,000 km) in
diameter, although most are much smaller. Most of them orbit the Sun in the asteroid belt, which lies between the orbits of Mars and Jupiter. Cometary nuclei exist in a huge cloud (called the Oort Cloud) that surrounds the planetary part of the solar system. They are made of frozen water and dust and are a few miles in diameter. Occasionally, a comet is deflected from the Oort Cloud on to a long, elliptical path that brings it much closer to the Sun. As the comet approaches the Sun, the cometary nucleus starts to vaporize in the heat, producing both a brightly shining coma (a huge sphere of
gas and dust around the nucleus), and a gas tail, and a dust tail. Meteoroids are small chunks of stone or stone and iron, which are fragments of asteroids or comets. Meteoroids range in size from tiny dust particles to objects tens of meters across. If a meteoroid enters the Earth’s atmosphere, it is heated by friction and appears as a glowing streak of light called a meteor (also known as a shooting star). Meteor showers occur when the Earth passes through the trail of dust particles left by a comet. Most meteoroids burn up in the atmosphere. The remnants of the few that are large enough to reach the Earth’s surface are termed meteorites.

Neptune and Pluto

 Neptune and Pluto:Neptune is the farthest planet from the Sun, at an average distance of about 2.8 billion miles (4.5 billion km). Neptune is the smallest of the giant planets and is thought to consist of a small rocky core surrounded by a mixture of liquids and gases. Several transient cloud features have been observed in its atmosphere. The largest of these were the Great Dark Spot, which was as wide as
the Earth, the Small Dark Spot, and the Scooter. The Great and Small Dark Spots were huge storms that were swept around the planet by winds of about 1,200 miles (2,000 km) per hour. The Scooter was a large area of cirrus cloud. Neptune has six tenuous rings and 13 known moons. Triton is the largest Neptunian moon and the coldest object in the solar system, with a temperature of -390°F (-240°C). Unlike most moons in the solar system, Triton orbits its mother planet in the opposite direction of the planet’s rotation. The region extending out from Neptune’s orbit is populated by Kuiper Belt objects and dwarf planets. They make a doughnutshaped belt called the Kuiper Belt. The Kuiper Belt objects are a mix of rock and ice, irregular in shape, and less than 600 miles (1,000 km) across. The larger dwarf planets, which include Pluto, are almost round bodies. Pluto was the first object discovered beyond Neptune and was considered a planet until the dwarf planet category was introduced in 2006. It is made of rock and ice and is 1,365 miles (2,274 km) across. It has three known moons. The largest, Charon, is about half Pluto’s size and the two probably had a common origin.

Uranus

Uranus:Uranus is the seventh planet from the Sun and the third largest, with a diameter of about 32,000 miles (51,000 km). It is thought to consist of a dense mixture of different types of ice and gas around a solid core. Its atmosphere contains traces of methane, giving the planet a blue-green hue, and the temperature at the cloud tops is about -350°F (-210°C). Uranus is the most featureless planet to have been closely observed: only a few icy clouds of methane have been seen so far. Uranus is unique among the planets in that its axis of rotation lies close to its orbital plane. As a result of its strongly tilted rotational axis, Uranus rolls on its side along its orbital path around the Sun, whereas other planets spin more or less upright. Uranus is encircled by main rings that consist of rocks
interspersed with dust lanes and two distant outer rings made of dust. The rings contain some of the darkest matter in the solar system and are extremely narrow, making them difficult to detect: most of them are less than 6 miles (10 km) wide, whereas most of Saturn’s rings are thousands of miles in width. There are 27 known Uranian moons, all of which are icy and most of which are farther out than the rings. The 13 inner moons are small and dark, with diameters of less than 100 miles (160 km), and the five major moons are between about 290 and 1,000 miles (470 and 1,600 km) in diameter. The major moons have a wide variety of surface features. Miranda has the most varied surface, with cratered areas broken up by huge ridges and cliffs 12 miles (20 km) high. Beyond these are nine much more distant moons with diameters less than 90 miles (150 km).

Saturn

Saturn:Saturn is the sixth planet from the Sun. It is a gas giant almost as big as Jupiter, with an equatorial diameter of about 75,000 miles (120,500 km). Saturn is thought to consist of a small core of rock and ice surrounded by an inner mantle of metallic hydrogen (liquid hydrogen that acts like a metal). Outside the inner mantle is an outer mantle of liquid hydrogen that merges into a gaseous atmosphere. Saturn’s clouds form belts and zones similar to those on Jupiter, but obscured by overlying haze. Storms and eddies, seen as red or white ovals, occur in the clouds. Saturn has an extremely thin but wide system of rings that is about half a mile (1 km) thick but extends outward to about 260,000 miles (420,000 km) from the planet’s surface. The main rings comprise thousands of narrow ringlets, each made of icy rock lumps that range in size from tiny particles to chunks several yards across. The D, E, and G rings are very faint, the F ring is brighter, and the A, B, and C rings are
bright enough to be seen from Earth with binoculars. In 2009, a huge dust ring was discovered 4 million miles (6 million km) beyond the main system. Saturn has more than 60 known moons, some of which orbit inside the rings and are thought to exert a gravitational influence on the shapes of the rings. Unusually, seven of the moons are co-orbital—they share an orbit with another moon. Astronomers believe that such co-orbital moons may have originated from a single satellite that broke up.

Jupiter

Jupiter:Jupiter is the fifth planet from the Sun and the innermost of the four giant planets. It is the largest and the most massive planet, with a diameter about 11 times that of the Earth and a mass about 2.5 times the combined mass of the seven other planets. Jupiter is thought to have a small rocky core surrounded by an inner mantle of metallic hydrogen (liquid hydrogen that acts like a metal). Outside the inner mantle is an outer mantle of liquid hydrogen and helium that merges into the gaseous atmosphere. Jupiter’s rapid rate of rotation causes the clouds in its atmosphere to form belts and zones that encircle the planet parallel to the equator. Belts are dark, low-lying, relatively warm cloud layers, and zones are bright, high-altitude, cooler cloud layers. Within the belts and zones, turbulence causes the formation of cloud features such as white ovals and red spots, both of which are huge storm systems. The most prominent cloud feature is a storm called the Great Red Spot, which consists of a spiraling column of clouds three times wider than the Earth that rises about five miles (8 km) above the upper cloud layer. Jupiter has a thin, faint, main ring, inside which is a tenuous halo ring of tiny particles. Beyond the main ring’s outer edge is a broad and faint two-part gossamer ring. There are 63 known Jovian moons. The four largest moons (called the Galileans) are Ganymede, Callisto, Io, and Europa. Ganymede and Callisto are cratered and icy. Europa is smooth and icy and is thought to have a subsurface water ocean. Io is covered in bright red, orange, and yellow splotches. This coloring is caused by sulfurous material from active volcanoes that shoot plumes of lava hundreds of miles above the surface.

Mars

Mars:Mars,known as the red planet, is the fourth planet from the Sun and the outermost rocky planet. In the 19th century, astronomers first observed what were thought to be signs of life on Mars. These signs included apparent canal-like lines on the surface, and dark patches that were thought to be vegetation. It is now known that the “canals” are an optical illusion, and the dark patches are areas where the red dust that covers most of the planet has been blown away. The fine dust particles are
often whipped up by winds into dust storms that occasionally obscure almost all the surface. Residual fine dust in the atmosphere gives the Martian sky a pinkish hue. The northern hemisphere of Mars has many large plains formed of solidified volcanic lava, whereas the southern hemisphere has many craters and large impact basins. There are also several huge, extinct volcanoes, including Olympus Mons, which, at 370 miles (600 km) across and 15 miles (25 km) high, is the largest known volcano in the solar system. The surface also has many canyons and branching channels. The canyons were formed by movements of the surface crust, but the channels are thought to have been formed by flowing water that has now dried up. The Martian atmosphere is much thinner than Earth’s, with only a few clouds and morning mists. Mars has two tiny, irregularly shaped moons called Phobos and Deimos. Their small size indicates that they may be asteroids that have been captured by the gravity of Mars.

The Moon

The Moon:The moon is the earth's only natural satellite. It is relatively large for a moon, with a diameter of about 2,155 miles (3,470 km)—just over a quarter that of the Earth. The Moon takes the same time to rotate on its axis as it takes to orbit the Earth (27.3 days), and so the same side (the near
side) always faces us. However, the amount of the surface we can see—the phase of the Moon—
depends on how much of the near side is in sunlight. The Moon is dry and barren, with negligible atmosphere and water. It consists mainly of solid rock, although its core may contain molten rock or iron. The surface is dusty, with highlands covered in craters caused by meteorite impacts, and lowlands in which large craters have been filled by solidified lava to form dark areas called maria or “seas.” Maria occur mainly on the near side, which has a thinner crust than the far side. Many of the craters are rimmed by mountain ranges that form the crater walls and can be thousands of feet high.

The earth

The earth:The earth is the third of the eight planets that orbit the Sun. It is the largest and densest rocky planet, and the only one known to support life. About 70 percent of the Earth’s surface is covered by water, which is not found in liquid form on the surface of any other planet. There are four main layers: the inner core, the outer core, the mantle, and the crust. At the heart of the planet the solid inner core has a temperature of about 11,900°F (6,600°C). The heat from this inner core causes material in the molten outer core and mantle to circulate in convection currents. It is thought that these convection currents generate the Earth’s magnetic field, which extends into space as the magnetosphere. The Earth’s atmosphere helps screen out some of the harmful radiation from the Sun, stops most meteoroids from reaching the planet’s surface, and traps enough heat to prevent extremes of cold. The Earth has one natural satellite, the Moon, which is thought to have formed when a huge asteroid impacted Earth in the distant past.

Venus

Venus:Venus is a rocky planet and the second planet from the Sun. Venus spins slowly backwards as it orbits the Sun, causing its rotational period to be the longest in the solar system, at about 243 Earth days. It is slightly smaller than Earth and probably has a similar internal structure, consisting of a semisolid metal core, surrounded by a rocky mantle and crust. Venus is the brightest object in the sky after the Sun and Moon because its clouds reflect sunlight strongly. The main component of the atmosphere is carbon dioxide, which traps heat in a greenhouse effect far stronger than that on Earth. As a result, Venus is the hottest planet, with a maximum surface temperature of about 900°F (480°C). The thick cloud layers contain droplets of sulfuric acid and are driven around the planet by winds at speeds of up to 220 miles (360 km) per hour. Although the planet takes 243 Earth days to rotate once, the high-speed winds cause the clouds to circle the planet in only four Earth days. The high temperature, acidic clouds, and enormous atmospheric pressure (about 90 times greater at the surface than that on Earth) make the environment extremely hostile. However, space probes have managed to land on Venus and photograph its dry, dusty surface. The Venusian surface has also been mapped by probes with radar equipment that can “see” through the cloud layers. Such radar maps reveal a terrain with craters, mountains, volcanoes, and areas where craters have been covered by plains of solidified volcanic lava. There are two large highland regions called Aphrodite Terra and Ishtar Terra.

Mercury

Mercury:Mercury is the nearest planet to the Sun, orbiting at an average distance of about 36 million miles (58 million km). Because Mercury is the closest planet to the Sun, it moves faster than any other planet, travelling at an average speed of nearly 30 miles (48 km) per second and completing an
orbit in just under 88 days. Mercury is very small (only 40 percent bigger than the Moon) and rocky. Most of the surface has been heavily cratered by the impact of meteorites, although there are also smooth, sparsely cratered lava-covered plains. The Caloris Basin is the largest crater, measuring about 800 miles (1,300 km) across. It is thought to have been formed when a 38-mile- (60-km-) diameter asteroid hit the planet, and is surrounded by concentric rings of mountains thrown up by the impact. The surface also has many clifflike ridges (called rupes) that are thought to have been formed when the hot core of the young planet cooled and shrank about four billion years ago, buckling the planet’s surface in the process. The planet rotates about its axis very slowly, taking nearly 59 Earth days to complete one rotation. As a result, a solar day (sunrise to sunrise) on Mercury is about 176 Earth days—twice as long as the 88-day Mercurian year. Mercury has extreme surface temperatures, ranging from a maximum of 800°F (430°C) on the sunlit side to -270°F (-170°C) on the dark side. At nightfall, the temperature drops very quickly because the planet’s atmosphere is almost nonexistent. It consists only of minute amounts of helium and hydrogen captured from the solar wind, plus traces of other gases.

The Sun

The Sun:The sun is the star at the center of the solar system. It is about five billion years old and will continue to shine as it does now for about another five billion years. The Sun is a yellow main sequence star (see pp. 22-23) about 870,000 miles (1.4 million km) in diameter. It consists almost entirely of hydrogen and helium. In the Sun’s core, hydrogen is converted to helium by nuclear
fusion, releasing energy in the process. The energy travels from the core, through the radiative and convective zones, to the photosphere (visible surface), where it leaves the Sun in the form of heat and light. On the photosphere there are often dark, relatively cool areas called sunspots, which usually appear in pairs or groups and are caused by the cooling effect of the magnetic field. Other types of
solar activity are flares, which are usually associated with sunspots, and prominences. Flares are sudden discharges of high-energy radiation and atomic particles. Prominences are huge loops or
filaments of gas extending into the solar atmosphere; some last for hours, others for months. Beyond the photosphere is the chromosphere (inner atmosphere) and the extremely rarified corona (outer atmosphere), which extends millions of miles into space. Tiny particles that escape from the corona give rise to the solar wind, which streams through space at hundreds of miles per second. The chromosphere and corona can be seen from Earth when the Sun is totally eclipsed by the Moon.

The solar system

The solar system consists of a central star (the Sun) and the bodies that orbit it. These bodies include eight planets and their more than 160 known moons; dwarf planets; Kuiper Belt objects; asteroids; comets; and meteoroids. The solar system also contains interplanetary gas and dust. The planets fall into two groups: four small rocky planets near the Sun (Mercury, Venus, Earth, and Mars); and four planets farther out, the giants (Jupiter, Saturn, Uranus, and Neptune). Between the rocky planets and giants is the asteroid belt, which contains thousands of chunks of rock orbiting the Sun. Beyond Neptune is the Kuiper Belt and, more distant, the Oort Cloud. Most of the bodies in the planetary part of the solar system move around the Sun in elliptical orbits located in a thin disk around the Sun’s equator. All the planets orbit the Sun in the same direction (counterclockwise when viewed from above) and all but Venus and Uranus also spin about their axes in this direction. Moons also spin as they, in turn, orbit their planets. The entire solar system orbits the center of our galaxy, the Milky Way.

Neutron stars and black holes

Neutron stars and black holes form from the stellar cores that remain after stars have exploded as supernovae (see pp. 26-27). If the remaining core is between about one and a half and three solar masses, it contracts to form a neutron star. If the remaining core is greater than about three solar masses, it contracts to form a black hole. Neutron stars are typically only about 6 miles (10 km) in diameter and consist almost entirely of subatomic particles called neutrons. Such stars are so dense that a teaspoonful would weigh about a billion tons. Neutron stars are observed as pulsars, so-called because they rotate rapidly and emit two beams of radio waves, which sweep across the sky and are detected as short pulses. Black holes are characterized by their extremely strong gravity, which is so powerful that not even light can escape; as a result, black holes are invisible. However, they can be
detected if they have a close companion star. The gravity of the black hole pulls gas from the other star, forming an accretion disk that spirals around the black hole at high speed, heating up and emitting radiation. Eventually, the matter spirals in to cross the event horizon (the boundary of the black hole), thereby disappearing from the visible universe.

Massive stars

Massive stars:Massive stars have a mass at least three times that of the Sun, and some stars are as massive as about 50 Suns. A massive star evolves in a similar way to a small star until it reaches the main sequence stage (see pp. 24-25). During its life as a main sequence star, it shines steadily until the hydrogen in its core has fused to form helium. This process takes billions of years in a small star, but only millions of years in a massive star. A massive star then becomes a red supergiant, which
initially consists of a helium core surrounded by outer layers of cooling, expanding gas. Over the next few million years, a series of nuclear reactions form different elements in shells around an iron core. The core eventually collapses in less than a second, causing a massive explosion called a supernova, in which a shock wave blows away the outer layers of the star. Supernovae shine brighter than an
entire galaxy for a short time. Sometimes, the core survives the supernova explosion. If the surviving core is between about one and a half and three solar masses, it contracts to become a tiny, dense
neutron star. If the core is greater than three solar masses, it contracts to become a black hole.

Small stars

Small stars:Small stars have a mass of up to about one and a half times that of the Sun. They begin to form when a region of higher density in a nebula condenses into a huge globule of gas and dust that contracts under its own gravity. Within a globule, regions of condensing matter heat up and begin to glow, forming protostars. If a protostar contains enough matter, the central temperature reaches about 27 million °F (8 million °C). At this temperature, nuclear reactions in which hydrogen fuses to form helium can start. This process releases energy, which prevents the star from contracting more and also causes it to shine; it is now a main sequence star. A star of about one solar mass remains on the main sequence for about 10 billion years, until much of the hydrogen in the star’s core has been converted into helium. The helium core then contracts, and nuclear reactions continue in a shell around the core. The core becomes hot enough for helium to fuse to form carbon, while the outer
layers of the star expand and cool. The expanding star is known as a red giant. When the helium in the core runs out, the outer layers of the star may be blown away as an expanding gas shell called a planetary nebula. The remaining core (about 80 percent of the original star) is now in its final stages. It becomes a white dwarf star that gradually cools and dims. When it finally stops shining altogether, the dead star will become a black dwarf.

Stars

Stars:Stars are bodies of hot, glowing gas that are born in nebulae. They vary enormously in size, mass, and temperature: diameters range from about 450 times smaller to over 1,000 times bigger than that of the Sun; masses range from about a twentieth to over 50 solar masses; and surface temperatures range from about 5,500ºF (3,000ºC) to over 90,000ºF (50,000ºC). The color of a star is determined by its temperature: the hottest stars are blue and the coolest are red. The Sun, with a surface temperature of 10,000ºF (5,500ºC), is between these extremes and appears yellow. The
energy emitted by a shining star is usually produced by nuclear fusion in the star’s core. The brightness of a star is measured in magnitudes—the brighter the star, the lower its magnitude. There are two types of magnitude: apparent magnitude, which is the brightness seen from Earth, and absolute magnitude, which is the brightness that would be seen from a standard distance of 10 parsecs (32.6 light-years). The light emitted by a star may be split to form a spectrum containing a series of dark lines (absorption lines). The patterns of lines indicate the presence of particular chemical elements, enabling astronomers to deduce the composition of the star’s atmosphere. The magnitude and spectral type (color) of stars may be plotted on a graph called a Hertzsprung-Russell diagram, which shows that stars tend to fall into several well-defined groups. The principal groups are main sequence stars (those which are fusing hydrogen to form helium), giants, supergiants, and white dwarfs.

Stars of southern skies

Stars of southern skies:When you look at the southern sky, you look toward the Galactic center, which has a huge population of stars. As a result, the Milky Way appears brighter in the southern sky than in the northern sky. The southern sky is rich in nebulae and star clusters. It contains the Large and Small Magellanic Clouds, which are two of the nearest galaxies to our own. Stars make fixed patterns in the sky called constellations. However, the constellations are only apparent groupings of stars, since the distances to the stars in a constellation may vary enormously. The shapes of constellations may change over many thousands of years due to the relative motions of stars. The movement of the constellations across the sky is due to the Earth’s motion in space. The daily rotation of the Earth causes the constellations to move across the sky from east to west, and the orbit of the Earth around the Sun causes different areas of sky to be visible in different seasons. The
visibility of areas of sky also depends on the location of the observer. For instance, stars near the celestial equator may be seen from either hemisphere at some time during the year, whereas stars
close to the celestial poles (the celestial South Pole is at the center of the map shown here) can never be seen from the opposite hemisphere.

Stars of northern skies

Stars of northern skies: When you look at the northern sky, you look away from the densely populated Galactic center, so the northern sky generally appears less bright than the southern sky. Among the best-known sights in the northern sky are the constellations Ursa Major (the Great Bear) and Orion. Some ancient civilizations believed that the stars were fixed to a celestial sphere surrounding the Earth, and modern maps of the sky are based on a similar idea. The North and South Poles of this imaginary celestial sphere are directly above the North and South Poles of the Earth, at the points where the Earth’s axis of rotation intersects the sphere. The celestial North Pole is at the center of the map shown here, and Polaris (the North Star) lies very close to it. The celestial equator marks a projection of the Earth’s equator on the sphere. The ecliptic marks the path of the Sun across the sky as the Earth orbits the Sun. The Moon and planets move against the background of the stars because the stars are much more distant; the nearest star outside the solar system is more than 50,000 times farther away than the planet Jupiter.

Nebulae and star clusters

Nebulae and star clusters:A Nebula is a cloud of dust and gas inside a galaxy.Nebulae become visible if the gas glows, or if the cloud reflects starlight or obscures light from more distant objects. Emission nebulae shine because their gas emits light when it is stimulated by radiation from hot young stars. Reflection nebulae shine because their dust reflects light from stars in or around the nebula. Dark nebulae appear as silhouettes because they block out light from shining nebulae or stars behind them. Two types of nebula are associated with dying stars: planetary nebulae and supernova remnants. Both consist of expanding shells of gas that were once the outer layers of a star. A planetary nebula is a gas shell drifting away from a dying stellar core. A supernova remnant is a gas shell moving away from a stellar core at great speed following a violent explosion called a supernova (see pp. 26-27). Stars are often found in groups known as clusters. Open clusters are loose groups of a few thousand young stars that were born from the same cloud and are drifting apart. Globular clusters are densely packed, roughly spherical groups of hundreds of thousands of older stars.

The Milky Way

The milky way:The milky way is the name given to the faint band of light that stretches across the night sky. This light comes from stars and nebulae in our galaxy, known as the Milky Way Galaxy or simply as “the Galaxy.” The Galaxy is believed to be a barred spiral, with a dense central bar of stars encircled by four arms spiraling outward and surrounded by a less dense halo. We cannot see the spiral shape because the solar system is in one of the spiral arms, the Orion Arm (also called the Local Arm). From our position, the center of the Galaxy is completely obscured by dust clouds; as a result, optical maps give only a limited view of the Galaxy. However, a more complete picture can be
obtained by studying radio, infrared, and other radiation. The central part of the Galaxy is relatively small and dense and contains mainly older red and yellow stars. The halo is a less dense region in which the oldest stars are situated; some of these stars are as old as the Galaxy itself (possibly 13 billion years). The spiral arms contain main sequence stars and hot, young, blue stars, as well as nebulae (clouds of dust and gas inside which stars are born). The Galaxy is vast, about 100,000 light-years across (a light-year is about 5,870 billion miles/9,460 billion km); in comparison, the solar system seems small, at about 12 light-hours across (about 8 billion miles/13 billion km). The entire Galaxy is rotating in space, although the inner stars travel faster than those farther out. The Sun, which is about two-thirds out from the center, completes one lap of the Galaxy about every 220 million years.