Redshift

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

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

II

WHAT IS REDSHIFT?

Light is made up of waves, and redshift is a change, caused by the object’s motion, in the wavelength of light radiated by an object. Redshifts occur because of a phenomenon scientists call the Doppler effect. The Doppler effect occurs when a wave-emitting object moves toward or away from an observer, and the observer sees or hears the waves differently than he or she would if the object were stationary relative to the observer. If a light-emitting object is moving away from an observer, each wave of light leaves the object from a point slightly farther away from the observer than the previous wave did. Therefore, the distance between waves (called the wavelength) that the observer sees is longer than it would be if the object were stationary. Austrian physicist Christian Johann Doppler described this effect in sound waves in the mid-1800s, and it became known as the Doppler effect for all types of waves.

In visible light, red light has the longest wavelength, and violet light the shortest. The light of an object moving away from an observer is shifted toward a longer wavelength, or toward the color red. The light from an object moving toward an observer is shifted toward the color violet. Astronomers most often use the Doppler effect to measure the velocity of galaxies, which are almost all moving away from Earth, so their light is shifted toward the color red. This is why astronomers call the effect redshift.

Astronomers can study redshift by separating an object’s light into its different colors. This technique is called spectroscopy and is similar to the way water vapor in the atmosphere separates the whitish light of the Sun into its different colors in a rainbow. Once the light of an object has been separated into its colors, scientists call the resulting rainbowlike display a spectrum. Chemical elements present in a light-emitting object produce bright and dark lines called emission and absorption lines on the object’s spectrum. These lines appear because atoms of different elements can only emit and absorb light at certain wavelengths. Spectroscopy helps astronomers learn about the chemical elements that make up an object, and, through the study of redshifts, the movement of the object.

When the light of a star or galaxy is redshifted, its entire spectrum is shifted by the same amount. Astronomers need some sort of marker to tell how far the light has shifted. Emission and absorption lines in the spectrum, created by the elements that make up the star, serve as markers. Certain elements occur in almost every astronomical object and provide handy reference points for measuring redshift. For instance, astronomers know that hydrogen is present in most stars and that it forms a characteristic pattern—which includes absorption lines at certain wavelengths—in the spectrum of an object that isn’t moving with respect to Earth. If this same pattern appears but is shifted toward the red end of the spectrum, scientists know the object is moving away from Earth.

Astronomers begin measuring redshifts by determining how much a chosen reference point, such as an emission or absorption line, has shifted. They define redshift as the amount the line has shifted divided by the wavelength of the original reference point (the place in the spectrum where the line should appear). This number (often abbreviated z) is equal to the velocity (v) of the object divided by the speed of light (c), so the mathematical formula for redshift is z = v/c. The speed of light is 300,000 km/s (190,000 mi/s). If the redshift of a star is 0.0001, the velocity of the star would be 0.01 percent of the speed of light, or about 30 km/s (19 mi/s).

If the velocity of an object is close to the speed of light, the equation for redshift, z = v/c, is no longer as simple, because the rules of relativity apply. German-born American physicist Albert Einstein developed the special theory of relativity in 1905 to explain how objects behave when their speeds are near the speed of light. The formula for redshift for objects with relativistic speeds is

III

GALAXIES, REDSHIFTS, AND DISTANCES

Redshifts of galaxies allow astronomers to measure the distance from Earth to the galaxies. Knowing the distances to galaxies gives astronomers an idea of the way the universe is expanding and provides clues to the origin, evolution, and future of the universe. The relationship between the redshift (and therefore velocity) and distance of a galaxy is called Hubble’s law, which was named after American astronomer Edwin Hubble.

Hubble’s law states that the velocity (v) of a galaxy moving away from Earth is proportional to the galaxy’s distance (d) from Earth. As distance increases, velocity also increases. The constant value that relates velocity and distance is called Hubble’s constant and is usually written as H₀ or simply H. Hubble’s law, written mathematically, is v = H₀d.

Hubble identified the relationship between velocity and distance in 1929, but the numeric value of Hubble’s constant is still uncertain. Astronomers know that it falls between 64 and 78 kilometers per second per megaparsec (between 40 and 48 miles per second per megaparsec). A parsec is a unit of length equal to 30.86 trillion km (19.18 trillion mi), and a megaparsec is 1 million parsecs. Astronomers use these units to make redshift calculations easier.

IV

REDSHIFT AND COSMOLOGY

Redshift and Hubble’s law are vital tools to scientists who study the structure, evolution, and age of the universe. This field is called cosmology. Redshift provides astronomers with a good idea of the general motion of matter in the universe. Observing objects that do not follow Hubble’s law enables astronomers to see the motion of individual galaxies and groups of galaxies and provides useful information about the structure of the universe.

The gravitational pull of nearby galaxies affects the motion of a galaxy. Measurements of redshift reveal that the Great Andromeda Spiral Galaxy, one of the Milky Way’s nearest neighbors, is actually moving toward the Milky Way at about 50 km/s (about 30 mi/s). Neighboring groups of galaxies also affect each other’s motion. The Milky Way and its neighbors are called the Local Group. The Local Group’s neighbor, the Virgo Cluster, is moving away at only about 80 percent of the velocity predicted by Hubble’s law. Deviations from Hubble’s law (also called the Hubble flow) provide one of the best means of calculating the total density of matter in the universe.

Astronomers can also use redshift to identify the oldest and most distant objects in the observable universe. Astronomers believe that quasars are the most distant objects in the universe, because they have some of the largest redshifts. Quasars are objects in space that strongly emit radio waves. Astronomers originally named these objects quasars, which stands for quasi-stellar (or starlike) radio source, because they appear as points of light, like stars, in photographs of the sky. When astronomers began studying quasars in radio and other wavelengths, however, they discovered that quasars are not really starlike at all. They emit far more radiation, especially radio-wavelength radiation, than stars do, and quasars have huge redshifts. Their redshifts are so large that the radiation they emit in the ultraviolet range (with wavelengths shorter than visible light) reaches Earth in the infrared range (with wavelengths longer than visible light). The redshift for some of the most distant quasars is about 5.0, meaning that the shift in wavelength is about five times greater than the wavelength itself. A quasar with a redshift of 5.0 would be between about 3000 Mpc and 6000 Mpc away from Earth—so far away that light from the quasar would take between 9 billion and 19 billion years to reach Earth. Astronomers believe that quasars may be huge black holes, or regions that are so dense that not even light can escape their gravitational pull, surrounded by swirling matter. The matter swirling around black holes is very hot and is moving very quickly. Under these conditions, matter can produce light. This may be the source of quasars’ radiation.

If Hubble’s law holds for most of the age of the universe, Hubble’s constant would give an accurate age of the universe. The universe’s age would be the inverse of the constant (1 divided by Hubble’s constant), or between 12 billion and 16 billion years. However, astronomers have evidence that Hubble’s constant probably is not really constant—that the rate of expansion of the universe has changed and will keep changing as the universe evolves. The estimated age of the universe is actually only about 14 billion years.

 

Radio Astronomy

The Very Large Array
Radio telescopes detect electromagnetic radiation from space in wavelengths ranging from about 1 mm (0.04 in) to more than 1 km (0.6 mi). Since radio telescopes are only sensitive to electromagnetic radiation with a relatively long wavelength, signals from a group of telescopes pointing at the same object can be combined, dramatically improving resolution. For example, the Very Large Array (VLA) in Socorro, New Mexico, has 27 dishes whose individual signals can be combined to form a single high-resolution image.
Radio Astronomy, branch of astronomy in which celestial objects and astrophysical phenomena are studied by examining their emission of electromagnetic radiation in the radio portion of the spectrum. SeeAstronomy; Astrophysics; Electromagnetic Radiation; Spectroscopy; Spectrum.

II

HISTORY

Robert W. Wilson
American physicist and radio astronomer Robert W. Wilson won the 1978 Nobel Prize in physics. While attempting to measure the intensity of radiation from a single, specific point in the sky, he discovered cosmic microwave background radiation.
Unsuccessful attempts to detect celestial radio emission were made during the latter part of the 19th century. The American radio engineer Karl G. Jansky, while working at Bell Telephone Laboratories, in 1932, was the first to detect radio noise from the region near the center of the Milky Way, during an experiment to locate distant sources of terrestrial radio interference. The distribution of this galactic radio emission was mapped by the American engineer Grote Reber, using a 9.5-m (31-ft) paraboloid that he built in his backyard in Wheaton, Illinois. In 1943 Reber also discovered the long-sought-after radio emission from the Sun. It was later realized, however, that solar radio emission had been detected a few years earlier, when strong solar bursts had interfered with the operation of British, American, and German radar systems designed to detect aircraft.
As a result of the great improvements made during World War II in radio antennas and sensitive receivers, radio astronomy flourished in the 1950s. Radio scientists adapted their wartime radar techniques to the construction of a variety of radio telescopes in Australia, the United Kingdom, the Netherlands, the United States, and the USSR, and the interest of professional astronomers was soon aroused by a series of remarkable discoveries.
Discrete sources of radio emission were cataloged in increasing numbers, and beginning in the 1950s many radio sources were identified with distant visible galaxies. In 1963 the continuing investigation of very small radio sources led to the discovery of quasi-stellar radio sources, called quasars (see Quasar), which, because of redshift of unprecedented magnitude, could be placed at enormous distances from the Earth. Soon afterward, in 1965, the American radio astronomers Arno Penzias and Robert W. Wilson announced the discovery of a 3 K (-454° F) cosmic background radio emission, which has many implications for theories of the origin and evolution of the universe (see Cosmology). An entirely new type of radio source, the pulsar, was discovered in 1968 and was quickly identified as a rapidly rotating neutron star (see Star).
For many years radio astronomers concentrated on studying relatively long wavelengths near 1 m (about 3.3 ft), for which large antenna structures and sensitive receivers were easy to build. As techniques were developed to build larger and more precise structures, and as sensitive short-wavelength receiving equipment was perfected, the wavelength bands down to 1 mm (about 0.04 in) received increased attention. At the same time, the development of space technology (see Space Exploration) allowed observations to be made at very long wavelengths from above the ionosphere, which is normally opaque to radiation longer than about 20 m (about 66 ft).

III

PRINCIPLES OF RADIO ASTRONOMY

Cosmic radio emission, insofar as is known, comes entirely from natural processes, although from time to time radio telescopes are also used to search (so far unsuccessfully) for possible sources of radio emission from extraterrestrial intelligence (see Exobiology). Several physical mechanisms are recognized that produce the observed radio emission.

A

Types of Emission

Because of the random motions of electrons, all bodies emit thermal, or heat, radiation characteristic of their temperature. Careful measurements of the intensity and spectrum of emissions are used to calculate the temperature of distant celestial bodies, such as the planets in the Earth’s solar system, as well as of hot clouds of ionized gas located throughout the Galaxy.
Radio astronomy measurements, however, are often concerned with the much more intense nonthermal emission arising from charged particles such as electrons and positrons moving through weak galactic and intergalactic magnetic fields. When the particle energy is so high that its velocity is close to the speed of light, the radio emission from these “ultra-relativistic” particles is referred to as synchrotron radiation, a term borrowed from the high-energy physics laboratory, where this type of radiation was first discovered.
Both the synchrotron (nonthermal) and thermal radio sources radiate over a wide range of wavelengths. By contrast, a third category of matter—excited atoms, ions, and molecules—radiate at discrete wavelengths characteristic of the atom or molecule and the state of excitation. Wide-range radio emission is referred to as continuum emission, and discrete radio emission as line emission.

B

Radio Telescopes

Arecibo Radio Telescope
The Arecibo Observatory in Puerto Rico contains the largest single stationary radio telescope in the world. Because it remains stationary, the Arecibo telescope uses Earth’s rotation to turn its field of view across the sky. Radio waves bounce off the bowl of the telescope and into the detecting platform suspended above the bowl.
Radio wavelengths are relatively long, extending from about 1 mm (about 0.04 in) to more than 1 km (about 0.6 mi), and radio telescopes must be extremely large in order to focus the incoming signals to produce a sharp radio image. The world’s largest stationary radio telescope, Arecibo Observatory in Puerto Rico, is a bowl-shaped dish 305 m (1000 ft) in diameter. The largest fully steerable parabolic dish-type antennas are 50 to 100 m (about 165 to 330 ft) in diameter, and they have a resolution of about 1 arc minute, equivalent to that of the unaided human eye at optical wavelengths. Incoming radio waves are focused by the parabolic surface onto a small horn antenna that leads to an extremely sensitive radio receiver. These receivers, although similar in principle to the home radio, are able to detect signals as weak as 10-17 W. The critical parts of the receiver are often cooled to temperatures close to absolute zero in order to obtain the best possible performance. For spectral line observations, specialized receivers are used that can be tuned to as many as 1000 frequencies simultaneously.

Radio Telescopes
The Very Large Array is a collection of parabolic dish antennas, located near Socorro, New Mexico. The 27 antennas are attached to a system of Y-shaped tracks; each track is 21 km (13 mi) in length. The individual signals from each telescope are combined into one high-resolution image, making the array the world's largest radio telescope.
In order to obtain higher resolution, arrays of antennas are used as interferometers (see Interferometer) giving resolutions of approximately 1 arc second, equivalent to that of large optical telescopes under ideal viewing conditions. The largest radio telescope of this type is the Very Large Array, or VLA, located on an isolated plain near Socorro, New Mexico. The VLA contains a total of 27 parabolic dishes, each 25 m (82 ft) in diameter, located along three 21-km (13-mi) arms in a Y configuration. Each antenna element contains its own receiver, and the signals from each receiver are sent to a central building where they are combined to form the high-resolution image by a technique that is known as aperture synthesis. Other interferometers may use antennas like huge television antennas. One such installation at Cambridge, England, uses 60 antennas to detect radiation at wavelengths of 2 m (6.6 ft).
Even higher resolutions may be achieved if individual antenna elements are spaced thousands of kilometers apart. With these spacings it becomes impractical to send the signals from each antenna directly to a common point. Instead, separate broadband tape recordings are made at each antenna, and the individual tapes are then shipped to a central processing facility. This technique of very long baseline interferometry (VLBI) involves using atomic clocks at each telescope to synchronize the individual recordings to an accuracy of better than one-millionth of a second. In this way, angular resolutions of one-thousandth of an arc second are achieved, equivalent to the apparent angular dimensions of a basketball at the distance of the moon. In 1984, the U.S. government appropriated funds for the construction of an installation called the very long baseline array (VLBA), a network of 10 radio antennas spread from the U.S.-Canadian border to Puerto Rico and from Hawaii to the U.S. Atlantic coast. The VLBA is expected to provide angular resolutions in the range of 200-millionths of an arc second. Canada and Australia are both planning similar programs.

IV

CLASSES OF RADIO SOURCES

Many discrete radio sources have been discovered and studied in our solar system, in our galaxy, and in the wide extent of the universe beyond our galaxy.

A

Solar System Radio Astronomy

The sun is the brightest radio source in the sky. Its radio emission is much more intense than would be expected from the thermal emission of its visible surface, which has a temperature near 6000 K (about 10,300° F). This is because most of the radio emission observed at longer radio wavelengths comes from the much hotter, but optically invisible, outer atmosphere, which has temperatures near 1,000,000 K (near 1,800,000° F). In addition to the thermal emission, numerous nonthermal storms and bursts occur, particularly during periods of high sunspot activity when the intensity of radio emission may dramatically increase by a factor of 1 million or more for brief periods of about an hour.
The only other source of natural nonthermal radio emission in the solar system is the planet Jupiter. At wavelengths near 15 m (about 49 ft), Jupiter emits strong bursts of radiation that come from relatively small regions, near the cloud surface, that rotate with the planet. The intensity of these bursts appears to be greatly influenced by the location of the satellite Io. In addition, Jupiter is surrounded by extensive radiation belts that radiate in the microwave band at wavelengths that are shorter than about 1 m (about 3.3 ft).
Thermal radiation has been observed to emanate from the surface or atmosphere of all of the planets except Pluto. These emissions have been used by instruments aboard spacecraft to derive information on planetary meteorological conditions and other phenomena.

B

Galactic Radio Sources

Radio Telescope Image
The Parkes 64-meter (210-foot) radio telescope in Australia produced this radio map of the Large Magellanic Cloud. This small, irregular galaxy is visible from the southern hemisphere. Regions of ionized hydrogen give off thermal emissions, shown as bright spots on this false-color image.
The Galaxy, or the Milky Way, emits radio waves as a result of synchrotron radiation from cosmic ray electrons moving through the weak galactic magnetic field. The 21-cm line emission from neutral hydrogen is also observed throughout the Galaxy. Small changes in the observed wavelength of the 21-cm line are caused by the motion of the hydrogen clouds toward or away from an observer. These changes are an example of the phenomenon known as the Doppler effect, or redshift. Clouds that are most distant from the center of the Galaxy revolve around the center with the greatest velocity, and observations of the Doppler effect are used to measure the velocity and locate the position of hydrogen clouds. In this way it has been possible to trace the shapes of the Milky Way’s spiral arms, which are not readily observed at optical wavelengths.
In addition to the diffuse background radiation, numerous discrete sources of radio emission exist in the Galaxy. These discrete sources include the following: supernova remnants, radio stars, emission nebulas, molecular clouds, and pulsars.
Supernova remnants are the clouds of debris remaining from stars that have exploded (see Supernova). Relativistic electrons produced in a supernova explosion are captured by the magnetic field surrounding the site of the explosion. As these electrons spiral around the magnetic field lines, they continue to radiate for thousands of years. In some cases the star itself continues to be a source of radio emission and is referred to as a radio star. Another important class of radio star comprises the binary (double) star systems that emit radio waves when mass is transferred from one component to the other. Radio stars are often X-ray sources as well.
Thermal radio emission is observed from clouds of ionized hydrogen (termed H II regions) located along the spiral arms of the Galaxy. When free electrons recombine with ions of hydrogen or other light elements, radio energy is released that can be observed as recombination lines in the radio portion of the spectrum.
Spectral lines also result from vibrational and rotational transitions of such interstellar molecules as water vapor (H2O), ammonia (NH3), formaldehyde (H2CO), and carbon monoxide (CO). More than 50 interstellar molecules are now known, including many complex and organic molecules. In some interstellar clouds, the radio molecular lines are unusually intense due to the maser (microwave amplification by the stimulated emission of radiation) effect (see Laser; Maser).
The intensity of most cosmic radio sources is steady, or only varies slowly with time. The pulsars, however, emit short periodic bursts or pulses of radiation about once per second. Although first discovered because of their intense pulsed radio emission, some were later found to emit optical and X-ray pulses as well. Pulsars are thought to form when stars like the sun collapse under their own gravity to dimensions of about 10 km (about 6 mi). The density then becomes extremely great, and electrons are stripped from their atoms, leaving a so-called neutron star.

C

Radio Galaxies

Most galaxies probably emit radio waves and do so at energies comparable to that of our own galaxy—about 1032 W. In the cases of the so-called radio galaxies, however, the radio emission is up to 100 million times stronger. Most of this energy originates not in the galaxies themselves but in clouds of superheated, ionized gases, or plasma, located hundreds of thousands or even millions of light-years away from the parent galaxy. These giant radio clouds may be 100 times the size of the galaxy itself and are among the largest known objects in the universe.
A great deal of energy is required to generate the powerful radio emissions from radio galaxies, and it may amount to a significant fraction of the total energy that would result from the nuclear burning of a whole galaxy. The origin of this energy and the manner in which it is converted to radio emissions have been major problems of astrophysics since the discovery of radio galaxies more than two decades ago.
Recent detailed pictures of radio galaxies, obtained with high resolution radio telescopes such as the VLA, often show a prominent jet of material connecting a bright, compact radio source at the galactic nucleus to the more extended radio lobes (clouds). It is widely speculated that these jets or beams transport energy away from the galactic nucleus to the radio-emitting plasma and that the source of energy lies in a massive object, possibly a black hole located at the galactic center. Frequently, a compact radio source is found at the center of radio galaxies. In one unusual radio galaxy observed in the mid-1980s, two bright clusters of stars near its center are emitting jets apparently braided together.

D

Quasars

Quasars (see Quasar) appear to radiate with the luminosity of hundreds of galaxies, but each quasar is smaller than a typical galaxy by a factor of nearly a million. Quasars have very large redshifts, and they are therefore believed to lie at great distances from the Milky Way. Because quasars appear to be so powerful, and because their radiation often varies rapidly, it was once thought they might be relatively nearby weak objects rather than distant powerful ones. However, evidence has accumulated supporting the cosmological interpretation of the redshifts. Radio galaxies, quasars, and bright objects called BL Lacertae objects are probably closely related phenomena.
Like the radio galaxies, some quasars are also surrounded by extended lobes of powerful radio emissions, but most of the radio emission from quasars usually comes from a bright core only a few light-years or less in diameter and coincident with the optically visible quasar.
When observed with very high resolution radio interferometers, this radio core is often found to consist of two or more smaller regions, which may appear to be moving away from each other with velocities considerably greater than the speed of light. Although these remarkably high velocities may seem at first to violate Albert Einstein’s special theory of relativity (see Relativity), they in fact can be explained as a result of motion just under the speed of light, which is directed almost toward the observer. Because the moving radio source is nearly catching up with the emitted radiation, the observed time interval between successive positions of relativistic jets of material appears shortened, and the velocity appears to be increased by a large factor over the true velocity. This phenomenon is termed apparent superluminosity.

V

COSMOLOGY

Because radio galaxies and quasars are such powerful radio sources, they can be detected from a great distance. Because of the long time it takes for signals to reach the Earth from distant radio sources, radio astronomers are able to see the universe as it appeared more than 10 billion years ago, or far back in time toward the origin of the universe—the so-called big bang. Unfortunately, determining the distance to a radio source is not possible from radio measurements alone, so that distinguishing between a powerful distant source and a relatively weak nearby one is impossible. The distance may be determined only if that source is optically identified with a galaxy or quasar that has a measurable redshift. Nevertheless, from studies of the statistical distribution of large numbers of radio sources, it appears that when the universe was only a few billion years old, the number of intense radio sources was much greater and their dimensions smaller.

Orbit astronomy and physics

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

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

II

LAWS OF MOTION

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

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

III

ORBITAL ELEMENTS

Elements of Orbits

Orbits of objects going around the sun are discussed in terms of their orientation with respect to three different planes. These are the plane of the orbit in question, the plane of the earth’s orbit (also known as the plane of the ecliptic), and the plane of the celestial equator. The elliptical orbit has center C and focus S. Six elements may be used to describe an orbit: size (periapsis distance SP), elongation (eccentricity e, which is the ratio CS/CP), longitude of the ascending node (angle Ω), argument of periapsis (angle ω), inclination (angle i), and the time when the orbiting body is at the periapsis.

Six orbital elements describe an orbit. The first two elements are size and elongation. The size of the orbit is given by the periapsis distance (SP) and the elongation of the orbit is given by the eccentricity ( e). For the ellipse in the accompanying figure, the eccentricity is the ratio CS/CP, where S is the focus and C the center of the ellipse. For elliptical orbits, e is greater than 0, but less than 1; for circular orbits, e is exactly 0; and for parabolic orbits, e is exactly 1. A body in a hyperbolic orbit—that is, when e is greater than 1—makes a single close passage to a central body and escapes along a so-called open orbit, never to return.

The next three orbital elements are concerned with the orbit's orientation. For this discussion, however, several parameters need to be defined: The reference plane for objects orbiting around the sun is the plane of the earth's orbit, also known as the plane of the ecliptic; the equinox (g) is the northbound intersection of the earth's orbit and the plane of the celestial equator; and the ascending node (N) is the northbound intersection of the orbit in question and the reference plane (see Coordinate System).

The three orbital elements that describe an orbit's orientation are the inclination (i), the longitude of the ascending node (Ω), and the argument of the periapsis (ω). The inclination is the angle between the reference plane and the orbit's plane. The longitude of the ascending node is the angle in the reference plane between the equinox and the ascending node. The argument of the periapsis measures the angular displacement in the plane of the orbit between the ascending node and the line that passes through the center of the orbit (C) and the periapsis (P). Finally, the sixth orbital element is the time when the celestial body in question is at the periapsis.

An orbit can also be described in terms of its semimajor axis (AC, CP, or a). This axis is half the long axis (AP) of the ellipse, or half the distance between the points of periapsis (P) and the apoapsis (A). The semimajor axis is longer than the periapsis distance (SP) and shorter than the apoapsis distance (AS), by an amount (CS) that is equal to the product of the semimajor axis and the eccentricity: CS = e(AC) = e(CP) = ea

IV

PERTURBATIONS

An orbit is perturbed when the forces are more complex than those between two spherical bodies. (Kepler's laws are exact only for unperturbed orbits.) The attraction between planets causes their elliptical orbits to change with time. The sun, for example, perturbs the lunar orbit by several thousand kilometers. Atmospheric drag causes the orbit of an earth satellite to shrink, and the oblate shape of the earth causes the direction of its node and perigee to change. The theory of relativity developed by German-born American physicist Albert Einstein explains an observed perturbation in the perihelion of the planet Mercury.

 

Interstellar Matter

Interstellar Matter

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

Crab Nebula

An exploding supernova star leaves behind a rapidly expanding cloud of gaseous material called a nebula. The Crab Nebula was produced when a star in the Milky Way galaxy exploded. Light from the supernova reached the earth in 1054. At the center of the Crab Nebula, a spinning pulsar star emits light of varying brightness. This illuminates the gaseous particles of the nebula, giving a cloudlike appearance.

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.

Orion Nebula

Located in the constellation Orion, 1,270 light years away from Earth, the Orion Nebula (M42) is a bright cloud of gas and dust where stars are in the process of being born. The Orion Nebula looks bright because it reflects light from the multiple star Theta Orionis, alongside it in this photograph. Radiation from new stars in the nebula lights up hydrogen in its outer regions, causing the gas to glow with its characteristic red color.

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.

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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).

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Other Galaxies

Galaxy M100

Stars make up only a part of the matter in a galaxy—some of the matter is in the form of interstellar dust. In this Hubble Space Telescope image of the core of galaxy M100, interstellar dust appears both as bright, hazy regions and dark areas. Interstellar matter can reflect, block, or absorb starlight.

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.

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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.

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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.

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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.

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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.

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Infrared Radiation

Stellar Nursery in Infrared

The Infrared Space Observatory (ISO) detected infrared radiation in space. It could see through clouds of interstellar dust because infrared radiation is not blocked by the dust as much as visible light is. The ISO took this picture of new stars forming out of a cloud of dust and gas. The stars are not visible to optical telescopes because the visible light that they emit is blocked by the dust surrounding them.

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.

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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.

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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.

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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.

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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.