Magnitude

Magnitude (astronomy), term used in astronomy to designate the brightness of a star. Magnitude, also called apparent magnitude, describes the brightness of a star as viewed from the earth. The term absolute magnitude refers to the brightness of a star as viewed from a standard distance of 10 parsecs, or about 32 light-years.

The ancient Alexandrian astronomer Ptolemy originally divided all visible stars into six magnitudes: the brightest were called first magnitude, those barely visible to the naked eye were called sixth magnitude, and the other visible stars were assigned intermediate positions. After the introduction of the telescope in the 17th century, this system of magnitudes was used and extended to the fainter stars in different ways by different astronomers. In the 19th century a standard system was finally adopted under which a star of any given magnitude is 2.512 times as bright as a star of the next higher magnitude; thus, for example, a star of the second magnitude is 2.512 times as bright as a star of the third magnitude. The advantage of this particular magnitude ratio, 2.512, is that it coincides closely with the Ptolemaic system; and because 2.512 is the fifth root of 100, a star of the first magnitude is exactly 100 times as bright as a star of the sixth magnitude, a star of the sixth magnitude is exactly 100 times as bright as a star of the 11th magnitude, and so on. The mean of the magnitudes of several hundred stars found in the Bonn Durchmusterung catalog, which was prepared by the German astronomer Friedrich Wilhelm August Argelander about 1860, was taken as the standard of the scale for calibration purposes.

With accurate instruments, such as bolometers and radiometers, astronomers today can measure differences as small as one-hundredth of a magnitude. Stars with magnitudes between 1.5 and 2.5 are called second-magnitude stars. Stars brighter than magnitude 1.5, of which there are 20, are called first-magnitude stars. Thus, the first-magnitude star Aldebaran has an actual magnitude of 1.1; the slightly brighter first-magnitude star Altair has a magnitude of 0.9. The brightest stars are brighter than magnitude zero. Sirius, the brightest star outside the solar system, has a magnitude of -1.6. The sun has a magnitude of -26.7, inasmuch as it is about 10 billion times as bright as Sirius in the earth’s sky.

Absolute magnitude, as opposed to apparent magnitude, indicates the brightness that a star would have if it were placed at a distance from the earth of ten parsecs, or 32.6 light-years. By rating stars in this way, astronomers are able to compare them with respect to intrinsic brightness. The sun, for example, has an absolute magnitude of +4.7. Sirius has an absolute magnitude of +1.4.

Binary Star

Binary Star, two stars that are bound to each other by gravity and orbit about a common center of mass. Binary star systems are quite common and the pairing of stars appears to be random in most cases. Astronomers estimate that approximately one-fourth of the visible stars belong to a binary system. The time it takes for one star to orbit the other can range from hours to centuries depending on the distance between the two stars and their masses. Some binary pairs, called interacting binary systems, are so close that they exchange material. Binary stars are very useful to astronomers because they are the only stars of which astronomers can directly determine mass.

Orbit

Orbit (astronomy and physics), path or trajectory of a body through space. A force of attraction or repulsion from a second body usually causes the path to be curved. A familiar type of orbit occurs when one body revolves around a second, strongly attracting body. In the solar system the force of gravity causes the moon to orbit about the earth and the planets to orbit about the sun, whereas in an atom electrical forces cause electrons to orbit about the nucleus. In astronomy, the orbits resulting from gravitational forces, which are discussed in this article, are the subject of the scientific field of celestial mechanics.

An orbit has the shape of a conic section—a circle, ellipse, parabola, or hyperbola—with the central body at one focus of the curve. When a satellite traces out an orbit about the center of the earth, its most distant point is called the apogee and its closest point the perigee. The perigee or apogee height of the satellite above the earth's surface is often given, instead of the perigee or apogee distance from the earth's center. The ending -gee refers to orbits about the earth; perihelion and aphelion refer to orbits about the sun; the ending -astron is used for orbits about a star; and the ending -apsis is used when the central body is not specified. The so-called line of apsides is a straight line connecting the periapsis and the apoapsis.

Topic:

• Laws of Motion

Orbit

Orbit->> LAWS OF MOTION

Early in the 17th century, the German astronomer and natural philosopher Johannes Kepler deduced three laws that first described the motions of the planets about the sun: (1) The orbit of a planet around the sun is an ellipse. (2) A straight line from the planet to the center of the sun sweeps out equal areas in equal time intervals as it goes around the orbit; the planet moves faster when closer to the sun and slower when distant. (3) The square of the period (in years) for one revolution about the sun equals the cube of the mean distance from the sun's center, measured in astronomical units.

The physical causes of Kepler's three laws were later explained by the English mathematician and physicist Isaac Newton as consequences of Newton's laws of motion and of the inverse square law of gravity. Kepler's second law, in fact, expresses the conservation of angular momentum. Moreover, Kepler's third law, in generalized form, can be stated as follows: The square of the period (in years) times the total mass (measured in solar masses) equals the cube of the mean distance (in astronomical units). This last law permits the masses of the planets to be calculated by measuring the size and period of satellite orbits.

Astronomical Unit

Astronomical Unit (AU), unit of distance used in the measurement of orbits and trajectories within the solar system. One AU is the average distance between the earth and the sun. Its value has been established as, roughly, 149,600,000 km (92,956,000 mi) by means of radar-ranging studies of nearby celestial objects such as Venus or passing asteroids; these studies have enabled astronomers to determine the scale of the solar system with great accuracy.

Nebula

Nebula, in astronomy, a localized conglomerate of the gaseous and finely divided dust particles that are spread throughout interstellar space. Before the invention of the telescope, the term nebula (Latin, “cloud”) was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were called nebulas.

Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation.

Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighboring stars. They include some of the most striking objects in the sky, such as the Great nebula in Orion (the middle “star” in the sword). The tremendous streams of matter in the diffuse nebulas are intermingled in violent, chaotic currents. Many thousands of luminous nebulas are known. Spectral studies show that light emanating from them consists of reflected light from stars and also, in so-called emission nebulas, of stimulated radiation of ionized gases and dust from the nebulas themselves.

Dark, diffuse nebulas are observed as nonluminous clouds or faintly luminous, obscuring portions of the Milky Way and too distant from the stimulation of neighboring stars to reflect or emit much light of their own. One of the most famous dark nebulas is the Horsehead nebula in Orion, so named for the silhouette of the dark mass in front of a more luminous nebular region. The longest dark rift observed on photographic plates of the star clouds of the Milky Way is a succession of dark nebulas. Both dark nebulas and luminous nebulas are considered likely sites for the processes of dust-cloud condensation and the formation of new stars.

Globular Cluster

Globular Cluster, compact, spherical group of stars, containing many thousands or even millions of stars.

Astronomers have found more than 200 globular clusters in the Milky Way Galaxy (the earth’s galaxy). Most galaxies contain globular clusters and some galaxies contain thousands of such clusters. Most of the known globular clusters in the Milky Way move around the center of the galaxy in orbits that take them far outside the Milky Way. By finding the center of their orbits, the American astronomer Harlow Shapley of Harvard University located the Milky Way’s center in 1918.

Globular clusters are the oldest structures associated with our galaxy. They contain only Population II stars—the oldest stars in the universe. All globular clusters in the Milky Way and the neighboring Andromeda Galaxy seem to be about the same age, suggesting they were created by conditions within galaxies while the galaxies were young.

The diameters of globular clusters average about 50 light-years. The stars within a cluster are very densely packed near its center and may be only a few billion kilometers away from each other.

Galaxy

Galaxy, a massive ensemble of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center. Astronomers estimate that there are about 125 billion galaxies in the universe. All the stars visible to the unaided eye from Earth belong to Earth’s galaxy, the Milky Way. The Sun, with its associated planets, is just one star in this galaxy. Besides stars and planets, galaxies contain clusters of stars; atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, and silicon, among others; and cosmic rays (see Interstellar Matter).

Galaxy M100
The spiral galaxy M100 is located between 35 million and 80 million light-years from earth. The Hubble Space Telescope captured this image of the core of M100 after repairs were made to the telescope in December 1993.



Galaxies M86 and M84
The elliptical galaxies M86 (center) and M84 (right) are members of the Virgo cluster of galaxies, located about 50 million light-years away from our smaller cluster, the Local Group. Elliptical galaxies are populated by older stars and contain little interstellar matter. They are usually the brightest galaxies.

Great Andromeda Spiral Galaxy

Great Andromeda Spiral Galaxy, also known as M 31, large spiral galaxy in the constellation Andromeda, about 2.2 million light-years from Earth.

The Great Andromeda Spiral Galaxy is the largest nearby galactic neighbor to the Milky Way Galaxy, Earth’s home galaxy. Because it is so near, it appears very bright, with a total magnitude (a measure of its brightness as seen from Earth) of 3.4, and is easily visible to the naked eye on a clear, dark night. Its immense diameter of about 200,000 light-years makes it appear five times larger than the full Moon in our sky.

The Great Andromeda Spiral Galaxy is the most studied of external galaxies because astronomers can view features in it that they believe are also present in the Milky Way, but are made invisible by the Milky Way’s thick intervening clouds of dust. Astronomers study the Andromeda Galaxy’s spiral arms; the birth of stars, dense spherical groups of stars called globular clusters, as well as looser star groupings called open clusters; interstellar matter; and supernova remnants. The mass of the Andromeda Galaxy is believed to be equivalent to between 300 billion and 400 billion Suns.

From the 1970s to the 1990s, observations made with ground-based telescopes and with the Hubble Space Telescope showed that the galaxy appears to have a double nucleus—two bright areas near its center, instead of the usual single bright spot. In 1999 astronomers mapping the motion of stars near the galaxy’s center discovered that the stars orbit the center in an elliptical path instead of in the more usual circular path found in most other galaxies. The stars move much more slowly at one end of the ellipse than at the other. Astronomers believe that stars bunch up at the slow point in the orbit, like cars in a traffic jam, creating one of the bright areas. The other bright area, at the other end of the ellipse, occurs because stars pass close to the black hole (a region of space so dense that not even light can escape its gravitational pull) at the center of galaxy. Astronomers believe that all galaxies have a similar, giant black hole at their center. Matter from the stars is more likely to fall into the black hole at this close point in the stars’ orbit, and matter accelerating toward the black hole glows to produce the second bright area.

The first recorded observation of this galaxy was in ad 905 by Persian astronomer al-Sufi, who described it as the “little cloud” in his Book of Fixed Stars (964). The telescopic discovery of this object is often attributed to German astronomer Simon Marius who described the soft glow of the object in 1611 or 1612.

Andromeda

Andromeda (astronomy), in astronomy, large constellation of the northern hemisphere situated just south of the constellation Cassiopeia and west of the constellation Perseus. Andromeda contains no stars of the first magnitude but is noted as the area of sky containing the Andromeda Galaxy, a member of the local group to which our own Milky Way belongs. At a distance of 2.2 million light-years, the Andromeda Galaxy is both the nearest spiral galaxy and the most distant object that can be seen with the naked eye. Before its nature was determined by means of powerful telescopes, it was erroneously believed to be a nebula, or cloud of interstellar matter. Through telescopes it is seen to have two small companion galaxies of elliptical form.

Open Cluster

Open Cluster, or galactic cluster, group of associated stars that travel together through space.

Astronomers have cataloged about 1200 open clusters in the earth’s galaxy, each containing from ten to many hundreds of stars (see Milky Way). Astronomers have also discovered hundreds of open clusters in other galaxies. The stars in a galaxy usually orbit in a plane around a common center, and almost all open clusters lie close to the plane of the galaxy in which they’re located. Clusters range from about 5 light-years to about 70 light-years in diameter. Often a thin, misty light, caused by reflection of starlight off the cosmic dust and gas in the cluster, envelops an entire cluster.

Open clusters are classified by the number of stars in them and by the degree to which the stars are concentrated toward the center of the cluster. All clusters in the same class are roughly the same size. The stars in an open cluster are usually relatively young, and the cluster is considered the same age as the stars in it. The ages of the Milky Way’s known open clusters range from about 1 million years old (the Orion Nebula) to about 5 billion years old (cluster NGC 188).

The American astronomer Robert J. Trumpler of the Lick Observatory carried out a study of open clusters in 1930. He found that the widths of the open clusters that he measured seemed to increase as the open clusters’ distances from the earth increased. He conjectured that astronomers had been overestimating the distance between the earth and clusters because the brightness of the clusters was being obscured by interstellar matter. When he revised the distance measurements for the clusters, their calculated widths became more uniform. His investigations provided the first clear evidence of the existence of cosmic dust and gas throughout the galaxy.

Great Orion Nebula

Great Orion Nebula or Orion Nebula, or M 42, diffuse nebula, or cloud of gas and dust, located in the constellation Orion. With its brightest parts having an apparent magnitude (a measure of brightness as seen from the earth) of 4.0, the Great Orion Nebula can be seen with the unaided eye as part of Orion’s sword, which hangs from his belt.

The Great Orion Nebula looks like a star when it is seen with the unaided eye, but even a small telescope reveals its cloudy nature. The entire Great Orion Nebula as seen from the earth is about 60 arc minutes wide, or about four times as wide as the full moon. The Great Orion Nebula is about 1500 light-years from the earth and it is about 30 light-years in diameter.

Unlike some diffuse nebulae, which are visible only because they reflect the light of nearby stars, the Great Orion Nebula not only reflects light, but it also emits light. Emission nebulae like the Great Orion Nebula are so huge that they provide enough gas and dust to create new stars. Some of these young stars are so hot and massive that the energy they give off excites the hydrogen atoms in the nebula and makes them glow. Astronomers estimate that the bright center of the nebula is a nursery for about 700 young stars. Using the Hubble Space Telescope (HST) to view this nebula, astronomers have found more than 150 protoplanetary systems, or stars surrounded by a ring of gas and dust that may be the beginning of a solar system.

The Great Orion Nebula was initially thought to be a star, but in 1610 French lawyer Nicholas-Claude Fabri de Peiresc discovered that it was actually a nebula. This discovery was confirmed independently in 1618 by Czech astronomer Rennus Cysatus. In 1769 French astronomer Charles Messier added the Great Orion Nebula to his Catalogue of Nebulae and Star Clusters (1771-1784), a list of astronomical objects that may be mistaken for comets. Because a dark strip of dust unevenly divides the Great Orion Nebula, Messier assigned the nebula two numbers. The smaller part of the nebula is known as M 43, but M 42 is usually assumed to refer to the entire nebula.

Orion

Orion (astronomy), constellation located on the celestial equator east of Taurus. It is an oblong configuration with three stars in line near its center. It is represented on pictorial charts as the figure of Orion, the hunter in Greek mythology, standing with uplifted club. Three bright stars represent his belt and three fainter stars aligned south of the belt represent his sword. Alpha (a) Orionis, or Betelgeuse, is located in the left corner of the oblong, corresponding to Orion's shoulder. Beta (β) Orionis, or Rigel, is diagonally opposite Betelgeuse. A nebula surrounding the three stars marking Orion's sword is one of the most conspicuous bright nebulas in the heavens.

In Greek mythology, Orion is the handsome giant and mighty hunter, the son of Poseidon, god of the sea, and Euryale, the Gorgon. Orion fell in love with Merope, the daughter of Oenopion, king of Chios, and sought her in marriage. Oenopion, however, constantly deferred his consent to the marriage, and Orion attempted to gain possession of Merope by violence. Incensed at his behavior, her father, with the aid of the god Dionysus, put Orion into a deep sleep and blinded him. Orion then consulted an oracle, who told him he could regain his sight by going to the east and letting the rays of the rising sun fall on his eyes. His sight restored, he lived on Crete (Kríti) as the huntsman of the goddess Artemis. One version of Orion's story relates that the goddess eventually killed him because she was jealous of his affection for Aurora, goddess of the dawn. After Orion's death, Artemis placed him in the heavens as a constellation.

Equator

Equator, in astronomy, the great circle in which the plane of the equator of the earth intersects the celestial sphere. The celestial equator is the line from which the declination of stars and planets is measured. See Ecliptic.

Magellanic Clouds

Magellanic Clouds, small, irregular galaxies that lie relatively near the Milky Way galaxy. The Large Magellanic Cloud (LMC), in the constellation Dorado (the Goldfish), and the Small Magellanic Cloud (SMC), in the constellation Tucana (the Toucan), are visible to the unaided eye in the southern hemisphere and as far north as 16° North latitude. They became known in Europe through descriptions made in 1521 by the Portuguese navigator Ferdinand Magellan, after whom they were named. The LMC lies about 150,000 light-years away from our sun, and the SMC lies about 173,000 light-years away. In the early 1980s, another galaxy, called the Mini Magellanic Cloud (MMC), was determined to lie about 20,000 light-years beyond the SMC, in the same line of sight. Apparently it was torn from the SMC by a near encounter with the LMC about 200 million years ago. A supernova was observed in the LMC in 1987.

Cassiopeia

Cassiopeia (astronomy), northern constellation, near the celestial pole. It is distinguished by a group of five stars, of second to fourth magnitude, in the form of a rough letter W. The brightest supernova on record appeared in the constellation in 1572 and was observed by the Danish astronomer Tycho Brahe. Brighter than the planet Venus, for about 16 months Cassiopeia was visible to the naked eye even at noon. It is named for the mythological Ethiopian queen Cassiopeia, the mother of Andromeda.

Transit

Transit, passage of one heavenly body over the disk of another, as of Mercury or Venus over the disk of the sun, or of a satellite over its primary. A transit of Mercury or Venus can take place only when either planet passes the sun at the time the sun is near one of the nodes of the planet; that is, when Venus or Mercury is in inferior conjunction, or is closer to earth than to the sun. The transit of Venus was first recorded in 1639. In 1679 the English astronomer Edmond Halley pointed out that these transits could be used to determine the distance of the sun. Usually two transits of Venus occur within 8 years of one another; then, after a lapse of 105 or 122 years, another two transits occur within 8 years. Transits of Mercury occur about 13 times in each century.

The term transit also refers to an instrument for measuring the passage of an object past the local meridian (see Transit Instrument). Likewise, the passage of an object past the local meridian is called a transit.

Mirfak

Mirfak, sometimes spelled Mirphak or Marfak, brightest star in the constellation Perseus, the Warrior. The star is also known as Algenib and is designated Alpha Persei. The name Mirfak is derived from the Arabic phrase Mirfaq al Thurayya, the “Elbow Nearest the Many Little Ones.” This refers to Mirfak’s position in Perseus near the Pleiades star cluster. Mirfak lies near the north celestial pole, a point in the sky about which the stars in the northern hemisphere appear to rotate. This appearance is actually due to the rotation of the earth. Observers who are north of latitude 40° north can see Mirfak all night long throughout the year, circling the north celestial pole. For this reason, Mirfak is called a north circumpolar star. Observers in the southern hemisphere can see Mirfak low in the northern sky from September through November.

Stars that are visible to the unaided eye, such as Mirfak, belong to the earth’s home galaxy, the Milky Way, and tend to be very bright or relatively close. Mirfak is a very bright star with an intrinsic luminosity, or total light output, that rates an absolute magnitude of -5.1 (bright stars have low or even negative magnitude values). This magnitude corresponds to the light output of about 9000 suns. Mirfak is about 630 light-years from earth. For comparison, the most distant easily seen stars are up to 5000 light-years from the earth. At Mirfak’s distance, it shines in the earth’s night sky with an apparent magnitude—a measure of how bright it appears to an observer on the earth—of +1.79, making it one of the 50 brightest stars in the night sky.

Mirfak owes its brightness to a moderately high surface temperature and very large size. Mirfak’s surface temperature is 6200° C (11,000°F), which is about 10 percent greater than the surface temperature of the sun and gives the star a yellow-white color. Its diameter is estimated at 124 million km (77 million mi), which is about 90 times greater than the diameter of the sun. From its composition, temperature, and size, astronomers classify Mirfak as a supergiant star—an older star that has used up the hydrogen fuel in its core and is now burning hydrogen in its outer shell and helium in its core. These changes have caused it to grow much larger than its original size, which has increased its luminosity tremendously.

Mirfak is the brightest star of a large cluster of about 70 loosely bound stars collectively known as the Alpha Persei Association. This cluster, which covers an area of sky equal to six full moons, is easily visible through binoculars. Many of its stars are in pairs, trios, and quartets. Studies indicate that the average age of the stars in the Alpha Persei Association is only about 51 million years, younger than many diamonds found on the earth.

Pleiades

Pleiades (astronomy), loose cluster of 400 to 500 stars, about 415 light-years from the solar system in the direction of the constellation Taurus. The stars are about 1 light-year apart, on the average, and photographs show them to be surrounded by a nebulosity that shines by their reflected light. The cluster was named by the ancient Greeks after the “Seven Sisters” of mythology. Observers have claimed to be able to see with the naked eye as many as 12 of the stars in the cluster.

Taurus

Taurus (astronomy) (Latin for “bull”), a constellation, represented pictorially by the forequarters of a bull. It is a zodiacal constellation—that is, a constellation located along the ecliptic, the apparent annual path of the sun across the sky (see Zodiac). Taurus contains the two famous star groups known as the Hyades, which includes the brilliant red star Aldebaran, and the Pleiades. It also contains the Crab Nebula, associated with the spectacular supernova of the year ad1054.

Supernova

Supernova, violent explosion that occurs when a large star uses up its supply of fuel, collapses under its own weight, and explodes. A shock wave from this catastrophic event expands into space, followed by a shell of material from the star’s atmosphere. The material blown off contains chemical elements created throughout the star’s lifetime. Debris from supernovas enriches the chemistry of interstellar space with material that becomes part of new stars and planets. See also Astronomy; Interstellar Matter.

Supernovas are rare phenomena—fewer than five supernovas in our Milky Way galaxy have been visible from Earth in the last 1,000 years. Some supernovas can be bright enough to see with the naked eye during the day. They may continue glowing for several weeks or even months after the explosion. Thick clouds of interstellar dust hide some supernovas, but astronomers can detect those by the radio waves that the supernova emits. See also Radio Astronomy.

Supernovas occur in all galaxies, not just the Milky Way. Supernovas that occur outside the Milky Way are bright enough to stand out against the other stars in the galaxy. However, they are usually not bright enough to pick out without a telescope. A typical supernova can produce as much light and other forms of electromagnetic radiation as billions of stars. Electromagnetic radiation is energy carried through space by electric and magnetic waves. The length of these waves determines the properties of the radiation. In addition to the radiation energy a supernova produces, the force of the explosion releases ten times more energy into the motion of the particles that the explosion blows outward. These tiny particles, called neutrinos, carry away a hundred times more energy than the electromagnetic radiation. Astronomers discover about ten supernovas in distant galaxies each year.

Topics:

• Formation of a Supernova
• After the Supernova
• Studying Supernovas

Supernova

Supernova->> FORMATION OF A SUPERNOVA

The mass of a star determines whether it will end its life in a supernova explosion. During the courses of their lifetimes, all stars convert hydrogen to helium in thermonuclear fusion reactions in their cores. Thermonuclear fusion reactions occur when the intense heat and gravitational force in a star’s nucleus force hydrogen atoms together. The atoms merge, or fuse together, creating helium atoms and releasing large amounts of energy in the form of electromagnetic radiation and heat. Massive stars have faster rates of fusion than smaller stars, so large stars may use up their fuel faster. After most of the hydrogen is used up, a star goes into a carbon-building phase, in which nuclear fusion turns the helium into carbon. After the helium is exhausted, most stars gradually cool until they no longer emit radiation.

When a star eight or ten times more massive than the Sun exhausts its helium, however, the nuclear burning cycle is far from complete. In these stars, the carbon core shrinks under its own weight, and its temperature rises high enough to fuse carbon into oxygen, neon, silicon, sulfur, and finally, iron.

Iron is the most stable element formed in stars, and even the intense heat and pressure of a stellar nucleus cannot force iron atoms to fuse into heavier elements. The thermonuclear process at the star’s core is essentially complete. At this point, the outward pressure produced by the reactions can no longer balance the inward gravitational attraction between atoms. As a result, all the core can do is collapse under its own weight. As it does so, the star implodes, transforming gravitational energy into kinetic energy, or energy of motion. The core of the star collapses in on itself, but as it does so, it transfers to the star’s atmosphere kinetic energy that sends the atmosphere exploding outward from the star’s core. The particles of the star’s atmosphere begin moving rapidly away from the star, tearing apart the star’s atmosphere.

Astronomers know of several variations of supernovas, but they all fall into one of two main types. The two kinds of supernovas are called Type I and Type II and are differentiated mostly by the presence of hydrogen in their debris. Type I supernovas tend to be older stars that have completely exhausted their hydrogen. Type II supernovas come from younger stars that have used up the hydrogen in their nucleus but have large amounts of hydrogen in their atmospheres. Astronomers can measure what elements exist in a star by examining its light because atoms of different elements emit and absorb electromagnetic radiation at different wavelengths. By separating a star’s light into its wavelengths, astronomers can tell which wavelengths are missing or especially bright, and therefore what elements are present in the star.

Supernova

Supernova->> AFTER THE SUPERNOVA

All supernova explosions produce clouds of debris and release huge amounts of energy, but Type I supernovas typically completely destroy their parent stars, while Type II explosions usually leave the stellar core behind. The stellar atmosphere of both types expands into space and appears as luminous clouds years, or even centuries, later. These clouds are called supernova remnants. The Crab Nebula is one of the most spectacular supernova remnants.

The fate of the stellar core left behind by a Type II supernova depends on the mass of the original star. Normal atoms are made up of positively charged particles called protons, particles with no electric charge called neutrons, and much smaller, negatively charged particles called electrons. If the original star had a mass about ten times that of the Sun, the core collapses with such force that its protons and electrons combine to form neutrons. The resulting body is composed entirely of neutrons, so astronomers call it a neutron star. Most neutron stars created by supernovas are pulsars. Pulsars are neutron stars that spin rapidly as they emit powerful beacons of radio waves. From Earth, these spinning beacons appear as pulses of radiation.

If the mass of the original star is greater than about ten solar masses, the nuclear forces that hold up a neutron star are too weak to resist the core’s gravitational pull. The core continues to collapse past the neutron star stage. It crushes itself until its mass is concentrated into a volume of space smaller than a typical city on Earth. At this point, the speed at which an object would have to travel to escape the core’s gravitation, like a space probe leaving Earth, is greater than the speed of light. No kind of matter, or even radiation, can reach this speed and escape, so these astronomical objects are invisible to the eye or to normal telescopes. Not only can matter not escape, but the collapsed core pulls in any matter or radiation that comes too close. Astronomers call this kind of an object a black hole because no light can escape its gravitational pull.

Supernova

Supernova->> STUDYING SUPERNOVAS

Chinese astronomers recorded supernovas visible from Earth as far back as ad 185. Probably the most well-known ancient supernova is the one that created the Crab Nebula in 1054. From Chinese and Japanese records, astronomers estimate that it was about 20 times as bright as any other star in the night sky. It was visible even during the day for several weeks after it first appeared.

The last time a supernova in the Milky Way galaxy became visible from Earth was October 1604. It was bright enough to be seen at night with the naked eye for more than a year. German mathematician Johannes Kepler made detailed observations of the supernova and carefully measured its position. Since then, astronomers have not seen any supernovas in the Milky Way. A number of supernovas have appeared in other galaxies, however.

One of the most important supernovas of the 20th century, and the brightest in the sky of the northern hemisphere since 1937, burst into view on March 28, 1993, in the galaxy M 81. Astronomers noticed the strange behavior of the parent star—a huge red-colored star called a red giant—before it exploded and were able to track its changes as it became a supernova.

On February 24, 1987, one of the closest supernovas in centuries occurred in the Large Magellanic Cloud, only 160,000 light-years from Earth. A light-year is a measure of distance equal to the distance that light travels in a year, or 9.5 trillion km (5.9 trillion mi). This supernova was visible from the southern hemisphere. Since this eruption, scientists have learned that its parent star may have once been a hot blue star with a mass about 20 times that of the Sun. The star probably swelled into a red giant star before it exploded.

Scientists are continually searching for and studying supernovas. Astronomers learn about the final evolutionary paths of massive stars from supernovas. In addition, supernovas give clues to the origin of the chemical elements that make up stars, planets, and even life. A supernova in a distant galaxy can even help astronomers measure the distance to the galaxy. To do this, astronomers examine the radiation emitted by the shell of material from the star’s atmosphere and use the information they gain to develop models of how wide the shell is. They then compare the width of their model to the apparent width of the shell as viewed from Earth to estimate the distance to the supernova remnant and to its parent galaxy.

Star

Star (astronomy), massive shining sphere of hot gas. Of all the stars in the universe, our Sun is the nearest to Earth and the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy, the massive ensemble of stars that contains our solar system (the Sun and its nine planets).

About 5,000 stars can be seen with the naked eye, although not all of these stars are visible at any given time or from any given place. With a small telescope, hundreds of thousands of stars can be seen. The largest telescopes disclose millions of galaxies, which may each contain over 200 billion stars. Modern astronomers believe there are more than 1 x 1022 stars in the universe (this number is very large, a 1 followed by 22 zeros). The largest stars, if placed at the Sun's position, would easily engulf Earth, Mars, Jupiter, and Saturn. The smallest white dwarf stars are about the size of Earth, and neutron stars are less than about 20 km (about 10 mi) in diameter.

All stars are composed of hot glowing gas. The outer layers of some stars are so empty that they can be described as red-hot vacuums. Other stars are so dense that a teaspoonful of the material composing the outer layers would weigh several tons. Stars are made chiefly of hydrogen and a smaller amount of helium. Even the most abundant of the other elements present in stars—oxygen, carbon, neon, and nitrogen—are generally present in very small quantities.

The Sun, our nearest star, is about 150 million km (about 93 million mi) from Earth. It appears different from the stars visible in the night sky because it is about 250,000 times closer to Earth than the next closest star. The next nearest star is Proxima Centauri, which is more than 30 trillion km (20 trillion mi) from Earth. While light from the Sun takes only about eight minutes to reach Earth, the farthest stars are so distant that their light takes billions of years to reach Earth.

The color of stars—ranging from the deepest red through all intermediate shades of orange and yellow to an intense white-blue—depends directly on their temperature. The coolest stars are red and the hottest stars are blue. Most stars make light by several different kinds of thermonuclear fusion, a process in which the nuclei of atoms combine to form a heavier element and release energy (see Nuclear Energy). One of the most common thermonuclear fusion processes occurs in stars when four hydrogen atoms combine into a helium atom, releasing energy that is transformed into light and heat.

In the 1990s astronomers discovered planets orbiting stars outside our solar system. Planets outside our solar system are difficult to detect, because they are much fainter than stars are. However, astronomers located these planets by measuring the wobble of a star’s motion created by the slight gravitational pull that is exerted on the star by a planet. Although scientists can only speculate how many Earthlike planets with continents and oceans exist in the universe, they believe that many stars have planetary systems (See also Gravity).

Planet

Planet, any major celestial body that orbits a star and does not emit visible light of its own but instead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planet are called asteroids or planetoids. In the solar system, there are nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Planets that orbit stars other than the Sun are collectively called extrasolar planets. Some extrasolar planets are nearly large enough to become stars themselves. Such borderline planets are called brown dwarfs. See also Planetary Science.

Meteor

Meteor, in astronomy, small solid body known as a meteoroid that enters a planet's atmosphere from outer space and is raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a cometlike train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder. Fainter meteors, called shooting or falling stars, usually occur singly and sporadically. At intervals, however, hundreds of such meteors occur simultaneously and appear to emanate from a fixed point. These swarms are called meteor showers and are named after the constellation in which they seem to have their point of origin. Some appear annually on the same days of each year and are called periodic showers; others occur infrequently at varying intervals. The periods of meteor showers generally coincide with those of certain comets. Most meteors are dissipated in flight and fall to the earth as dust; a meteor that reaches the surface of the earth or another planet is called a meteorite.

Meteoroid

Meteoroid, solid body orbiting the sun, becoming a meteor, or shooting star, if it enters the earth's atmosphere. The vast majority of meteroids are the size of grains of dust, but they range upward in size without any definite limit. The largest can have masses of thousands of tons. See Asteroids; Comets.

Asteroid

Asteroid, one of the many small or minor rocky planetoids that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter.

SIZES AND ORBITS

The largest representatives are 1 Ceres, with a diameter of about 1,003 km (about 623 mi), and 2 Pallas and 4 Vesta, with diameters of about 550 km (about 340 mi). The naming of asteroids is governed by the International Astronomical Union (IAU). After an astronomer observes a possible unknown asteroid, other astronomers confirm the discovery by observing the body over a period of several orbits and comparing the asteroid’s position and orbit to those of known asteroids. If the asteroid is indeed a newly discovered object, the IAU gives it a number according to its order of discovery, and the astronomer who discovered it chooses a name. Asteroids are usually referred to by both number and name.

About 200 asteroids have diameters of more than 97 km (60 mi), and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

Few scientists now believe that asteroids are the remnants of a former planet. It is more likely that asteroids occupy a place in the solar system where a sizable planet could have formed but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Originally perhaps only a few dozen asteroids existed, which were subsequently fragmented by mutual collisions to produce the population now present. Scientists believe that asteroids move out of the asteroid belt because heat from the Sun warms them unevenly. This causes the asteroids to drift slowly away from their original orbits.

Pallas

Pallas, the second largest asteroid, the second to be discovered. It was first observed by the German astronomer Heinrich Olbers in 1802. It is about 480 km (about 300 mi) in diameter, and it revolves about the sun in 1684 days.

Vesta

Vesta (astronomy), asteroid orbiting the Sun between the orbits of Mars and Jupiter at a mean distance of 2.36 astronomical units (about 353 million km/about 219.4 million mi) and having a diameter of approximately 385 km (about 240 mi). Vesta is the third largest asteroid and was the fourth asteroid to be discovered. German astronomer Heinrich Olbers found it in 1807. In 1999 the National Aeronautics and Space Administration (NASA) spacecraft Deep Space 1 made observations that suggested that asteroid 9969 Braille, which passes close to Earth, and Vesta are composed of similar materials. Scientists used the information to theorize about the relationship between Vesta and the many meteorites that have struck Earth over time.

Redshift

Redshift, change, or shift, in the light radiated by an object, such as a star or galaxy, that indicates the object’s motion. Scientists have used redshifts to measure the velocities (speed and direction) of distant galaxies. Knowing the velocities of galaxies helps astronomers understand how the universe is changing. This knowledge allows scientists to interpret the distant past of the universe and to predict the universe’s distant future.

Redshift only occurs when an object is moving. Another mechanism can also redden the light of astronomical objects, but it is not considered to be the same as redshift. Dust particles between stars are just the right size to scatter light with short wavelengths more than they scatter light with long wavelengths. As the light of a star passes through a cloud of dust on the light’s way to Earth, more of the long, red wavelengths get through the dust than the short, blue wavelengths do. This makes the star appear redder than it really is, but the light that reaches Earth is the true red light of the star and has not actually shifted. See also Interstellar Matter.

Milky Way

Milky Way, the large, disk-shaped aggregation of stars, or galaxy, that includes the Sun and its solar system. In addition to the Sun, the Milky Way contains about 400 billion other stars. There are hundreds of billions of other galaxies in the universe, some of which are much larger and contain many more stars than the Milky Way.

The Milky Way is visible at night, appearing as a faintly luminous band that stretches across the sky. The name Milky Way is derived from Greek mythology, in which the band of light was said to be milk from the breast of the goddess Hera. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. All of the individual stars that are distinct in the sky lie within the Milky Way Galaxy.

From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horizon. It extends through the constellations Perseus, Cassiopeia, and Cepheus. In the region of the Northern Cross it divides into two streams: the western stream, which is bright as it passes through the Northern Cross, fades near Ophiuchus, or the Serpent Bearer, because of dense dust clouds, and appears again in Scorpio; and the eastern stream, which grows brighter as it passes southward through Scutum and Sagittarius. The brightest part of the Milky Way extends from Scutum to Scorpio, through Sagittarius. The center of the galaxy lies in the direction of Sagittarius and is about 25,000 light-years from the Sun (a light-year is the distance light travels in a year, about 9.46 trillion km or 5.88 trillion mi).

Topics:

• Structure
• Types of Stars
• Rotation

Structure of Milky Way

Galaxies have three common shapes: elliptical, spiral, and irregular. Elliptical galaxies have an ovoid or globular shape and generally contain older stars. Spiral galaxies are disk-shaped with arms that curve around their edges, making these galaxies look like whirlpools. Spiral galaxies contain both old and young stars as well as numerous clouds of dust and gas from which new stars are born. Irregular galaxies have no regular structure. Astronomers believe that their structures were distorted by collisions with other galaxies.

Astronomers classify the Milky Way as a large spiral or possibly a barred spiral galaxy, with several spiral arms coiling around a central bulge about 10,000 light-years thick. Stars in the central bulge are close together, while those in the arms are farther apart. The arms also contain clouds of interstellar dust and gas. The disk is about 100,000 light-years in diameter and is surrounded by a larger cloud of hydrogen gas. Surrounding this cloud in turn is a spherical halo that contains many separate globular clusters of stars mainly lying above or below the disk. This halo may be more than twice as wide as the disk itself. In addition, studies of galactic movements suggest that the Milky Way system contains far more matter than is accounted for by the visible disk and attendant clusters—up to 2,000 billion times more mass than the Sun contains. Astronomers have therefore speculated that the known Milky Way system is in turn surrounded by a much larger ring or halo of undetected matter known as dark matter.

Milky Way: Types of Stars

The Milky Way contains both the so-called type I stars, brilliant, blue stars; and type II stars, giant red stars. Blue stars tend to be younger because they burn furiously and use up all of their fuel within a few tens of millions of years. Red stars are usually older, and use their fuel at a slower rate that they can sustain for tens of billions of years. The central Milky Way and the halo are largely composed of the type II population. Most of this region is obscured behind dust clouds, which prevent visual observation. Astronomers have been able to detect light from this region at other wavelengths in the electromagnetic spectrum, however, using radio and infrared telescopes and satellites that detect X rays. Such studies indicate compact objects near the galactic center, probably a massive black hole. A black hole is an object so dense that nothing, not even light, can escape its intense gravity. The center of the galaxy is home to clouds of antimatter particles, which reveal themselves by emitting gamma rays when they meet particles of matter and annihilate. Astronomers believe the antimatter particles provide more evidence for a massive black hole at the Milky Way’s center.

Observations of stars racing around the center also suggest the presence of a black hole. The stars orbit at speeds up to 1.8 million km/h (1.1 million mph)—17 times the speed at which Earth circles the Sun—even though they are hundreds of times farther from the center than Earth is from the Sun. The greater an object’s mass, the faster an object orbiting it at a given distance will move. Whatever lies at the center of the galaxy must have a tremendous amount of mass packed into a relatively small area in order to cause these stars to orbit so quickly at such a distance. The most likely candidate is a black hole.

Surrounding the central region is a fairly flat disk comprising stars of both type II and type I; the brightest members of the latter category are luminous, blue supergiants. Imbedded in the disk, and emerging from opposite sides of the central region, are the spiral arms, which contain a majority of the type I population together with much interstellar dust and gas. One arm passes in the vicinity of the Sun and includes the great nebula in Orion.

Milky Way Rotation

The Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, and the spiral arms trail in the same direction. The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system the period of rotation is more than 200 million years. The speed of the solar system due to the galactic rotation is about 220 km/sec (about 140 mi/sec).

Perseus

Perseus (astronomy), constellation in the northern hemisphere of the celestial sphere. Perseus is near the constellations Cassiopeia, Andromeda, Aries, and Taurus. The constellation represents Perseus, a mortal hero of Greek mythology.

Perseus’s brightest star is the yellow super giant Mirfak. The constellation’s second brightest star, Algol, is the best known of all eclipsing binary stars. It loses and regains more than twice its normal brightness every three days. In 1901 light from an exploding star in the center of the constellation was detected by observers on the earth. The star increased in brightness more than 100,000 times, making it one of the brightest novas of recent centuries. Within three days it faded.

Perseus is visible throughout all of the northern hemisphere and the northern two-thirds of the southern hemisphere. It reaches its highest point in the night sky in December.

Constellation

Constellation (astronomy), in astronomy, any of 88 imagined groupings of bright stars that appear on the celestial sphere and that are named after religious or mythological figures, animals, or objects. The term also refers to the delimited areas on the celestial sphere that contain the named groups of stars.

The oldest known drawings of constellations are motifs on seals, vases, and gaming boards from the Sumerians, indicating that constellations may have been developed as early as 4000 bc. The constellation Aquarius was named by the Sumerians after their god of heaven An, who pours the waters of immortality upon the earth. The division of the zodiac into 12 equal signs was known around 450 BC by the Babylonians. The northern constellations known today are little different from those known by the Chaldeans and the ancient Egyptians, Greeks, and Romans. Homer and Hesiod mentioned constellations, and the Greek poet Aratus of Soli (circa 315-c. 245 bc) gave a verse description of 44 constellations in his Phaenomena. The Alexandrian astronomer and mathematician Ptolemy, in his Almagest, described 48 constellations, of which 47 are known today by the same name.

In the past many other peoples have grouped stars in constellations, although their arrangements usually did not correspond to those of the ancients. Some Chinese constellations, however, resemble those of the ancients, indicating the possibility of a common origin.

At the end of the 16th century the first explorers of the South Seas mapped the southern sky, which was largely unknown to the ancients. New constellations were added by a Dutch navigator, Pieter Dirckz Keyser, who participated in the exploration of the East Indies in 1595. Subsequently, other southern constellations were added by the German astronomer Johann Bayer, who published the first extensive star atlas in the Western world, the Uranometria; by Johannes Hevelius; and by the French astronomer Nicolas Louis Lacaille. Many others proposed new constellations, but astronomers finally settled on a list of 88. The boundaries of constellations, however, remained a matter of discussion until 1930, when definitive boundaries were fixed by the International Astronomical Union.

The genitive forms of the names of constellations, preceded by a Greek letter, are used to designate about 1300 bright stars; this system was introduced by Johann Bayer. The famous star Algol in the constellation Perseus, for example, is called Beta Persei.

Zodiac

Zodiac, imaginary belt in the celestial sphere, extending about 8° on either side of the ecliptic, the apparent path of the Sun among the stars. The width of the zodiac was determined originally so as to include the orbits of the Sun and Moon and of the five planets (Mercury, Venus, Mars, Jupiter, and Saturn) that were known to the people of ancient times. The zodiac is divided into 12 sections of 30° each, which are called the signs of the zodiac. Starting with the vernal equinox and then proceeding eastward along the ecliptic, each of the divisions is named for the constellation situated within its limits in the 2nd century bc. The names of the zodiacal signs are Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes. Because of the precession of the equinoxes about the ecliptic, a 26,000-year cycle, the first point of Aries retrogrades about 1° in 70 years, so that the sign Aries today lies in the constellation Pisces. In about 24,000 years, when the retrogression will have completed the entire circuit of 360°, the zodiacal signs and constellations will again coincide.

It is believed that the zodiacal signs originated in Mesopotamia as early as 2000 bc. The Greeks adopted the symbols from the Babylonians and passed them on to the other ancient civilizations. The Egyptians assigned other names and symbols to the zodiacal divisions. The Chinese also adopted the 12-fold division, but called the signs rat, ox, tiger, hare, dragon, serpent, horse, sheep, monkey, hen, dog, and pig. Independently, the Aztec people devised a similar system.

See also Astrology.

Celestial Sphere

Celestial Sphere, imaginary sphere of the heavens, with the earth at its center. The sphere forms the basis for the coordinate systems used in assigning positions to objects observed in the sky. It is also used for designating time intervals and for navigation.

The equatorial system of coordinates establishes a grid in the celestial sphere that is based on the earth's equator and poles, projected outwards to intersect with the sphere. Because the earth is moving around the sun, the appearance of celestial objects such as stars changes on the sphere from day to day. Thus one particular moment of the year is assigned as the time when the celestial grid is established. This moment is the vernal equinox, when the sun's disk passes directly above the equator and marks the beginning of spring in the northern hemisphere (see Ecliptic). Celestial latitude is called declination, and celestial longitude is called right ascension in this equatorial system. Right ascension is measured from the zero-hour circle established by the vernal equinox. The yearly path traced by the sun across the celestial sphere forms a great circle, called the ecliptic, on the sphere. A coordinate system that establishes a grid on the celestial sphere using the ecliptic rather than the equator as its reference plane is also sometimes employed, as are other systems.

The apparent daily movement of the celestial sphere about the earth, caused by the earth's own rotation, is actually about four minutes shorter than the mean solar day.

Day

Day, in chronology, period of time required for one rotation of a celestial body, especially the earth, on its axis. This period is shorter or longer depending on whether the sun or another star is used as a reference point; thus, the sidereal day—the time it takes for the earth to rotate once relative to a star not the sun—is 4 min shorter than the mean solar day. The solar day, measured by the interval between meridian passages of the sun, varies in length because of the variation in speed of the earth in its orbit. In consequence, the length of the solar day is averaged over the period of a year, and the mean solar day thus obtained is used for all civil and many astronomical purposes. Each type of day is divided into exactly 24 hr that vary in length proportionately to the respective type of day.

The civil day now begins at midnight, local time. In ancient times, the Babylonian day began with sunrise and with sunset among the Athenians and Jews. The day is still often regarded as starting with sunset in ecclesiastical (particularly Jewish ecclesiastical) usage; until recently, the astronomical day started at noon, and the Julian day still starts at noon.

In common usage day, as distinct from night, is the period of natural light between dawn and dusk. The period of daylight, most nearly constant near the equator, varies with the latitude and the season, reaching a maximum of 24 hr in the polar zones in summer, a phenomenon known as the midnight sun.

Ecliptic

Ecliptic, in astronomy, the apparent great-circle annual path of the sun in the celestial sphere, as seen from the earth. It is so named because eclipses occur only when the moon is on or near this path. The plane of this path, called the plane of the ecliptic, intersects the celestial equator (the projection of the earth's equator on the celestial sphere) at an angle of about 23°27’. This angle is known as the obliquity of the ecliptic and is approximately constant over a period of millions of years, although at present it is decreasing at the rate of 48 seconds of arc in each century and will decrease for several millenniums until it reaches 22°54’, after which it will again increase.

The two points at which the ecliptic intersects the celestial equator are called nodes or equinoxes. The sun is at the vernal equinox about March 21 and at the autumnal equinox about September 23. Halfway on the ecliptic between the equinoxes are the summer and winter solstices. The sun arrives at these points about June 21 and December 22, respectively. The names of the four points correspond to the seasons beginning in the northern hemisphere on these dates. The equinoxes do not occur at the same points of the ecliptic every year, for the plane of the ecliptic and the plane of the equator revolve in opposite directions. The two planes make a complete revolution with respect to each other once every 25,868 years. The movement of the equinoxes along the ecliptic is called the precession of the equinoxes. A correction for precession must be applied to celestial charts to find the true position of the stars at any given time.

The ecliptic is also used in astronomy as the fundamental circle for a system of coordinates called the ecliptic system. Celestial latitude is measured north and south of the ecliptic; celestial longitude is measured east and west of the vernal equinox.

In astrology, the ecliptic is divided into 12 arcs of 30° each, called the signs of the Zodiac. These signs, or “houses of heaven,” are named after the constellations through which the ecliptic passes.

See also Eclipse.

Eclipse

Eclipse, in astronomy, the obscuring of one celestial body by another, particularly that of the sun or a planetary satellite. Two kinds of eclipses involve the earth: those of the moon, or lunar eclipses; and those of the sun, or solar eclipses . A lunar eclipse occurs when the earth is between the sun and the moon and its shadow darkens the moon. A solar eclipse occurs when the moon is between the sun and the earth and its shadow moves across the face of the earth. Transits and occultations are similar astronomical phenomena but are not as spectacular as eclipses because of the small size of these bodies as seen from earth. (see Transit).

Topics:

• Lunar Eclipses
• Solar Eclipses
• Frequency of Eclipses
• Observation of Eclipses

Eclipse

Eclipse->>LUNAR ECLIPSES

The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra. The approximate mean length of the umbra is 1,379,200 km (857,000 mi); at a distance of 384,600 km (239,000 mi), the mean distance of the moon from the earth, it has a diameter of about 9170 km (about 5700 mi).

A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the center, it is obscured for about 2 hours. If it does not pass through the center, the period of totality is less and may last for only an instant if the moon travels through the very edge of the umbra.

A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial eclipse can range from near totality, when most of the moon is obscured, to a slight or minor eclipse, when only a small portion of the earth’s shadow is seen on the passing moon. Historically, the view of the earth’s circular shadow advancing across the face of the moon was the first indication of the shape of the earth.

Before the moon enters the umbra in either total or partial eclipse, it is within the penumbra and the surface becomes visibly darker. The portion that enters the umbra seems almost black, but during a total eclipse, the lunar disk is not completely dark; it is faintly illuminated with a red light refracted by the earth’s atmosphere, which filters out the blue rays. Occasionally a lunar eclipse occurs when the earth is covered with a heavy layer of clouds that prevent light refraction; the surface of the moon is invisible during totality.

Black Hole

Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black.

PROPERTIES

The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity—that is, a dimensionless object of infinite density.

Earth

Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans.

Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.

Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.

Topics:

• Earth, The Solar System, and the Galaxy
• Earth's Atmosphere
• Earth's Surface
• Earth's Interior
• Earth's Future

Earth, The Solar System, and the Galaxy

Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.

Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.

Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is somewhat less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune.

Earth has one natural satellite, the Moon. The Moon orbits the Earth, completing one revolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than 4 billion years ago when a large meteorite or small planet struck the Earth.
As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min 4.1 sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s center.

Earth’s axis of rotation is inclined (tilted) 23.5° relative to its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur in between these orientations.

Solar System: The Sun and the Solar Wind

The Sun is a typical star of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun’s hot, dense interior (see Nuclear Energy). Although this nuclear fusion is transforming 600 million metric tons of hydrogen each second, the Sun is so massive (2 × 1030 kg, or 4.4 × 1030 lb) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth.

For all the Sun’s steadiness, it is an extremely active star. On its surface, dark sunspots bounded by intense magnetic fields come and go in 11-year cycles and sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth. A continuous stream of protons, electrons, and ions also leaves the Sun and moves out through the solar system. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the Moon’s surface by piloted United States Apollo spacecraft (see Space Exploration; Apollo program).

The Sun’s activity also influences the heliopause, a region of space that astronomers believe marks the boundary between the solar system and interstellar space. The heliopause is a dynamic region that expands and contracts due to the constantly changing speed and pressure of the solar wind. In November 2003 a team of astronomers reported that the Voyager 1 spacecraft appeared to have encountered the outskirts of the heliopause at about 86 AU from the Sun. They based their report on data that indicated the solar wind had slowed from 1.1 million km/h (700,000 mph) to 160,000 km/h (100,000 mph). This finding is consistent with the theory that when the solar wind meets interstellar space at a turbulent zone known as the termination shock boundary, it will slow abruptly. However, another team of astronomers disputed the finding, saying that the spacecraft had neared but had not yet reached the heliopause.

Solar System: The Major Planets

Nine major planets are currently known. They are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto does not belong to either group, and there is an ongoing debate as to whether Pluto should be categorized as a major planet.

Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477°C (about 890°F).

Earth is the only planet known to have abundant liquid water and life. However, in 2004 astronomers with the National Aeronautics and Space Administration’s Mars Exploration Rover mission confirmed that Mars once had liquid water on its surface. Scientists had previously concluded that liquid water once existed on Mars due to the numerous surface features on the planet that resemble water erosion found on Earth. Mars’s carbon dioxide atmosphere is now so thin that the planet is dry and cold, with polar caps of frozen water and solid carbon dioxide, or dry ice. However, small jets of subcrustal water may still erupt on the surface in some places.

Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-colored clouds, and its immense magnetosphere, rings, and satellites make it a planetary system unto itself. One of Jupiter’s largest moons, Io, has volcanoes that produce the hottest surface temperatures in the solar system. At least four of Jupiter’s moons have atmospheres, and at least three show evidence that they contain liquid or partially frozen water. Jupiter’s moon Europa may have a global ocean of liquid water beneath its icy crust.

Saturn rivals Jupiter, with a much more intricate ring structure and a similar number of satellites. One of Saturn’s moons, Titan, has an atmosphere thicker than that of any other satellite in the solar system. Uranus and Neptune are deficient in hydrogen compared with Jupiter and Saturn; Uranus, also ringed, has the distinction of rotating at 98° to the plane of its orbit. Pluto seems similar to the larger, icy satellites of Jupiter or Saturn. Pluto is so distant from the Sun and so cold that methane freezes on its surface. See also Planetary Science.

Solar System: Other Orbiting Bodies

The asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Ceres, which has a diameter of 1,003 km (623 mi), to microscopic grains. Some asteroids are perturbed, or pulled by forces other than their attraction to the Sun, into eccentric orbits that can bring them closer to the Sun. If the orbits of such bodies intersect that of Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth’s Moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On Earth that record has eroded away, except for a few recently found impact craters.

Some meteors and interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases typically 5 to 10 km (about 3 to 6 mi) in diameter. Comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the Sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter’s strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley’s Comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the fragments was released through massive explosions, some resulting in fireballs larger than Earth.

Comets circle the Sun in two main groups, within the Kuiper Belt or within the Oort cloud. The Kuiper Belt is a ring of debris that orbits the Sun beyond the planet Neptune. Many of the comets with periods of less than 500 years come from the Kuiper Belt. In 2002 astronomers discovered a planetoid in the Kuiper Belt, and they named it Quaoar.

Many of the objects that do not fall into the asteroid belts, the Kuiper Belt, or the Oort cloud may be comets that will never make it back to the Sun. The surfaces of the icy satellites of the outer planets are scarred by impacts from such bodies. The asteroid-like object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross the path of Earth’s orbit may be the rocky remains of burned-out comets. Chiron and similar objects called the Centaurs probably escaped from the Kuiper Belt and were drawn into their irregular orbits by the gravitational pull of the giant outer planets, Jupiter, Saturn, Neptune, and Uranus.

The Sun was also found to be encircled by rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light, a faint glow that appears in the east before dawn and in the west after dusk. Another ring, lying only two solar widths away from the Sun, was discovered in 1983.

Solar System: Movements of the Planets and Their Satellites

If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury and Pluto have obviously inclined orbits. Pluto’s orbit is so elliptical that it is sometimes closer than Neptune to the Sun.

The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found. Jupiter, Saturn, and Neptune each have at least one satellite that moves around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun. (Some satellites of Saturn have done the same with smaller bodies.) The comets exhibit a roughly spherical distribution of orbits around the Sun.

Within this maze of motions, some remarkable patterns exist: Mercury rotates on its axis three times for every two revolutions about the Sun; no asteroids exist with periods (intervals of time needed to complete one revolution) 1/2, 1/3, …, 1/n (where n is an integer) the period of Jupiter; the three inner Galilean satellites of Jupiter have periods in the ratio 4:2:1. These and other examples demonstrate the subtle balance of forces that is established in a gravitational system composed of many bodies.