Moon

Moon

I

INTRODUCTION

Moon, name given to the only natural satellite of Earth. The Moon is the second brightest object in Earth’s sky, after the Sun, and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons.

Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 12 percent of the light that falls on it. This reflectivity, called albedo, of 0.12 is similar to that of coal dust.

The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Moon’s mass is only 1.2 percent of Earth’s mass. The average density of the Moon is only three-fifths that of Earth, and gravity at the lunar surface is only one-sixth as strong as gravity at sea level on Earth. The Moon moves in an elliptical (oval-shaped) orbit around Earth at an average distance of 384,403 km (238,857 mi) and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in 27 days 7 hours 43 minutes. For the Moon to go from one phase to the next similar phase—as seen from Earth—requires 29 days 12 hours 44 minutes. This period is called a lunar month. The Moon rotates once on its axis in the same period of time that it circles Earth, accounting for the fact that virtually the same portion of the Moon (the “near side”) is always turned toward Earth.

II

THE MOON SEEN FROM EARTH

As the Moon orbits Earth in a counterclockwise direction, Earth itself rotates counterclockwise (from west to east) on its axis and revolves around the Sun in a counterclockwise orbit. All of these motions combined determine when and how the Moon appears in the sky to an observer on Earth. Seen from a single spot on Earth, the Moon rises about 50 minutes later every day. Since the Moon has moved 13.8 degrees further in its orbit in 24 hours, the Earth has to turn an extra 13.8 degrees on its axis for the Moon to rise above the horizon again.

The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half of Earth has day while the other half has night. Thus, there is no permanent “dark side of the Moon,” which is sometimes confused with the Moon’s far side—the side that always faces away from Earth. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the near side is completely in shadow. About a week after a new moon, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted near side; a week afterward, in its last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than Earth; it is new when it is closer. When it is more than half illuminated, it is said to be in gibbous phase. When it is less than half illuminated, it is said to be in crescent phase. The Moon is said to be waning as it progresses from full to new, and to be waxing as it proceeds from new to full.

The Moon is in the sky about 12 hours a day. At new moon it is in the sky during daylight hours, rising just after dawn. At full moon it is visible throughout the night, rising at sunset. The phases of the Moon match its position in the sky. New moon is noticeable when the Moon is close to the western horizon at sunset. The full moon occurs when the Moon is above the eastern horizon at sunset about 14 days later. The dark phase of the Moon occurs when the Moon is in the daytime sky with its shaded night side facing Earth. Its unseen presence can be revealed in a spectacular way if the dark Moon passes directly in front of the Sun. When this happens, the view of the Sun is blocked and the Moon’s shadow falls on a small region of the surface of the Earth, an event called a solar eclipse.

By a cosmic coincidence, the apparent sizes of the disk of the Moon and the disk of the Sun are approximately the same (within about 0.5 of a degree) when seen from Earth. If the Moon’s orbit lay exactly in the plane of Earth’s orbit around the Sun, a solar eclipse would occur somewhere on Earth every month at new moon. However, the Moon’s orbit is tilted 5.1 degrees with respect to the plane of Earth’s orbit around the Sun. As a result, solar eclipses occur only about 2 to 5 times a year. Partial eclipses, when the Moon only partially covers the disk of the Sun, happen more often than total eclipses.

Another type of eclipse results when Earth comes directly between the Sun and the Moon. Lunar eclipses happen at full moon about twice a year and are visible over large areas of Earth. The round shadow of Earth passes over the Moon, giving it a red or copper hue from sunlight filtered through Earth’s atmosphere.

At any one time, an observer on Earth can see only 50 percent of the Moon’s entire surface. However, an additional 9 percent can be seen from time to time around the edges because the viewing angle from Earth changes slightly as the Moon moves through its elliptical orbit. This slight relative motion is called libration.

III

SURFACE OF THE MOON

Early observers of the Moon believed that the dark regions on its face were oceans, giving rise to their name maria (Latin for “seas”). This term is still used today although these regions are now known to be completely dry. The brighter regions were held to be continents. Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge.

The Moon has no movement of wind or water to alter its surface, yet it was geologically active in the past and is still not totally unchanging. Craters cover the surface, and meteors continue to create new craters. Micrometeorites also slowly erode surface features and alter the lunar soil. Billions of years ago volcanic eruptions sculpted large areas of the surface. Volcanic features such as maria, domes (low, rounded, circular hills), and rilles (channels or grooves) are still discernable. Small amounts of gas from deep in the Moon may still reach the surface. Scientists have also recently discovered possible evidence of ice in permanently shadowed areas of the surface. Such ice could have come from comet impacts.

A

Craters

The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full Moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface.

The biggest of the Moon’s craters were created by the impacts of large remnants from the formation of the planets billions of years ago when the young solar system still contained many such remnants. Astronomers, however, have directly observed meteorites forming small craters on the Moon’s surface. Seismometers operating on the lunar surface have also recorded signals indicating between 70 and 150 meteorite impacts per year, with projectile masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although neither as often nor as violently as in the distant past.

B

Volcanic Features

Maria, domes, rilles, and a few craters display indisputable characteristics of volcanic origin. Maria are plains of dark-colored rock that cover approximately 40 percent of the Moon's visible hemisphere. The maria formed when molten rock erupted onto the surface and solidified between 3.16 billion and 3.96 billion years ago. This rock resembles terrestrial basalt, a volcanic rock type widely distributed on Earth, but the rock that formed the maria has a higher iron content and contains unusually large amounts of titanium. The largest of the maria is Oceanus Procellarum, an oval-shaped plain on the near side of the Moon 2,500 km by 1,500 km wide.

Photographs of the side of the Moon not visible from Earth have revealed a startling fact: The far side generally lacks the maria that are so conspicuous a feature of the visible side. This probably reflects the fact that the Moon’s crust is thicker on the far side than on the near side, and therefore the lavas that form the maria were more easily erupted through the thinner crust of the near side.

Rilles are of two basic types: sinuous and straight. Sinuous rilles are meandering channels that are probably lava drainage channels or collapsed lava tubes formed by large lava flows. Straight rilles are large shallow troughs caused by movement of the Moon’s crust; they may be up to a thousand kilometers long and several kilometers wide.

Domes are small rounded features that range from 8 to 16 km (5 to 10 mi) in diameter and from 60 to 90 m (200 to 300 ft) in height. Domes, thought to be small inactive volcanoes, often contain a small rimless pit on their tops.

Magnetic and other measurements indicate a current temperature at the Moon’s core as high as 1600°C (2900°F), above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid. However, no recent eruptions of liquid rock have been observed and the Moon evidently has had no volcanic activity on its surface over the last 1 billion years. At most, trapped gas from deep in the Moon may still reach the surface in some places.

Astronomers reported possible evidence of “out-gassing” on the surface of the Moon in the last 1 to 10 million years in a paper published in 2006. The unusually bright soil around a feature 3 km (2 mi) wide on the Moon’s equator indicates some process has turned over fresh regolith that has not had enough time to be “weathered” by solar wind and micrometeorites. Called Ina, the feature was first photographed from Apollo spacecraft orbiting the Moon in the 1970s, and was later examined by the Clementine probe. Gases from inside the Moon may have erupted on the surface, lifting and exposing fresh lunar soil. Scientists do not know the exact source and nature of the gases. At least three other lunar features that look similar to Ina have been identified. They may have been formed by bursts of gas, as well.

C

Ice

Temperatures on most of the Moon’s surface are too extreme for water or ice to exist, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn. Temperatures in permanently shadowed areas near the lunar poles, however, may consistently be as low as -220°C (-364°F). Comets and micrometeoroids that strike the Moon release gases that contain water. The gases would form an extremely thin atmosphere that would then migrate to the coldest regions of the poles and condense out, forming ice that combines with the lunar soil. In 1996 a team working with data gathered by the Clementine spacecraft announced that frozen water may exist in one of these shadowed areas near the Moon’s south pole. Clementine was a joint venture by the Department of Defense and the National Aeronautics and Space Administration (NASA). The spacecraft’s radar showed what may be an 8,000 sq km (3,000 sq mi) area covered with a mixture of dirt and ice crystals. Clementine was launched in 1994 and gathered data for four months.

NASA launched the Lunar Prospector spacecraft toward the Moon in 1998. Prospector returned data that appeared to confirm the Clementine discovery and suggested that a significant amount of water exists in the dark areas near the lunar poles in the form of ice crystals mixed with soil. The evidence was indirect, however, and consisted of finding elevated levels of hydrogen, a component of water, around the poles. Estimates of the possible amount of water on the Moon varied widely, from 10 million to 6 billion metric tons.

In 1999, at the end of the Lunar Prospector’s mission, scientists programmed the spacecraft to crash at a specific spot likely to contain water, hoping that the debris that rose with the impact would contain detectable water vapor. Although no water was detected after the crash, scientists could not conclude that no water existed on the Moon. They acknowledged several other possible explanations for the result: The spacecraft might have missed its target area, the telescopes used to observe the crash might have been aimed incorrectly, or the magnitude of the impact created by the Lunar Prospector spacecraft may have been insufficient to generate a large cloud of water vapor.

In 2003 researchers used the giant Arecibo Observatory radio telescope to bounce radar signals off the surface of craters at the Moon’s poles. The returned radar signal indicated that large, thick layers of ice were not present. The findings failed to rule out the existence of smaller amounts of ice at the lunar poles preserved in thin layers or as scattered ice crystals mixed with dust. The Arecibo Observatory conducted a higher resolution radar study of the lunar south pole in 2006 and found that similar radar signals came from both sunlit and shaded areas. The issue of ice at the lunar poles was not resolved, however. NASA’s Lunar Reconnaissance Orbiter (LRO), scheduled for launch in 2008, will carry a special satellite called LCROSS (Lunar CRater Observation and Sensing Satellite) that will look for evidence of water in the debris plume when the LRO’s booster stage crashes into Shackleton Crater at the south pole.

IV

ORIGIN OF THE MOON

Measuring the ages of lunar rocks has revealed that the Moon is about 4.6 billion years old, or about the same age as Earth and probably the rest of the solar system. Before the modern age of space exploration, scientists had three major models for the origin of the Moon. The fission from Earth model proposed that the young, molten Earth rotated so fast that it flung off some material that became the Moon. The formation in Earth orbit model claimed that the Moon formed independently, but close enough to Earth to orbit the planet. The formation far from Earth model proposed that the Moon formed independently in orbit around the Sun but was subsequently captured by Earth’s gravity when it passed close to the planet. None of these three models, however, is entirely consistent with current knowledge of the Moon. In 1975, having studied Moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation: a giant, planetary impact.

The giant impact model proposes that early in Earth’s history, well over 4 billion years ago, Earth was struck by a large planet-sized body sometimes referred to as Theia. Early estimates for the size of this object were comparable to the size of Mars, but other research suggests that the object may have been more massive and that it struck Earth at a glancing angle. The catastrophic impact blasted portions of Earth and the impacting body into Earth’s orbit, where debris from the impact eventually coalesced to form the Moon. After years of research on lunar rocks during the 1970s and 1980s, this model became the most widely accepted one for the Moon’s origin.

The giant impact model seems to account for most of the available evidence: the similarity in composition between Earth and Moon indicated by analysis of lunar samples, the near-complete global melting of the Moon (and possibly Earth) in the distant past, and the simple fact that the other models are all inadequate to one degree or another. Research continues on the ramifications of such a violent lunar origin to the early history of Earth and the other planets.

Similar giant impacts may have affected the planets Mercury and Venus, but without forming moons–at least none that have survived. Mercury may have had most of its outer crust blasted away, leaving a dense iron core. Venus’s slow backward (retrograde) rotation may have been caused by one or more collisions with planet-sized bodies.

V

MAGNETIC PROPERTIES OF THE MOON

The Moon has no global magnetic field as does Earth. Some lunar rocks are weakly magnetic, indicating that they solidified in the presence of a magnetic field. The Moon has small, local magnetic fields that seem to be strongest in areas that are on opposite hemispheres from large basins. The origin of these local magnetic fields is unknown. Some scientists speculate that the magnetic fields were induced by the extreme shock pressures associated with the large asteroid collisions that created the basins. Others believe that the Moon originally had a global magnetic field generated by the movement of liquid metal in the core as on Earth. This global field shut down for some reason and only remnants of it exist in certain places on the lunar surface, preserved in material ejected by the asteroid collisions. The “fossil” magnetism found in some lunar rocks supports the former global field model, whereas the regional distribution of the magnetic surface anomalies tends to support the local field model. Regions of strong magnetic fields repel the charged particles that stream from the Sun in the solar wind. Scientists believe that interaction with the solar wind darkens the Moon, and that some lighter swirl-shaped regions of the Moon are protected by local magnetic fields.

VI

GRAVITATIONAL INFLUENCE OF THE MOON

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. The Moon, being much nearer to the Earth than the Sun, is the principal cause of tides. Because the force of gravity decreases with distance, the Moon exerts a stronger gravitational pull on the side of the Earth that is closer to it and a weaker pull on the side farther from it. The Earth does not respond to this variation in strength because the planet is rigid—instead, it moves in response to the average of the Moon’s gravitational attraction. The world’s oceans, however, are liquid and can flow in response to the variation in the Moon’s pull. On the side of the Earth facing the Moon, the Moon’s stronger pull makes water flow toward it, causing a dome of water to rise on the Earth’s surface directly below the Moon. On the side of the Earth facing away from the Moon, the Moon’s pull on the oceans is weakest. The water’s inertia, or its tendency to keep traveling in the same direction, makes it want to fly off the Earth instead of rotate with the planet. The Moon’s weaker pull does not compensate as much for the water’s inertia on the far side, so another dome of water rises on this side of the Earth. The dome of water directly beneath the Moon is called direct tide, and the dome of water on the opposite side of the Earth is called opposite tide.

As the Earth rotates throughout the day, the domes of water remain aligned with the Moon and travel around the globe. When a dome of water passes a place on the Earth, that place experiences a rise in the level of the ocean water, known as high tide or high water. Between successive high tides the water level drops. The lowest water level reached between successive high tides is known as low tide or low water. Low and high tides alternate in a continuous cycle. The variations that naturally occur in the level between successive high tide and low tide are referred to as the range of tide. At most shores throughout the world, two high tides and two low tides occur every lunar day, the average length of a lunar day being 24 hours, 50 minutes, and 28 seconds. One of these high tides is caused by the direct-tide dome and the other by the opposite-tide dome. Two successive high tides or low tides are generally of about the same height.

VII

LUNAR EXPLORATION

Throughout the 19th and 20th centuries, visual exploration with powerful telescopes yielded fairly comprehensive knowledge of the geography of the visible side of the Moon. The hitherto unseen far side of the Moon was first revealed to the world in October 1959 through photographs made by the Soviet Luna 3 spacecraft. These photographs showed that the far side of the Moon is similar to the near side except for the absence of large maria. Craters are now known to cover the entire Moon. In 1964 and 1966 photographs from U.S. spacecraft—Ranger 7 through 9 and Lunar Orbiter 1 through 5—further supported these conclusions. The entire Moon has about 3 trillion craters larger than 1 m (3 ft) in diameter.

The successful landings of the robotic U.S. Surveyor series spacecraft and the USSR Luna series in the 1960s, and then the manned landings on the lunar surface as part of the U.S. Apollo program, made direct measurement of the physical and chemical properties of the lunar surface a reality (see Space Exploration). The Apollo astronauts collected rocks, took thousands of photographs, and set up instruments on the Moon that radioed information back to Earth even after the astronauts departed. These instruments measured temperature and gas pressure at the lunar surface; heat flow from the Moon’s interior; molecules and ions of hot gases, called the solar wind, that stream out from the atmosphere of the Sun; the Moon’s magnetic field and gravity; seismic vibrations of the lunar surface caused by landslides, meteorite impacts, and so-called moonquakes; and the precise distance between Earth and the Moon.

All six manned landings on the Moon—Apollo 11, 12, 14, 15, 16, and 17—returned samples of rock and soil to Earth. These samples weighed a total of 384 kg (847 lb). The astronauts explored increasingly wider areas on the Moon with each successive flight, culminating with the 35 km (22 mi) explored using a lunar roving vehicle by the Apollo 17 crew. This final mission included the only geologist ever to walk on the Moon, Harrison (Jack) Schmitt. Analysis of the data and rocks obtained by the lunar missions continues.

In 1994, the joint Defense Department/NASA spacecraft Clementine orbited the Moon for 71 days, mapping the color and precise altitude of the lunar surface. From Clementine data, astronomers obtained their first global look at the topography and mineralogy of the Moon, finding that the Moon’s crust is indeed made of a low-iron, low-density rock called anorthosite and mapping the large, ancient basins that make up the structural framework of the Moon. Clementine also discovered possible evidence of ice on the Moon in the permanently dark areas near the south pole.

NASA sent a spacecraft of its own, an orbiter called Lunar Prospector, to the Moon in 1998. Lunar Prospector orbited around the Moon’s north and south poles and returned data until July 1999. The spacecraft mapped the gravitational field of the Moon, determined the distribution of radioactive elements in its crust, and found additional evidence that could indicate the presence of ice at the lunar poles. Scientists used the spacecraft right up to its final moments. They ended Prospector’s mission by programming it to crash into the Moon’s surface and then observed the cloud of debris that rose from the impact.

In 2003 the European Space Agency (ESA) launched it first lunar probe, called SMART-1 (Small Missions for Advanced Research and Technology). The solar-powered orbiter used an innovative form of ion propulsion. It carried a number of instruments to study the chemical elements that make up the Moon’s surface. At the end of its mission in 2006, the probe was deliberately crashed into the lunar surface. Earth-based telescopes studied the composition of the debris thrown up by the impact.

An ambitious series of international Moon probes have been planned. China’s first Moon probe, the lunar orbiter Chang’e No. 1 (pronounced CHAHNG-UH), was scheduled to be launched in 2007. It is named for the fairy maiden Chang’e, who traveled to the Moon in Chinese mythology. Based on the design of the Dongfanghong-3 communications satellite, the Chinese probe will send back photos and data about the Moon. The Chinese are working with Russian experts to build a robot Moon lander for launch by 2010.

India’s first Moon mission, called Chandrayaan-1 (pronounced CHUN-dry-ahn, meaning “Moon craft” in Sanskrit), is scheduled for launch in 2008. The orbiting probe will carry European scientific instruments and two instruments provided by NASA, the M³ (Moon Mineralogy Mapper) and the Mini SAR (Mini Synthetic Aperture Radar). M³ (pronounced “em cube”) is a highly advanced imaging spectrometer designed to map the entire lunar surface to study its mineral composition. Mini SAR is an imaging radar device designed to map the lunar poles and look for water ice. It will see into permanently shadowed areas at the poles and can distinguish the radar signature of ice from surface roughness.

Japan planned a 2007 launch date for SELENE (SELenological and ENgineering Explorer). The orbiter will map the Moon and deploy a small satellite to study the gravitational field on the far side of the Moon. The orbiter’s propulsion unit will later separate and land on the lunar surface.

NASA has proposed a series of unmanned Moon probes over the next decade, leading to a manned landing by 2020. The Lunar Reconnaissance Orbiter is slated for launch in 2008, carrying six different instruments. Also on board will be a small, separate satellite called LCROSS (Lunar CRater Observation and Sensing Satellite). The main probe will map small areas of the Moon’s surface in high resolution. The upper stage of the probe’s booster rocket will impact Shackleton Crater at the south pole, sending up a plume of debris that could reveal water ice. The LCROSS satellite will study the plume for signs of ice then crash into the crater itself, creating a second impact for observers on Earth. Possible future missions may include sample returns from the lunar surface.

The first step toward a return to manned exploration of the Moon came in 2006 with NASA’s official announcement of the design and contractor for the Orion space capsule, part of the Constellation program of manned space flights. Plans call for a lunar landing in 2020, with an unmanned cargo mission in 2019. A crew of four will reach the Moon in an Orion capsule accompanied by a lunar lander. The four astronauts will land on the surface and spend about a week on the Moon. By 2024 a permanent Moon base may be established, probably near the lunar south pole. China has also announced plans for manned flights to the Moon for sometime after 2020.

Although much has been learned about the Moon in the past few decades, much still remains mysterious. Understanding the Moon and its history is important for two reasons. First, the Moon is a natural laboratory to study the geological processes—meteorite impacts, volcanism, and large-scale movements of the crust—that have shaped all of the rocky planets. Second, the Moon’s ancient surface retains a record of events in this part of the solar system that has been erased from the much more active, dynamic surface of Earth. The impact record, which has been almost entirely erased on Earth, is especially clear on the Moon, and may contain important clues to the history of life on Earth. Thus, the Moon serves as a touchstone, allowing us to better comprehend the complex stories of all the planets in our solar system.

Reviewed By:
Paul D. Spudis

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Stellar Evolution

Black Hole

Black Hole

I

INTRODUCTION

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.

II

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.

III

FORMATION

Black holes are thought to form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their energy production is no longer available to resist contraction of the core to ever-higher densities. Two new types of pressure, electron and neutron pressure, arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the star is more than about five times as massive as the Sun, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.

In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material) circling the center of the galaxy M87 with an acceleration that indicated the presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black holes in the centers of dozens of galaxies and had found that the masses of the black holes were correlated with the masses of the parent galaxies. More massive galaxies tend to have more massive black holes at their centers. Learning more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.

The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. For black holes of sufficiently small mass it is possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping (see X Ray: Pair Production). The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.

The American astronomer Kip Thorne of California Institute of Technology in Pasadena, California, has evaluated the chance that black holes can collapse to form 'wormholes,' connections between otherwise distant parts of the universe. He concludes that an unknown form of 'exotic matter' would be necessary for such wormholes to survive.

Reviewed By:
Jay M. Pasachoff

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Supenova

Supernova

I

INTRODUCTION

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.

II

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 (see Nuclear Energy: Nuclear Fusion). 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 (see Spectroscopy).

III

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.

IV

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 (see Magellanic Clouds), 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.

Contributed By:
Dennis L. Mammana

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Characteristics of the Planets

Characteristics of the Planets

Characteristic

Mercury

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto*

Equatorial radius (Earth radii†)	0.3825	0.9488	1	0.5325	11.21	9.449	4.007	3.883	0.1874
Equatorial inclination (degrees)	0.01	2.64	23.5	25.2	3.13	26.7	82.2	28.3	57.4
Mass (Earth masses‡)	0.0553	0.8150	1	0.1074	317.8	95.16	14.54	17.15	0.0023
Average density (g/cm3)	5.4	5.2	5.5	3.9	1.3	0.69	1.3	1.6	1.8
Rotational period (days)	58.6	-240	1	1.03	0.414	0.444	-0.718	0.671	-6.4
Orbital period (years)	0.2408	0.6152	1	1.881	11.86	29.46	84.01	164.8	247.9
Average distance from the Sun (AUs)	0.3871	0.7233	1	1.524	5.203	9.59	19.10	30	39.30
Orbital eccentricity (ratio)	0.206	0.00674	0.0167	0.0935	0.0489	0.0576	0.0497	0.00995	0.248
Orbital inclination (degrees)	7	3.39	0.0003	1.85	1.30	2.49	0.772	1.77	17.2
Moons (number)	0	0	1	2	63	56	27	13	3
*Reclassified as a dwarf planet by the International Astronomical Union in 2006 †Planet's radius expressed as a multiple of Earth's radius (6,378 km) ‡Planet's mass expressed as a multiple of Earth's mass (5.974×1024 kg)

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Interstellar Matter

Interstellar Matter

I

INTRODUCTION

Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.

II

DISTRIBUTION OF INTERSTELLAR MATTER

Most astronomers believe that the Milky Way Galaxy condensed out of a huge cloud of gas. Most of the interstellar gas that now exists is presumably left over from the formation of the galaxy. This gas consisted mainly of the lighter elements hydrogen and helium, but heavier elements joined the gas as the galaxy evolved. These heavier elements, which are the products of various stars, are released into interstellar space as a star evolves or when a star explodes at the end of its life. Nuclear fusion reactions inside massive stars form most of the moderately heavy chemical elements—that is, those elements with atomic weights between that of lithium and iron. Supernova explosions, which mark the end of the lives of massive stars (see Supernova), form the heaviest naturally occurring elements, such as silver and lead. Some of these heaviest elements are also produced inside binary star systems.

Red giant stars—large, bright, relatively cool stars that evolve from stars like the sun—produce interstellar dust particles as their atmospheres expand and cool. Small particles of silica and carbon form in the atmosphere and drift into interstellar space. Atoms collect on the surface of these particles, adding to the particle size and sometimes forming molecules.

A

Nebulas

Many of the most beautiful examples of interstellar matter are in the form of nebulas, regions of gas and dust scattered through the galaxy. Many nebulas emit or reflect light in the visible part of the electromagnetic spectrum, and so are visible when viewed through a telescope. French astronomer Charles Messier cataloged many nebulas in the mid-1700s. Amateur astronomers, as well as professionals, often study nebulas. The high resolution of the Wide Field and Planetary Camera 2 on the Hubble Space Telescope has allowed astronomers to image all types of nebulas much more clearly than before.

Nebulas glow for one of two reasons—reflection or emission. Reflection nebulas are composed mostly of dust. When a reflection nebula occurs near a star or a group of hot stars, light from the stars illuminates the gas and dust to produce wispy, bluish patches. Emission nebulas are composed mostly of ionized hydrogen—hydrogen atoms that have lost their electrons. Energy from nearby stars heats the gas, making it emit a reddish light. A special class of nebulas, known as planetary nebulas, are composed of gas given off by stars like the sun in a late stage of their lifetimes. They are called planetary nebulas because early astronomers noticed that they looked like the faint disks of distant planets.

B

Galactic Halo

Most known nebulas occur in the plane of the galaxy. The galactic halo is a huge sphere that surrounds the plane of the galaxy. Astronomers believe that the halo must contain about 90 percent of the total mass of the galaxy. A fraction of that mass occurs in visible matter—mostly globular star clusters. About half of the halo’s mass is probably made up of small stars that are dark. Such stars have used up their nuclear fuel or are not massive enough to begin nuclear reactions. The rest of the halo’s matter may be interstellar matter in the form of interstellar dust or weakly interacting particles. Weakly interacting particles are nuclear particles that participate only in the weak interaction, one of the four ways matter interacts (the others are the strong interaction, gravity, and electromagnetic interaction).

C

Other Galaxies

Astronomers and cosmologists are actively studying interstellar matter in galaxies other than the Milky Way. Irregular galaxies such as the Large and Small Magellanic Clouds—satellites of our own galaxy—often have much interstellar matter. Spiral galaxies in general also have large amounts of interstellar matter—spiral galaxies that appear edge-on from Earth show dark lanes, or long, narrow dark patches where interstellar dust appears dark in silhouette against radiation from farther away. The Hubble Space Telescope is powerful enough to make detailed images of emission nebulas in nearby spiral galaxies. Studying interstellar matter in other galaxies helps astronomers understand the structure of our own galaxy.

III

EFFECTS OF INTERSTELLAR MATTER

While astronomers can detect some interstellar matter directly, they can also detect interstellar matter by how it changes the radiation that travels through it. Astronomers can then study the interstellar matter by measuring how it changes this radiation. Interstellar matter blocks, reflects, and absorbs radiation. Astronomers detect interstellar matter in a wide variety of ways, using instruments that are sensitive in many parts of the electromagnetic spectrum, from radio waves to X rays. See also Electromagnetic radiation.

A

Interstellar Dust

Interstellar dust produces effects that are quite different from interstellar gas. Dust particles can block all of the light from a source, or they can just block certain wavelengths. Dust may also reflect light that hits it, making light from a single star appear diffuse and cloudy. Dust particles can also emit their own radiation if they absorb enough energy from other sources. Glowing dust particles can also be detected in the infrared, even if they are invisible in the visible light part of the spectrum.

A1

Extinction

Interstellar dust makes up only about 1 percent of interstellar matter. Sometimes, it has sufficient density to absorb enough light that astronomers can see the silhouette of a cloud of dust. At other times, it blocks only a percentage of the light from behind it, a process known by astronomers as extinction. The long, narrow dark lanes in the Milky Way as seen from Earth are examples of extinction. The amount of extinction is different for different wavelengths of light.

A2

Reddening

Starlight that does not get completely absorbed by interstellar dust can still be changed by the dust’s effects. As light passes through less dense patches of interstellar dust, the dust particles scatter some of the light. The dust particles are of a particular size that scatters light of short wavelengths more than light of long wavelengths. In the visible light area of the spectrum, this means that more of the original red light (with a long wavelength) than the original blue light (with a short wavelength) gets through the dust. This makes distant stars appear redder than they actually are. Astronomers call this process reddening. Reddening is not related to the red shift caused by the movement of distant galaxies.

A3

Infrared Radiation

Interstellar dust blocks visible light, but the light and other radiation from stars also warms the dust and makes it emit energy as infrared radiation. Most infrared radiation does not pass through Earth's atmosphere, so astronomers use observatories at high altitude such as the Mauna Kea Observatory in Hawaii or observatories in space to study infrared radiation. See also Infrared Astronomy.

A4

Reflection

Interstellar dust often surrounds newly formed stars. The dust reflects light from the stars to produce a reflection nebula, a fuzzy patch of bluish light. The Pleiades star cluster is an example of a reflection nebula. A cluster of stars surrounded by a cloud of dust makes up the Pleiades. The dust reflects and diffuses the light from the stars into several clouds of light.

B

Interstellar Gas

Gas does not block as much radiation as dust does, but astronomers can detect the presence of interstellar gas because of the radiation it emits and absorbs.

B1

Radio Emissions

Much of the interstellar gas is neutral hydrogen—that is, hydrogen in its lowest energy state (also known as its ground state). An atom of neutral hydrogen has two possible orientations, depending on a property—called spin—of the atom’s single electron. When a hydrogen atom switches between these two versions of the ground state, it gives off a photon, or a packet of electromagnetic radiation, with a wavelength of 21 cm (8.3 in). This wavelength is in the radio area of the electromagnetic spectrum and can be detected with a radio telescope.

Astronomers have used this 21-cm radiation to map the distribution of gas in space. If the gas is moving relative to Earth, the radiation it produces will have a slightly different wavelength. Gas moving away from Earth will seem to produce radiation with a slightly longer wavelength, while gas moving toward the planet will appear to produce slightly shorter wavelengths. This shift in wavelength arises from the relative movement between the source of the radiation and the observer on Earth, and it is called a Doppler shift (see Doppler Effect). Studying the movement of gas enables astronomers to study the galaxy’s structure and see how the galaxy rotates.

The ground-state hydrogen atom is not the only atom or molecule that emits radio waves. Since the 1960s, radio astronomers have discovered about 100 types of molecules in interstellar space that emit radio waves. The intensity of these emissions and their Doppler shifts have contributed to mapping the Milky Way Galaxy and to determining the composition of the Milky Way and other galaxies.

B2

Emission and Absorption Lines

Astronomers can also study interstellar gas by using the fact that atoms emit or absorb radiation (such as light) when they change from one energy level to another. Atoms emit radiation when they drop from one energy level to a lower energy level and absorb radiation when they jump to a higher level. In the case of interstellar gas, the radiation they absorb is provided by the light of nearby stars.

In a cloud of interstellar gas, many atoms will make the same energy level change at the same time, creating enough change in radiation to allow astronomers to study the gas. Astronomers study radiation from interstellar gas by separating the radiation into its different wavelengths, or its spectrum, much as a prism will separate white light into the colors of a rainbow (Spectroscopy). Atoms of a particular element at a particular energy level will only emit or absorb radiation at very specific wavelengths, or colors in the case of visible light. Many atoms making the same energy-level change will show up on the spectrum as bright or dark lines. The bright lines, caused by atoms emitting radiation, are called emission lines. The dark lines, caused by atoms absorbing radiation at a particular wavelength, are called absorption lines. If the cloud of gas is moving relative to Earth, the lines may be shifted by the Doppler effect. Astronomers use the wavelengths at which emission or absorption lines occur to determine the types of atoms present and the speed and direction of the movement of the cloud.

Emission and absorption lines are not limited to radiation in the visible light range. Neutral hydrogen produces emission and absorption lines at some ultraviolet and some radio wavelengths. Molecular hydrogen (H₂, two hydrogen nuclei sharing their electrons) emit and absorb in the ultraviolet part of the spectrum. Some of the gas in the interstellar medium is hot, about 100,000° C (about 200,000° F). Gas this hot emits radiation in the X-ray range. Astronomers can determine the gas’s temperature by analyzing its spectrum. See also X-Ray Astronomy.

Contributed By:
Jay M. Pasachoff

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Galaxy

Galaxy

I

INTRODUCTION

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).

II

EARLY HISTORY OF THE STUDY OF GALAXIES

A Persian astronomer, al-Sufi, is credited with first describing the spiral galaxy seen in the constellation Andromeda. By the middle of the 18th century, only three galaxies had been identified. In 1780, the French astronomer Charles Messier published a list that included 32 galaxies. These galaxies are now identified by their Messier (M) numbers; the Andromeda galaxy, for example, is known among astronomers as M31.

Thousands of galaxies were identified and cataloged by the British astronomers Sir William Herschel, Caroline Herschel, and Sir John Herschel, during the early part of the 19th century. Since 1900 galaxies have been discovered in large numbers by photographic searches. Galaxies at enormous distances from Earth appear so tiny on a photograph that they can hardly be distinguished from stars. The largest known galaxy has about 13 times as many stars as the Milky Way.

In 1912 the American astronomer Vesto M. Slipher, working at the Lowell Observatory in Arizona, discovered that the lines in the spectrum of all galaxies were shifted toward the red spectral region (see Redshift; Spectroscopy). This was interpreted by the American astronomer Edwin Hubble as evidence that all galaxies are moving away from one another and led to the conclusion that the universe is expanding. It is not known if the universe will continue to expand or if it contains sufficient matter to slow down the galaxies gravitationally so they will eventually begin contracting to the point from which they arose. See Cosmology.

III

CLASSIFICATION OF GALAXIES

When viewed or photographed with a large telescope, only the nearest galaxies exhibit individual stars. For most galaxies, only the combined light of all the stars is detected. Galaxies exhibit a variety of forms. Some have an overall globular shape, with a bright nucleus. Such galaxies, called ellipticals, contain a population of old stars, usually with little apparent gas or dust, and few newly formed stars. Elliptical galaxies come in a vast range of sizes, from giant to dwarf.

In contrast, spiral galaxies are flattened disk systems containing not only some old stars but also large populations of young stars, much gas and dust, and molecular clouds that are the birthplace of stars (see Star). Often the regions containing bright young stars and gas clouds are arranged in long spiral arms that can be observed to wind around the galaxy. Generally a halo of faint older stars surrounds the disk; a smaller nuclear bulge often exists, emitting two jets of energetic matter in opposite directions.

Other disklike galaxies, with no overall spiral form, are classified as irregulars. These galaxies also have large amounts of gas, dust, and young stars, but no arrangement of a spiral form. They are usually located near larger galaxies, and their appearance is probably the result of a tidal encounter with the more massive galaxy. Some extremely peculiar galaxies are located in close groups of two or three, and their tidal interactions have caused distortions of spiral arms, producing warped disks and long streamer tails. Ring galaxies, for example, form when a small galaxy collides with the center of a spiral galaxy. An intense ring of stars forms at the outer edges of the new, combined galaxy. The Hubble Space Telescope (HST) has revealed many more ring galaxies than astronomers expected, suggesting that galactic collisions may be common.

Quasars are objects that appear stellar or almost stellar, but their enormous redshifts identify them as objects at very large distances (see Quasar; Radio Astronomy). They are probably closely related to radio galaxies and to BL Lacertae objects. The Hubble Space Telescope (HST) completed a survey of nearby galaxies in 1996 that revealed that all large galaxies may be homes to quasars early in the galaxy’s life. The HST survey showed that most of the galaxies contain massive black holes, which may be the next stage in galactic evolution.

IV

DETERMINATION OF EXTRAGALACTIC DISTANCES

In viewing a galaxy with a telescope, inferring its distance is impossible, for it may be a gigantic galaxy at a large distance or a smaller one closer to Earth. Astronomers estimate distances by comparing the brightness or sizes of objects in the unknown galaxy with those in Earth’s galaxy. The brightest stars, supernovas, star clusters, and gas clouds have been used for this purpose. Cepheid variables, stars the brightness of which varies periodically, are especially valuable because the period of pulsation is related to the intrinsic brightness of the star. By observing periodicity, the true brightness can be computed and compared with the apparent brightness; distance can then be inferred. Astronomers have learned that the speed of the stars as they orbit the center of their galaxy depends on the intrinsic brightness and mass of that galaxy. Rapidly rotating galaxies are extremely luminous; slowly rotating ones are intrinsically faint. If the orbital velocities of stars in a galaxy can be determined, then the distance of that galaxy can be inferred.

V

DISTRIBUTION OF GALAXIES

Galaxies are generally not isolated in space but are often members of small or moderate-sized groups or clusters, which in turn form large superclusters of galaxies. Earth’s galaxy, the Milky Way Galaxy, is one of at least 30 galaxies in what astronomers call the Local Group. The Milky Way and the Andromeda galaxies are the two largest members of the Local Group, each with hundreds of billions of stars. The Large, Small, and Mini Magellanic Clouds are nearby satellite galaxies, but each is small and faint, with about 100 million stars. See also Magellanic Clouds.

The Local Group is a member of the Local Supercluster. The nearest cluster is the Virgo cluster, which contains thousands of galaxies. The Virgo cluster is at or near the center of the Local Supercluster, and its gravitational pull on the Local Group is making this group recede more slowly than the expansion of the universe would normally cause it to recede.

Overall, the distribution of clusters and superclusters in the universe is not uniform. Instead, superclusters of tens of thousands of galaxies are arranged in long, stringy, lacelike filaments, arranged around large voids. The Great Wall, a galactic filament discovered in 1989, stretches across more than half a billion light-years of space. Cosmologists theorize that dark matter, material that neither radiates nor reflects light, has sufficient mass to generate the gravitational fields responsible for the heterogeneous structure of the universe.

The most distant galaxies known, near the edge of the observable universe, are blue because of the hot, young stars they contain. Observing these galaxies from Earth is difficult because the light and radiation they emit is mostly in the blue, violet, and ultraviolet range, a range that is mostly blocked by Earth’s atmosphere. Astronomers have obtained images of young galaxies using the Keck Telescope in Hawaii and the Hubble Space Telescope, which resides in an orbit high above Earth’s atmosphere and thus avoids atmospheric interference. Photos from the HST show galaxies that are as far as 13 billion light-years away from Earth, which means they formed soon after the universe formed about 13.7 billion years ago. The galaxies appear to be spherical in shape, and may be early precursors of elliptical and spiral galaxies.

VI

ROTATION OF SPIRAL GALAXIES

Stars and gas clouds orbit about the center of their galaxy. Astronomers believe that most galaxies spin around a black hole, a dense object with such a large gravitational pull that nothing nearby can escape, not even light. Using the HST in 1994, astronomers found the first evidence for a black hole in the center of a galaxy. In 1998 researchers found strong evidence that the Milky Way galaxy’s center, which is 28,000 light-years away from Earth, contains a black hole more than two million times the mass of the Sun. In 1999 a group of astronomers showed that the two bright spots at the center of the Andromeda galaxy were caused by stars speeding around a black hole, the real center of the galaxy.

Orbital periods are more than 100 million years. These motions are studied by measuring the positions of lines in the galaxy spectra. In spiral galaxies, the stars move in circular orbits, with velocities that increase with increasing distances from the center. At the edges of spiral disks, velocities of 300 km/sec (about 185 mi/sec) have been measured at distances as great as 150,000 light-years.

This increase in velocity with increase in distance is unlike planetary velocities in the solar system, for example, where the velocities of planets decrease with increasing distance from the sun. This difference tells astronomers that the mass of a galaxy is not as centrally concentrated as is the mass in the solar system. A significant portion of galaxy mass is located at large distances from the center of the galaxy, but this mass has so little luminosity that it has only been detected by its gravitational attraction. Studies of velocities of stars in external galaxies have led to the belief that much of the mass in the universe is not visible as stars. The exact nature of this dark matter is unknown at present. See also Cosmology.

VII

RADIATION FROM A GALAXY

Knowledge of the appearance of a galaxy is based on optical observations. Knowledge of the composition and motions of the individual stars comes from spectral studies in the optical region also. Because the hydrogen gas in the spiral arms of a galaxy radiates in the radio portion of the electromagnetic spectrum, many details of galactic structure are learned from studies in the radio region. The warm dust in the nucleus and spiral arms of a galaxy radiates in the infrared portion of the spectrum. Some galaxies radiate more energy in the optical region.

Recent X-ray observations have confirmed that galactic halos contain hot gas, gas with temperatures of millions of degrees. X-ray emission is also observed from objects as varied as globular clusters, supernova remnants, and hot gas in clusters of galaxies. Observations in the ultraviolet region also reveal the properties of the gas in the halo, as well as details of the evolution of young stars in galaxies. See X-Ray Galaxy.

VIII

ORIGINS OF GALAXIES

As the 21st century began, astronomers believed they were much closer to understanding the origins of galaxies. Observations made by the Cosmic Background Explorer (COBE) satellite, which was launched in 1989, confirmed predictions made by the big bang theory of the universe’s origin. COBE also detected small irregularities, or ripples, in the background radiation that uniformly pervades the universe. These ripples were thought to be clumps of matter that formed soon after the big bang. The clumps became the seeds from which galaxies and clusters of galaxies developed. The ripples were studied in more detail in limited regions of the sky by a variety of ground-based and balloon-based experiments. A more recent spacecraft, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), made even more accurate observations of these ripples across the entire sky. In 2003 WMAP’s results confirmed the existence of these galactic seeds, providing a full-sky map of the universe’s emerging galaxies.

Contributed By:
Vera C. Rubin

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Milky Way

Milky Way

I

INTRODUCTION

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).

II

STRUCTURE

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.

III

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 (see Radio Astronomy; Infrared Astronomy; X-Ray Astronomy). 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. See Nebula.

IV

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).

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

Eclipse

Eclipse

I

INTRODUCTION

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).

II

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.

III

SOLAR ECLIPSES

The length of the moon’s umbra varies from 367,000 to 379,800 km (228,000 to 236,000 mi), and the distance between the earth and the moon varies from 357,300 to 407,100 km (222,000 to 253,000 mi). Total solar eclipses occur when the moon’s umbra reaches the earth. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of the earth, so that the area in which a total solar eclipse is visible is never wider than that and is usually considerably narrower. The width of the penumbra shadow, or the area of partial eclipse on the surface of the earth, is about 4828 km (about 3000 mi). At certain times when the moon passes between the earth and the sun, its shadow does not reach the earth. At such times an annular eclipse occurs in which an annulus or bright ring of the solar disk appears around the black disk of the moon.

The shadow of the moon moves across the surface of the earth in an easterly direction. Because the earth is also rotating eastward, the speed of the moon shadow across the earth is equal to the speed of the moon traveling along its orbit, minus the speed of the earth’s rotation. The speed of the shadow at the equator is about 1706 km/h (about 1060 mph); near the poles, where the speed of rotation is virtually zero, it is about 3380 km/h (about 2100 mph). The path of a total solar eclipse and the time of totality can be calculated from the size of the moon’s shadow and from its speed. The maximum duration of a total solar eclipse is about 7.5 minutes, but these are rare, occurring only once in several thousand years. A total eclipse is usually visible for about 3 minutes from a point in the center of the path of totality.

In areas outside the band swept by the moon’s umbra but within the penumbra, the sun is only partly obscured, and a partial eclipse occurs.

At the beginning of a total eclipse, the moon begins to move across the solar disk about 1 hour before totality. The illumination from the sun gradually decreases and during totality (and near totality) declines to the intensity of bright moonlight. This residual light is caused largely by the sun’s corona, the outermost part of the sun’s atmosphere. As the surface of the sun narrows to a thin crescent, the corona becomes visible. At the moment before the eclipse becomes total, brilliant points of light, called Baily’s beads, flash out in a crescent shape. These points are caused by the sun shining through valleys and irregularities on the lunar surface. Baily’s beads are also visible at the instant when totality is ending, called emersion. Just before, just after, and sometimes during totality, narrow bands of moving shadows can be seen. These shadow bands are not fully understood but are thought to be caused by irregular refraction of light in the atmosphere of the earth. Before and after totality, an observer located on a hill or in an airplane can see the moon’s shadow traveling eastward across the earth’s surface like a swiftly moving cloud shadow.

IV

FREQUENCY OF ECLIPSES

If the earth’s orbit, or the ecliptic, were in the same plane as the moon’s orbit, two total eclipses would occur during each lunar month, a lunar eclipse at the time of each full moon, and a solar eclipse at the time of each new moon. The two orbits, however, are inclined, and, as a result, eclipses occur only when the moon or the sun is within a few degrees of the two points, called the nodes, where the orbits intersect.

Periodically both the sun and the moon return to the same position relative to one of the nodes, with the result that eclipses recur at regular intervals. The time of the interval, called the saros, is a little more than 6585.3 days or about 18 years, 9 to 11 days, depending on the number of intervening leap years, and 8 hours. The saros, known since the time of ancient Babylonia, corresponds almost exactly to 19 returns of the sun to the same node, 242 returns of the moon to the same node, and 223 lunar months. The disparity between the number of returns of the moon and the number of lunar months is the result of the nodes moving westward at the rate of 19.5° per year. An eclipse that recurs after the saros will be a duplicate of the earlier eclipse but will be visible 120° farther west on the earth’s surface, because of the rotation of the earth during the third of a day included in the interval. Lunar eclipses recur 48 or 49 times and solar eclipses 68 to 75 times before slight differences in the motions of the sun and moon eliminate the eclipse.

During one saros about 70 eclipses take place, usually 29 lunar and 41 solar; of the latter, usually 10 are total and 31 partial. The minimum number of eclipses that can occur in a given saros year is 2, the maximum 7, and the average is 4.

During the 20th century 375 eclipses took place: 228 solar and 147 lunar. The last total eclipse of the sun visible in the United States occurred over the state of Hawaii on July 11, 1991. The prior such eclipse occurred over the state of Washington on February 26, 1979. The next total eclipse will be visible from the U.S. in 2017.

V

OBSERVATION OF ECLIPSES

Many problems of astronomy can be studied only during a total eclipse of the sun. Among these problems are the size and composition of the solar corona and the bending of light rays passing close to the sun because of the sun’s gravitational field (see Relativity). The great brilliance of the solar disk and the sun-induced brightening of the earth’s atmosphere make observations of the corona and nearby stars impossible except during a solar eclipse. The coronagraph, a photographic telescope, permits direct observation of the edge of the solar disk at all times. Today, scientific solar eclipse observations are extremely valuable, particularly when the path of the eclipse traverses large land areas. An elaborate network of special observatories may provide enough data for months of analysis by scientists. Such data may provide information on how minute variations in the sun affect weather on earth, and how scientists can improve predictions of solar flares.

See also Astronomy.

Contributed By:
Fred Hess

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