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Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.

B

BACKGROUND RADIATION

Background radiation is the microwaves permeating the universe that are probably the remnants of the Big Bang. This background radiation accounts for a temperature of 2.7 K in space.


BAILY'S BEADS

Baily's beads (often spelled Bailey's beads) are bead-like bursts of light that appear about 15 seconds before and after totality during a solar eclipse. Baily's beads are caused by light shining through valleys on the edge of the moon. They were named for the British astronomer Francis Baily (1774-1844), one of the founders of the Royal Astronomical Society.

BAR

A bar is a unit of pressure. One bar is equal to the pressure that the Earth's atmosphere exerts at sea level.


BARNARD, E. E.

Edward Emerson Barnard (1857-1923) was an American astronomer who discovered Barnard's star (the star system second-closest to us) in 1916, 16 comets, and Amalthea, a moon of Jupiter, in 1892.


BARRED SPIRAL GALAXY

A barred spiral galaxy (abbreviated SB) is a spiral galaxy whose center is elongated or bar-shaped.


BAROMETER

A barometer is a device that measures air (barometric) pressure. It measures the weight of the column of air that extends from the instrument to the top of the atmosphere. There are two types of commonly-used barometers, mercury and aneroid (meaning "fluidless") . Early water barometers (also known as "storm glasses") date from the 17th century. The mercury barometer was invented by the Italian physicist Evangelista Torricelli (1608 - 1647), a pupil of Galileo, in 1643. Toricelli inverted a glass tube filled with mercury into another container of mercury; the mercury in the tube "weighs" the air in the atmosphere above the tube. The aneroid barometer (using a spring balance instead of a liquid) was invented by the French scientist Lucien Vidie in 1844.


BARYCENTER

The barycenter is the center of mass around which a system of objects rotate. For example, the Earth and the moon both revolve around their barycenter, which is 1707 km inside the Earth along the line between them. Pluto and its moon Charon revolve around their barycenter, which is 700 km inside Pluto along the line between them.

BARYON

Baryons (heavy particles) include protons and neutrons.

BASALT

Basalt is a type of igneous rock.

BAYER, JOHANNES

Johannes Bayer (1564-1617) was a Bavarian (German) astronomer who first named stars by assigning them to constellations and giving them Greek letters (alpha, beta, gamma, delta, etc.), in magnitude classes (by decreasing brightness). Bayer published Uranometria (a detailed star chart/catalog) in 1603.

BAYER SYSTEM

In each constellation, every star is classified using the Bayer system by assigning then Greek letters (alpha, beta, gamma, delta, epsilon, zeta, eta, theta, iota, kappa, lambda, mu, nu, xi, omicron, pi, rho, sigma, tau, upsilon, phi, chi, psi, and omega), in order of decreasing brightness. The Bayer system cannot go beyond 24 (the number of letters in the Greek alphabet). The brightest star in a constellation is alpha, the second-brightest is beta, the third is gamma, etc. Johannes Bayer (1564-1617) was a Bavarian (German) astronomer devised this system.

BEG, ULUGH

Ulugh Beg (1359-1449) was a Persian astronomer who cataloged 1012 stars and made detailed observations of the moon and planets. He also determined the inclination of the plane of the ecliptic.

BELINDA

Belinda is the ninth moon of Uranus. It has a diameter of about 68 km and orbits at roughly 75,255 km from Uranus; this moon was discovered in 1986 by Voyager 2. Belinda was named for a character in Alexander Pope's The Rape of the Lock.

BELL, JOCELYN

Susan Jocelyn Bell Burnell (1943- ) is an astronomer who discovered the existence of pulsars in 1967, while she was a graduate student at Cambridge University. A pulsar is a rapidly spinning neutron star that emits energy in pulses. BEll's graduate advisor (Anthony Hewish) was given a share of the 1974 Nobel Prize, but Bell was ignored. No one had any idea what these unusual objects were at the time, so the name little green men (LGM) was used. Soon, Thomas Gold suggested that pulsars were rapidly-spinning neutron stars, the remnants of a supernova.

BESSEL, FRIEDRICH

Friedrich Wilhelm Bessel (July 22, 1784-March 17, 1846) was a German astronomer and mathematician who cataloged about 50,000 stars, mathematically predicted the existence of a planet beyond Uranus (1840), was the first person to see the "motion" of a star due to parallax (observing 61 Cygni), was the first person to calculate the distance to a star (observing 61 Cygni - 10.3 light-years from Earth), realized that there were dark stars, devised the famous Bessel function (a mathematical function), and made many other contributions to science.

BETELGEUSE

(pronounced "beetle juice") Betelgeuse (alpha Orionis) is the second-brightest star in the constellation called Orion and one of the brightest stars in the sky. It is a supergiant star, reddish in color, and over 600 million miles in diameter (almost 1,000 times bigger than the Sun but cooler than the Sun). Betelgeuse is about 14,000 times brighter than the Sun. If Betelgeuse were at the center of our Solar System, it would extend beyond the orbit of Jupiter. It is 520 light-years from Earth. It is a variable star, varying in magnitude from 0.3 to 1.2 over a period of about 7 years, averaging about 0.70. It is the only star (other than our sun) for which we have surface images.

BIANCA

Bianca is the third moon of Uranus. It has a diameter of about 44 km and orbits at roughly 59,165 km from Uranus; this moon was discovered in 1986 by Voyager 2. Bianca was named for a character in Shakespeare's Taming of the Shrew.

BIG BANG THEORY

The big bang theory states that the universe began as a tiny but powerful explosion of space-time roughly 13.7 billion years ago (plus or minus 1 percent). In 1965, Arno Penzias and Robert Wilson used a horn antenna (in Crawford Hill, N.J.) and discovered the cosmic microwave background (CMB) radiation with a temperature of 2.7 Kelvin; this CMB was left-over from the early moments of the universe after the Big Bang (this was predicted by George Gamow and Ralph Alpher, in 1948). Contrast it with the Steady-State Theory.

BIG CRUNCH

The big crunch is the collapse of all of the matter in the universe - a reversal of the big bang. This is one hypothesized future of the universe, assuming that the universe is closed (having a mass density exceeding the critical mass density).


BIG DIPPER

The Big Dipper is a group of 7 stars (an asterism) contained in the Northern Hemisphere constellation called Ursa Major (The Great Bear). The two brightest stars in the Big Dipper (Dubhe and Merak) "point" to the North Star, Polaris.
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BILLION
A billion is a thousand million. Multicellular life evolved on Earth about a billion years ago. The universe is about 17 billion years old.


BINARY STAR

A binary star is really two stars that rotate around a common center of mass. About half of all stars are in a group of at least two stars.

BLACK BODY

A black body is an idealized body that is a perfect radiator and perfect absorber of electromagnetic radiation. A black body not only absorbs all wavelengths of energy and radiates at all wavelengths, but it does this at the maximum possible intensity for any given temperature. It doesn't necessarily look black. A star is a good approximation to a black body since stellar gases are very good absorbers of energy.

BLACK BODY RADIATION

Black body radiation is the radiation produced by a black body.

BLACK DROP EFFECT

The black drop effect is an optical effect that happens early in a transit of Venus, when Venus just travels in front of the solar disk. The light from the Sun is bent around Venus (it is refracted by Venus' dense atmosphere) making Venus look stretched-out.

BLACK DWARF

A black dwarf is a small, very dense, cold, dead star. It is made mostly of carbon. This dark star is what remains after a red giant star loses its outer layers, forming a planetary nebula and then a white dwarf. The nuclear core of a black dwarf is depleted. Black dwarfs are about the size of the Earth (but tremendously heavier)! Our sun will someday turn into a black dwarf.

BLACK HOLE

A black hole is a massive object (or region) in space that is so dense that within a certain radius (the Schwarzschild radius, which determines the event horizon), its gravitational field does not let anything escape from it, not even light. It is thought that giant stars (those with a mass over 3 times the mass of the Sun) will evolve into red supergiants, then supernova , and then black holes. It is thought that the typical black holes has a mass of roughly 10 times that of the Sun, but the range must be huge. For a typical black hole with a mass 10 times that of the Sun, the Schwarzschild radius would be roughly 18.6 miles (30 km). The phrase 'black hole' was coined by the physicist John Archibald Wheeler; before Wheeler, black holes were called 'frozen stars.' Astronomers think that there may be a black hole at the center of each galaxy.


BLAZAR

A blazar is a type of extreme quasar. These extremely energetic objects emit jets of gamma rays and other electromagnetic radiation.


BLUE GIANT STAR

A blue giant is a huge, very hot, very luminous, blue star. It is not a main sequence star but a post-main-sequence star. These incredibly hot stars burn helium. These giants have the spectral type O or B and are very rare and very bright. Blue giants have at least 18 times the mass of the Sun. Examples include Rigel and Regulus.


BLUE MOON

When a single month has two full moons, the second full moon is called a Blue Moon. Another definition of the blue mon is the third full moon that occurs in a season of the year which has four full moons (usually each season has only three full moons.)

BLUE SHIFT

The blue shift is a decrease in the wavelength of the light that is emitted from an object that is moving toward us. This decrease in wavelength makes the object appear to be bluer than it actually is. For example, when a star is travelling towards Earth, its light appears bluer (the light waves are shortened, shortening the wavelength). Compare with red shift.

BODE'S LAW

Bode's "Law" (also known as the Titus-Bode Law) is an interesting mathematical coincidence, and not a physical law. It is a numerical series that matches planetary distances from the Sun. The Titus-Bode series predicts the positions of all the planets in our Solar System except Neptune (plus it predicts a planet where the asteroid belt is). This relationship is named for Johann Titus and the German astronomer Elert Bode, who did their work in the late 1700's. For the Titus-Bode series for our Solar System, start with 0.4 (A.U.), then form the series by adding 0, 0.3, 0.6, 1.2, 2.4, etc. (doubling each time after the first). The results it predicts are: Mercury: 0.4 A.U., Venus 0.7 A.U., Earth 1.0 A.U., Mars 1.6 A.U., Asteroid Belt 2.8 A.U., Jupiter 5.2 A.U., Saturn 10.0 A.U., Uranus 19.6 A.U.


BOLIDE

A bolide is a meteor, asteroid, or comet that hits the Earth (or other planet or moon) and explodes.

BOLOMETER

A bolometer is an instrument that measures the total amount of radiant energy (not just visible light) received from a star or other celestial object.

BOLOMETRIC MAGNITUDE

The bolometric magnitude of a star includes the total radiation output of the star (not just visible light). It is the magnitude that a star would have if all of its energy were included in the measurement. It includes the entire electromagnetic spectrum: X-rays, ultraviolet, infrared, gamma rays, etc. The ordinary magnitude of a star only takes into account the visible light that the star emits.

BOND, WILLIAM C.

William Cranch Bond (1789-1859) was an American astronomer who, with William Lassell, discovered Saturn's moon Hyperion in 1848. He was the first director of the Harvard College Observatory.


BONDAR, ROBERTA

Roberta Lynn Bondar (1945-) was the first Canadian woman to go into space. Bondar was the payload specialist on NASA's space shuttle Discovery during Mission STS-42, January 22-30, 1992. A neurobiologist, Bondar is a professor at University of Western Ontario, Ontario, Canada.


BOÖTES

Boötes (the herdsman) is a large constellation in the Northern Hemisphere. The brightest star in Boötes is Arcturus, a red giant that is the fourth brightest star in the sky.


BOW SHOCK

A bow shock is a supersonic shock wave that is formed as the solar wind interacts with the outermost layer of a planet's magnetosphere (or a highly conducting ionosphere). At this boundary onthe sun-ward side, the solar wind plasma is deflected around the planet and is slowed to subsonic speed by the planet's magnetic field.


BRAHE, TYCHO

Tycho (Tyge) Brahe (1546-1601) was a Danish astronomer who made extensive and seminal calculations of the orbits of the planets. His work (done without a telescope) was the basis upon which Kepler made his revolutionary orbital formulas. He worked with Kepler for a few years before his death, and Kepler edited his principal work, Astronomiae Instauratae Progymnasmata, ("Exercises Toward a Restored Astronomy"). He also observed the "new star" (really a nova) in Cassiopeia in 1572. He observed a comet in 1577 and was realized that it was not in the atmosphere, but was in space. He corrected most astronomical quantities. Although he incorrectly believed that the Earth was at the center of the universe and that the Sun and the stars revolved around the Earth, he did accept part of the Copernican theory, that the other planets orbit the Sun. He had a nose made of gold; he lost most of his real nose in a duel about mathematics!

BRECCIA

Breccia is a type of rock that is composed of rough, angular pieces of broken-up, older rocks. Impact craters produce breccia.


BRIGHT-LINE SPECTRUM

A bright-line spectrum (also called an emission spectrum) is a discontinuous spectrum (discrete bright bands) that is emitted by atoms or molecules. The emission spectrum is characteristic of the chemicals which are emitting the photons. It looks like a series of bright lines on a black background (this is called spectroscopy).
Magnitude Difference Brightness Ratio
0 1
1 2.5
2 6.3
2.5 10
4 40
5 100
7.5 1,000
10 10,000

BRIGHTNESS RATIO

The brightness ratio is a comparison of the brightness of two celestial objects (like stars or planets). The brightness ratio is defined as 2.512 raised to the difference of the magnitudes of the objects (since the magnitude scale is logarithmic and the difference in each successive magnitude is a factor of about 2.512 times). For example, a 1st magnitude star is 2.512 times brighter than a 2nd magnitude star (their brightness ration is 2.512). For another example, if two objects have magnitudes of 2 and 7 (a difference of 5), their brightness ratio is 2.512 raised to the 5th power, which is about 100.

BROWN DWARF

A brown dwarf is a "star" whose mass is too small to have nuclear fusion occur at its core (the temperature and pressure at its core are insufficient for fusion) - a failed star. A brown dwarf is not very luminous. It is usually regarded as having a mass between 1028 kg and 84 x 1028 kg. It continues to cool down and contract, turning into a compact dark object that is not easily detected.

BRUNO, GIORDANO

Giordano (Filippo) Bruno (1548-1600) was an Italian philosopher, poet, and priest who spread the ideas of Copernicus and his own ideas that there were an infinity of worlds in the universe and that the stars were other suns. He was burned at the stake for heresy.

B-TYPE STAR

B-type stars are very hot and blue; they burn helium. Temperatures of B-type stars range from 11,000 - 25,000 K. Their mass averages 18 times the mass of the Sun and the average luminosity is 20,000 times that of the Sun. Examples include Rigel, Spica, and Regulus.
BUNSEN, ROBERT
The laboratory Bunsen burner was invented by Robert Wilhelm Bunsen in 1855. Bunsen (1811-1899) was a German chemist and teacher. He invented the Bunsen burner for his research in isolating chemical substances - it has a high-intensity, non-luminous flame that does not interfere with the colored flame emitted by chemicals being tested.

For more information on Bunsen, click here.


BURNEY, VENETIA

The name Pluto was suggested by Venetia Burney of England, who was 11 years old at the time. She suggested the name to her grandfather, who was Librarian at Oxford. He passed her idea on to the astronomers who were trying to name the newly-discovered planet. The name Pluto was announced on May 30, 1930, by the Lowell Observatory.
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BYA
"bya" stands for billions of years ago.
Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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