Infrared Astronomy

Infrared Astronomy

Star-Forming Region

The Flame Nebula, also called NGC 2024, is a region of dust and gas in the constellation Orion. Some of the dust and gas is in the process of condensing into stars. This photograph was taken in the infrared range of electromagnetic radiation. Infrared radiation has wavelenghts slightly longer than those of visible light.

Infrared Astronomy, the detection and study of infrared radiation emanating from objects in outer space. Infrared radiation is another form of electromagnetic energy, similar to visible light and radio waves, but differing by its wavelength (or frequency); all such waves travel at the speed of light in a vacuum. Infrared wavelengths begin around 0.0007 mm just beyond the reddest light that the human eye can detect, this is called the “near” infrared, and grow in size to about 0.35 mm in the “far” infrared. Wavelengths larger than this belong to the sub-millimeter, microwave, and radio parts of the electromagnetic spectrum.

Infrared observations are important in astrophysics for several reasons. Infrared radiation penetrates more easily through the vast stretches of interstellar gas and dust clouds than does visible and ultraviolet light, revealing regions hidden to normal telescopes. Young stars are surrounded by a cocoon of gas and dust which can make them invisible, but their heat warms the dust grains and produces infrared radiation which escapes to reveal their presence. Infrared radiation is also called “thermal” or heat radiation. Many molecules, such as carbon monoxide (CO) and hydrogen (H₂), and tiny solid particles known as dust grains, are best studied at infrared wavelengths. Finally, the expansion of the universe changes (or redshifts) the visible light emitted by the most distant galaxies into red and infrared light.

II

INFRARED TELESCOPES AND DETECTORS

Infrared Telescope

Infrared telescopes detect radiation that has wavelengths longer than the light that humans can see. Infrared radiation enters the telescope and reflects off of a large mirror on the bottom of the telescope, then off of a smaller mirror. Detectors and instruments beneath the mirrors record the radiation. Infrared telescopes must be kept at very low temperatures to prevent their own heat from producing infrared radiation that could interfere with observations.

Infrared telescopes look similar to normal optical telescopes. Light is collected and focused by a large curved mirror onto a smaller secondary mirror and then into the scientific instrument (camera or spectrometer) for analysis. Most often, the secondary mirror is very small and gold-coated because gold offers better reflectance in the infrared than the normal aluminized mirrors do. The secondary mirror is mechanized so that it can tilt back and forth rapidly, up to 20 times per second, allowing the detector in the science instrument to compare the signals from the source plus sky background and the sky background alone. This technique is called “chopping” and is very effective for detecting a faint signal against a very strong background. Infrared telescopes have an open structure, without the black tubes or baffles common to normal telescopes, since these emit infrared radiation. For an infrared telescope in space the baffles can be cooled to reduce infrared emissions. One of the largest telescopes dedicated to ground-based infrared astronomy is the 152-inch (3.8-m) United Kingdom Infrared Telescope (UKIRT) at the Mauna Kea Observatory in Hawaii, but many other telescopes possess infrared instruments, especially for near-infrared work. In recent years, infrared astronomy has been revolutionized by the introduction of tiny imaging devices called “infrared arrays” making it possible to take pictures at these invisible wavelengths and display them on computer screens.

Everything that is warm emits infrared radiation. A star like the Sun emits most of its radiant energy as visible (yellow) light. Redder and cooler stars at half the Sun’s temperature have a peak emission at near-infrared wavelengths. Merely warm objects, like telescopes and Earth’s atmosphere, don’t emit light but they give off profuse amounts of infrared radiation with a peak emission around a wavelength of 0.01 mm. Since they are much closer to the infrared camera or detector, these sources of infrared radiation swamp any signals from distant planets, stars, and galaxies. Likewise, the optical and mechanical parts of the scientific instrument will emit strong infrared radiation. The solution to this problem is to cool the detector, and all the optics, inside a vacuum chamber. Temperatures around that of liquid nitrogen (77 degrees Kelvin (K) above absolute zero) are needed for near-infrared detection whereas temperatures near liquid helium (only 4 K) are required for far-infrared detectors.

To eliminate the large, unwanted background of infrared radiation it would be necessary to cool down the entire telescope and remove Earth’s atmosphere. For a space-based telescope, however, there is no atmosphere and no problem of condensation and ice formation if the telescope itself is cooled. Ground-based infrared telescopes need to be located at very high, dry, and cold sites like Mauna Kea, Hawaii, to minimize the thermal background. This is effective for near-infrared work. Far-infrared observations are best done from space or from high-flying airplanes.

III

THE INFRARED UNIVERSE

Orion Nebula in Infrared

This image of the Orion Nebula was taken in infrared radiation—radiation with wavelengths longer than visible light—and given false visible colors. The Orion Nebula is a cloud of gas and dust that surrounds new stars. The young stars light up the nebula and cause it to glow.

Infrared radiation emanating from the outer planets and their moons reveals much about their temperatures and compositions. Jupiter emits more infrared radiation than expected from absorption of sunlight, indicating that it has an internal source of heat energy. Infrared observations of Jupiter’s moon Io show thermal hot spots caused by the active volcanoes on its surface.

Stars of lower mass than the Sun are less luminous and much cooler. They emit most of their energy at infrared wavelengths. While these red dwarf stars are much more plentiful than the rare and short-lived high-mass stars, the very lowest mass objects—called brown dwarfs—have been hard to find because they are so faint at visible light wavelengths. Infrared astronomy has played a key role in the search for these objects which lie on the boundary between true stars and giant Jupiter-like planets.

Although interstellar space is a vacuum, it is not completely empty. Tiny solid particles called dust grains can be found around and between the stars. These particles are formed from heavier elements (like silicon and iron) produced deep inside an earlier generation of massive stars. Dust grains absorb ultraviolet and visible light, making the most distant stars appear redder and fainter, while also heating the dust slightly above the temperature of space and causing it to glow at far-infrared wavelengths. Dust obscures our view of distant parts of our own Milky Way Galaxy, including the center of the galaxy. The central regions can only be observed at infrared and radio wavelengths. Recent ground-based and space-based observations of the center of the galaxy have revealed for the first time clear evidence of dynamic motion around a massive, invisible source believed to be a black hole with a mass over one million times that of the Sun. Dust also obscures our view of young, recently-formed stars. As a cloud of gas and dust contracts to form a star it tends to flatten into a disk due to rotation. Eventually the disk might dissipate or it may allow planets to form. The disk both hides the starlight and is heated by it. Infrared observations reveal the hidden source and provide knowledge of how stars and planetary systems form.

Spiral galaxies similar to our own contain lots of dust and star-forming regions making them difficult to study except with infrared instruments. Some classes of galaxies are exceptionally luminous in the infrared because they have so much dust and have absorbed so much ultraviolet energy from the central sources within the nucleus of the galaxy. Distant galaxies are fainter and redder than nearby ones. The reddening in this case is not caused by intergalactic dust but by the expansion of the universe discovered by Edwin Hubble. Traveling at 186,000 mi every second (300,000 km/s), it takes light billions of years to reach us from the most distant galaxies, and therefore we see these galaxies as they were when they were young and not as they are now. During this time space itself has expanded and the wavelength of the radiation has been stretched or “redshifted” by such a large amount that the galaxy is no longer readily detectable at visible light wavelengths because all its energy is now in the infrared.

IV

HISTORY AND CURRENT RESEARCH

Saturn’s Atmosphere

This infrared photo of the planet Saturn has been color coded to indicate the cloud level in Saturn’s atmosphere. Violet and blue represent areas in which Saturn’s atmopshere is clear down to the main cloud layer. Green and yellow show layers of haze above the main cloud layer (yellow represents thicker haze). Red and orange indicate the highest level of clouds, thicker than the haze. White areas are areas of the atmosphere with high levels of water vapor. The bright dots at the upper right and lower left of the picture are Saturn’s satellites Tethys and Dione, respectively. The Hubble Space Telescope took this image in 1998.

Modern infrared astronomy began in the 1950s when simple photoelectric detectors made from lead sulphide became available and were used to survey the sky at infrared wavelengths for new sources. Later, germanium detectors were used to open up the study of much longer infrared wavelengths with the aid of rocket, balloon, and airplane surveys. In the late 1970s, several large ground-based telescopes dedicated to infrared astronomy were built which included the 3.8-m United Kingdom Infrared Telescope (UKIRT) and the National Aeronautics and Space Administration (NASA) 3-m Infrared Telescope Facility (IRTF) both on Mauna Kea, Hawaii, at 14,000 ft above sea level. With the launch of the Infrared Astronomical Satellite (IRAS), by the United States, the United Kingdom, and The Netherlands in 1983, infrared astronomy took another leap forward. This mission surveyed the entire sky at wavelengths of 12, 25, 60, and 100 microns (1 micron is a millionth of a meter) until its onboard supply of liquid helium ran out. A short time later infrared astronomy was revolutionized by the first introduction of devices that could take infrared images. The advent of sensitive infrared cameras inspired many traditional observatories to convert for infrared work, especially in the near infrared where city lights don’t cause problems and the thermal background from the telescope is minimal.

Uranus and Its Rings

The planet Uranus rotates on an axis that is tilted nearly horizontal. Other planets in the solar system have axes that are more vertical. This infrared image taken by the Hubble Space Telescope shows Uranus's rings orbiting in the plane of the planet's tipped equator. The colors are not real and are used to bring out details such as clouds in the atmosphere and the shape of the rings. The white disks are moons.

Infrared astronomy from space received two boosts when the Infrared Space Observatory (ISO) was launched in December 1995 and when the Hubble Space Telescope was refurbished in 1997 with NICMOS, the Near Infrared Camera and Multi-Object Spectrograph. Both of these missions used infrared array detectors. Another important satellite which used infrared techniques for part of its mission was the Cosmic Background Explorer (COBE). The Spitzer Space Telescope, launched in 2003, is the largest and most sensitive infrared space telescope ever launched.

Future infrared missions include SOFIA, the Stratospheric Observatory for Infrared Astronomy. SOFIA is a modified Boeing 747 with a 2.5-m telescope on board, due for commissioning in 2004.

Gravitation

Gravitation

Gravitation, the force of attraction between all objects that tends to pull them toward one another. It is a universal force, affecting the largest and smallest objects, all forms of matter, and energy. Gravitation governs the motion of astronomical bodies. It keeps the moon in orbit around the earth and keeps the earth and the other planets of the solar system in orbit around the sun. On a larger scale, it governs the motion of stars and slows the outward expansion of the entire universe because of the inward attraction of galaxies to other galaxies. Typically the term gravitation refers to the force in general, and the term gravity refers to the earth's gravitational pull.

Gravitation is one of the four fundamental forces of nature, along with electromagnetism and the weak and strong nuclear forces, which hold together the particles that make up atoms. Gravitation is by far the weakest of these forces and, as a result, is not important in the interactions of atoms and nuclear particles or even of moderate-sized objects, such as people or cars. Gravitation is important only when very large objects, such as planets, are involved. This is true for several reasons. First, the force of gravitation reaches great distances, while nuclear forces operate only over extremely short distances and decrease in strength very rapidly as distance increases. Second, gravitation is always attractive. In contrast, electromagnetic forces between particles can be repulsive or attractive depending on whether the particles both have a positive or negative electrical charge, or they have opposite electrical charges (see Electricity). These attractive and repulsive forces tend to cancel each other out, leaving only a weak net force. Gravitation has no repulsive force and, therefore, no such cancellation or weakening.

The gravitational attraction of objects for one another is the easiest fundamental force to observe and was the first fundamental force to be described with a complete mathematical theory by the English physicist and mathematician Sir Isaac Newton. A more accurate theory called general relativity was formulated early in the 20th century by the German-born American physicist Albert Einstein. Scientists recognize that even this theory is not correct for describing how gravitation works in certain circumstances, and they continue to search for an improved theory.

II

EARTH'S GRAVITATION

Gravitation plays a crucial role in most processes on the earth. The ocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives weather patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space.

The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size (see Star, Black Hole).

A

Acceleration

Freefall

Falling objects accelerate in response to the force exerted on them by Earth’s gravity. Different objects accelerate at the same rate, regardless of their mass. This illustration shows the speed at which a ball and a cat would be moving and the distance each would have fallen at intervals of a tenth of a second during a short fall.

If an object held near the surface of the earth is released, it will fall and accelerate, or pick up speed, as it descends. This acceleration is caused by gravity, the force of attraction between the object and the earth. The force of gravity on an object is also called the object's weight. This force depends on the object's mass, or the amount of matter in the object. The weight of an object is equal to the mass of the object multiplied by the acceleration due to gravity.

A bowling ball that weighs 16 lb is actually being pulled toward the earth with a force of 16 lb. In the metric system, the bowling ball is pulled toward the earth with a force of 71 newtons (a metric unit of force abbreviated N). The bowling ball also pulls on the earth with a force of 16 lb (71 N), but the earth is so massive that it does not move appreciably. In order to hold the bowling ball up and keep it from falling, a person must exert an upward force of 16 lb (71 N) on the ball. This upward force acts to oppose the 16 lb (71 N) downward weight force, leaving a total force of zero. The total force on an object determines the object's acceleration.

If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.

The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the center of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earth's poles than at the equator.

III

EARLY IDEAS ABOUT GRAVITATION

The ancient Greek philosophers developed several theories about the force that caused objects to fall toward the earth. In the 4th century bc, the Greek philosopher Aristotle proposed that all things were made from some combination of the four elements, earth, air, fire, and water. Objects that were similar in nature attracted one another, and as a result, objects with more earth in them were attracted to the earth. Fire, by contrast, was dissimilar and therefore tended to rise from the earth. Aristotle also developed a cosmology, that is, a theory describing the universe, that was geocentric, or earth-centered, with the moon, sun, planets, and stars moving around the earth on spheres. The Greek philosophers, however, did not propose a connection between the force behind planetary motion and the force that made objects fall toward the earth.

At the beginning of the 17th century, the Italian physicist and astronomer Galileo discovered that all objects fall toward the earth with the same acceleration, regardless of their weight, size, or shape, when gravity is the only force acting on them. Galileo also had a theory about the universe, which he based on the ideas of the Polish astronomer Nicolaus Copernicus. In the mid-16th century, Copernicus had proposed a heliocentric, or sun-centered system, in which the planets moved in circles around the sun, and Galileo agreed with this cosmology. However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. Like the Greek philosophers, he saw no connection between the force behind planetary motion and gravitation on earth.

In the late 16th and early 17th centuries the heliocentric model of the universe gained support from observations by the Danish astronomer Tycho Brahe, and his student, the German astronomer Johannes Kepler. These observations, made without telescopes, were accurate enough to determine that the planets did not move in circles, as Copernicus had suggested. Kepler calculated that the orbits had to be ellipses (slightly elongated circles). The invention of the telescope made even more precise observations possible, and Galileo was one of the first to use a telescope to study astronomy. In 1609 Galileo observed that moons orbited the planet Jupiter, a fact that could not reasonably fit into an earth-centered model of the heavens.

The new heliocentric theory changed scientists' views about the earth's place in the universe and opened the way for new ideas about the forces behind planetary motion. However, it was not until the late 17th century that Isaac Newton developed a theory of gravitation that encompassed both the attraction of objects on the earth and planetary motion.

IV

NEWTON'S THEORY OF GRAVITATION

Gravitational Forces

Because the Moon has significantly less mass than Earth, the weight of an object on the Moon’s surface is only one-sixth the object’s weight on Earth’s surface. This graph shows how much an object that weighs w on Earth would weigh at different points between the Earth and Moon. Since the Earth and Moon pull in opposite directions, there is a point, about 346,000 km (215,000 mi) from Earth, where the opposite gravitational forces would cancel, and the object's weight would be zero.

To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object is motion at a constant speed on a straight line, and that it takes a force to slow down, speed up, or change the path of an object. Newton also invented calculus, a new branch of mathematics that became an important tool in the calculations of his theory of gravitation.

Newton proposed his law of gravitation in 1687 and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles' masses divided by the square of the distance between them. The gravitational force between two objects can be expressed by the following equation: F= GMm/d² where F is the gravitational force, G is a constant known as the universal constant of gravitation, M and m are the masses of each object, and d is the distance between them. Newton considered a particle to be an object with a mass that was concentrated in a small point. If the mass of one or both particles increases, then the attraction between the two particles increases. For instance, if the mass of one particle is doubled, the force of attraction between the two particles is doubled. If the distance between the particles increases, then the attraction decreases as the square of the distance between them. Doubling the distance between two particles, for instance, will make the force of attraction one quarter as great as it was.

According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts along the line between their centers. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the center of gravity, at which all the force of gravity can be considered to be acting.

Newton's law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. It is therefore known as the universal law of gravitation. In order to know the strength of gravitational forces in general, however, it became necessary to find the value of G, the universal constant of gravitation. Scientists needed to perform an experiment, but gravitational forces are very weak between objects found in a common laboratory and thus hard to observe. In 1798 the English chemist and physicist Henry Cavendish finally measured G with a very sensitive experiment in which he nearly eliminated the effects of friction and other forces. The value he found was 6.754 x 10^-11 N-m²/kg²—close to the currently accepted value of 6.670 x 10^-11 N-m²/kg² (a decimal point followed by 10 zeros and then the number 6670). This value is so small that the force of gravitation between two objects with a mass of 1 metric ton each, 1 meter from each other, is about 67 millionths of a newton, or about 15 millionths of a pound.

Gravitation may also be described in a completely different way. A massive object, such as the earth, may be thought of as producing a condition in space around it called a gravitational field. This field causes objects in space to experience a force. The gravitational field around the earth, for instance, produces a downward force on objects near the earth surface. The field viewpoint is an alternative to the viewpoint that objects can affect each other across distance. This way of thinking about interactions has proved to be very important in the development of modern physics.

A

Planetary Motion

Newton's law of gravitation was the first theory to accurately describe the motion of objects on the earth as well as the planetary motion that astronomers had long observed. According to Newton's theory, the gravitational attraction between the planets and the sun holds the planets in elliptical orbits around the sun. The earth's moon and moons of other planets are held in orbit by the attraction between the moons and the planets. Newton's law led to many new discoveries, the most important of which was the discovery of the planet Neptune. Scientists had noted unexplainable variations in the motion of the planet Uranus for many years. Using Newton's law of gravitation, the French astronomer Urbain Leverrier and the British astronomer John Couch each independently predicted the existence of a more distant planet that was perturbing the orbit of Uranus. Neptune was discovered in 1864, in an orbit close to its predicted position.

B

Problems with Newton's Theory

Frames of Reference

A situation can appear different when viewed from different frames of reference. Try to imagine how an observer's perceptions could change from frame to frame in this illustration.

Scientists used Newton's theory of gravitation successfully for many years. Several problems began to arise, however, involving motion that did not follow the law of gravitation or Newtonian mechanics. One problem was the observed and unexplainable deviations in the orbit of Mercury (which could not be caused by the gravitational pull of another orbiting body).

Another problem with Newton's theory involved reference frames, that is, the conditions under which an observer measures the motion of an object. According to Newtonian mechanics, two observers making measurements of the speed of an object will measure different speeds if the observers are moving relative to each other. A person on the ground observing a ball that is on a train passing by will measure the speed of the ball as the same as the speed of the train. A person on the train observing the ball, however, will measure the ball's speed as zero. According to the traditional ideas about space and time, then, there could not be a constant, fundamental speed in the physical world because all speed is relative. However, near the end of the 19th century the Scottish physicist James Clerk Maxwell proposed a complete theory of electric and magnetic forces that contained just such a constant, which he called c. This constant speed was 300,000 km/sec (186,000 mi/sec) and was the speed of electromagnetic waves, including light waves. This feature of Maxwell's theory caused a crisis in physics because it indicated that speed was not always relative.

Albert Einstein resolved this crisis in 1905 with his special theory of relativity. An important feature of Einstein's new theory was that no particle, and even no information, could travel faster than the fundamental speed c. In Newton's gravitation theory, however, information about gravitation moved at infinite speed. If a star exploded into two parts, for example, the change in gravitational pull would be felt immediately by a planet in a distant orbit around the exploded star. According to Einstein's theory, such forces were not possible.

Though Newton's theory contained several flaws, it is still very practical for use in everyday life. Even today, it is sufficiently accurate for dealing with earth-based gravitational effects such as in geology (the study of the formation of the earth and the processes acting on it), and for most scientific work in astronomy. Only when examining exotic phenomena such as black holes (points in space with a gravitational force so strong that not even light can escape them) or in explaining the big bang (the origin of the universe) is Newton's theory inaccurate or inapplicable.

V

EINSTEIN'S THEORY OF RELATIVITY

In 1915 Einstein formulated a new theory of gravitation that reconciled the force of gravitation with the requirements of his theory of special relativity. He proposed that gravitational effects move at the speed of c. He called this theory general relativity to distinguish it from special relativity, which only holds when there is no force of gravitation. General relativity produces predictions very close to those of Newton's theory in most familiar situations, such as the moon orbiting the earth. Einstein's theory differed from Newton's theory, however, in that it described gravitation as a curvature of space and time.

In Einstein's general theory of relativity, he proposed that space and time may be united into a single, four-dimensional geometry consisting of 3 space dimensions and 1 time dimension. In this geometry, called spacetime, the motions of particles from point to point as time progresses are represented by curves called world lines. If there is no gravity acting, the most natural lines in this geometry are straight lines, and they represent particles that are moving always in the same direction with the same speed—that is, particles that have no force acting on them. If a particle is acted on by a force, then its world line will not be straight. Einstein also proposed that the effect of gravitation should not be represented as the deviation of a world line from straightness, as it would be for an electrical force. If gravitation is present, it should not be considered a force. Rather, gravitation changes the most natural world lines and thereby curves the geometry of spacetime. In a curved geometry, such as the two-dimensional surface of the earth, there are no straight lines. Instead, there are special curves called geodesics, an example of which are great circles around the earth. These special curves are at each point as straight as possible, and they are the most natural lines in a curved geometry. The effect of gravitation would be to influence the geodesics in spacetime. Near sources of gravitation the space is strongly curved and the geodesics behave less and less like those in flat, uncurved spacetime. In the solar system, for example, the effect of the sun and the earth is to cause the moon to move on a geodesic that winds around the geodesic of the earth 12 times a year.

A

Testing Einstein's Theory

Einstein's theory required verification, but the level of precision in measurement needed to distinguish between Einstein's theory and Newton's theory was difficult to achieve in the early 20th century and remains so today. There were two predictions, however, that could be tested. One involved deviations in the orbit of Mercury. Astronomers had observed that the ellipse of Mercury's orbit itself rotated—that is, the point nearest the sun, called the perihelion, slowly advanced around the sun. The rate of advancement predicted by Newton's theory differed slightly from what astronomers had measured, but Einstein's theory predicted the correct rate.

The second test involved measuring the bending of light as it passed around the sun. Both Newton's and Einstein's theories predicted that light would be deflected by gravitation. But the amount of deflection predicted by the two theories differed. The light to be measured in such a test originates in distant stars. However, under ordinary conditions the sun's brightness prevents scientists from observing the light from these stars. This problem disappears during an eclipse, when the moon blocks the sun's light. In 1919 a special British expedition took photographs during an eclipse. Scientists measured the deflection of starlight as it passed by the sun and arrived at numbers that agreed with Einstein's prediction. Subsequent eclipse observations also have confirmed Einstein's theory.

Other physicists have proposed relativistic theories of gravitation to compete with Einstein's. In these competing theories, almost all of which are geometrical like Einstein's, gravitational effects move at the speed c. They differ mostly in the mathematical details. Even using the technology of the late 20th century, scientists still find it very difficult to test these theories with experiments and observations. But Einstein's theory has passed all tests that have been made so far.

B

Applications of Einstein's Theory

Einstein's general relativity theory predicts special gravitational conditions. The Big Bang theory, which describes the origin and early expansion of the universe, is one conclusion based on Einstein's theory that has been verified in several independent ways.

Another conclusion suggested by general relativity, as well as other relativistic theories of gravitation, is that gravitational effects move in waves. Astronomers have observed a loss of energy in a pair of neutron stars (stars composed of densely packed neutrons) that are orbiting each other. The astronomers theorize that energy-carrying gravitational waves are radiating from the pair, depleting the stars of their energy. Very violent astrophysical events, such as the explosion of stars or the collision of neutron stars, can produce gravitational waves strong enough that they may eventually be directly detectable with extremely precise instruments. Astrophysicists are designing such instruments with the hope that they will be able to detect gravitational waves by the beginning of the 21st century.

Another gravitational effect predicted by general relativity is the existence of black holes. The idea of a star with a gravitational force so strong that light cannot escape from its surface can be traced to Newtonian theory. Einstein modified this idea in his general theory of relativity. Because light cannot escape from a black hole, for any object—a particle, spacecraft, or wave—to escape, it would have to move past light. But light moves outward at the speed c. According to relativity, c is the highest attainable speed, so nothing can pass it. The black holes that Einstein envisioned, then, allow no escape whatsoever. An extension of this argument shows that when gravitation is this strong, nothing can even stay in the same place, but must move inward. Even the surface of a star must move inward, and must continue the collapse that created the strong gravitational force. What remains then is not a star, but a region of space from which emerges a tremendous gravitational force.

VI

OTHER MODERN THEORIES

Einstein's theory of gravitation revolutionized 20th-century physics. Another important revolution that took place was quantum theory. Quantum theory states that physical interactions, or the exchange of energy, cannot be made arbitrarily small. There is a minimal interaction that comes in a packet called the quantum of an interaction. For electromagnetism the quantum is called the photon. Like the other interactions, gravitation also must be quantized. Physicists call a quantum of gravitational energy a graviton. In principle, gravitational waves arriving at the earth would consist of gravitons. In practice, gravitational waves would consist of apparently continuous streams of gravitons, and individual gravitons could not be detected.

Einstein's theory did not include quantum effects. For most of the 20th century, theoretical physicists have been unsuccessful in their attempts to formulate a theory that resembles Einstein's theory but also includes gravitons. Despite the lack of a complete quantum theory, it is possible to make some partial predictions about quantized gravitation. In the 1970s, British physicist Stephen Hawking showed that quantum mechanical processes in the strong gravitational pull just outside of black holes would create particles and quanta that move away from the black hole, thereby robbing it of energy.

A

Theory of Everything

An important trend in modern theoretical physics is to find a theory of everything (TOE), in which all four of the fundamental forces are seen to be really different aspects of the same single universal force. Physicists already have unified electromagnetism and the weak nuclear force and have made progress in joining these two forces with the strong nuclear force (see Grand Unification Theories). However, relativistic gravitation, with its geometric and mathematically complex character, poses the most difficult challenge. Einstein spent most of his later years searching for an all-encompassing theory by trying to make electromagnetism geometrical like gravitation. The modern approach is to try to make gravitation fit the pattern of the other fundamental forces. Much of this work involves looking for mathematical patterns. A TOE is difficult to test using experiments because the effects of a TOE would be important only in the rarest circumstances.

Galaxy

Galaxy

Galaxy Group

This galaxy group, named Hickson Compact Group (HCG) 87, is about 400 million light-years from Earth. The galaxies are interacting gravitationally, influencing one another’s structure and evolution. The image was taken by the Gemini South telescope at Cerro Pachón, Chile.

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

Andromeda Galaxy

The Andromeda Galaxy, a spiral galaxy similar to our own Milky Way Galaxy, is the farthest object from Earth visible to the naked eye. Its whirlpool of stars can be seen from the Northern Hemisphere in the constellation Andromeda. The Milky Way and Andromeda galaxies are part of a group of galaxies called the Local Group, which in turn is part of larger group called the Virgo Cluster.

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

Milky Way

Spiral galaxies, such as our own Milky Way, have a relatively flat disk shape with spiral arms. This false-color image looks toward the center of the Milky Way, located 30,000 light-years away. Bright star clusters are visible in the image along with darker areas of dust and gas.

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.

Hoag’s Object

A ring of young, massive, blue stars surrounds a nucleus of older, yellow stars in the galaxy known as Hoag’s Object. Astronomers speculate that this unusual separation is the result of a collision with another galaxy. Hoag’s Object lies 600 million light-years away in the constellation Serpens.

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

Distant Galaxies

In January 1996 astronomers were able to prove that there are five times more galaxies in the universe than previously thought. Helping them in this conclusion was the Deep Field image taken by the orbiting Hubble Space Telescope. Although the image covers only a tiny speck of the sky, it is packed with 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.

Colliding Clusters of Galaxies

Galaxy cluster 1E 0657-556, called the "Bullet Cluster," is actually two giant groups of galaxies that collided head-on. This composite picture was created from images taken by different space telescopes in X-ray and visible light. Astronomers think the collision made the dark matter around the galaxies visible, indicated by the blue regions, which bend the light from more distant galaxies in the background. The pink regions are hot gas stripped away in the collision. Dark matter is a still unidentified substance that makes up about 23 percent of the universe. It is thought to surround most galaxies, affecting their shapes by its gravity.

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.