history of astronomy

The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:
The Answer
The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when Galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the Newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.
There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, Ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, Copernicus (Polish, 1500s) proposed the heliocentric system, Kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin Hubble (American, died ~1940) who discovered that the Universe is expanding.
The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho Brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.
To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:









Imagine the Universe! Dictionary
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[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 ]
(Note - Greek letters are written out by name - alpha, beta etc.)
A
absorption
The process in which light or other electromagnetic radiation gives up its energy to an atom or molecule.
absorption line spectrum
A spectrum showing dark lines at some narrow color regions (wavelengths). The lines are formed by atoms absorbing light, which lifts their electrons to higher orbits.
accretion
Accumulation of dust and gas onto larger bodies such as stars, planets and moons.
accretion disk
A relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.
active galactic nuclei (AGN)
A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.
Tell me about AGN!
Tell me more about AGN!
angstrom
A unit of length equal to 0.00000001 centimeters. This may also be written as 1 x 10-8 cm (see scientific notation).
angular momentum
A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A spinning body also possesses spin angular momentum.
apastron
The point of greatest separation between two stars which are in orbit around each other. See binary stars. Opposite of periastron.
aphelion
The point in its orbit where a planet is farthest from the Sun. Opposite of perihelion.
apoapsis
The point in an orbit when the two objects are farthest apart. Special names are given to this orbital point for commonly used systems: see apastron, aphelion, and apogee.
apogee
The point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee.
arc minute
An angular measurement equal to 1/60th of a degree.
arc second
An angular measurement equal to 1/60th of an arc minute or 1/3600th of a degree.
Ariel V
A UK X-ray mission, also known as UK-5
ASCA
The Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
ASD
Astrophysics Science Division, located at NASA's Goddard Space Flight Center. The scientists, programmers and technicians working here study the astrophysics of objects which emit cosmic ray, x-ray and gamma-ray radiation.
ASM
All Sky Monitor. An instrument designed to observe large areas of the sky for interesting astronomical phenomena. An ASM measures the intensity of many sources across the sky and looks for new sources. Many high-energy satellites have carried ASM detectors, including the ASM on Vela 5B, Ariel V, and the Rossi X-ray Timing Explorer.
Astro-E/Astro-E2
A X-ray/gamma-ray mission built jointly by the United States and Japan. Astro E was destroyed in February 2000, when a Japanese M-5 rocket failed to lift the instrument into orbit. A replacement mission, Astro-E2, was succesfully launched in July 2005, and subsequently renamed Suzaku.
Tell me more about Astro-E.
Tell me more about Astro-E2/Suzaku.
astronomical unit (AU)
149,597,870 km; the average distance from the Earth to the Sun.
astronomy
The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.
astrophysics
The part of astronomy that deals principally with the physics of the universe, including luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium.
atmosphere
The gas that surrounds a planet or star. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.
AXAF
The Advanced X-ray Astrophysics Facility. AXAF was renamed Chandra X-ray Observatory, CXO, and launched in July 1999.
Tell me more about AXAF.
B
Balmer lines (J. Balmer)
Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second (or first excited) state and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer.
baryon
Any of the subatomic particles which interact via the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.
BATSE
BATSE (Burst and Transient Source Experiment) was an instrument aboard the Compton Gamma Ray Observatory that detected and located gamma-ray bursts in the sky.
BBXRT
The Broad Band X-Ray Telescope, which was flown on the Astro-1 space shuttle flight (Dec. 1990)
Tell me more about BBXRT.
Big Bang
A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion (referred to as the "Big Bang").
binary stars
Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.
Tell me about X-ray binary stars.
Tell me more about X-ray binary stars.
black dwarf
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.
black hole
An object whose gravity is so strong that not even light can escape from it.
Tell me about X-rays from black holes.
Tell me about gamma rays from black holes and neutron stars.
Tell me more about black holes.
black-hole dynamic laws; laws of black-hole dynamics
1. First law of black hole dynamics:
For interactions between black holes and normal matter, the conservation laws of mass-energy, electric charge, linear momentum, and angular momentum, hold. This is analogous to the first law of thermodynamics.
2. Second law of black hole dynamics:
With black-hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease. This is analogous to the second law of thermodynamics, with the surface areas of the black holes being a measure of the entropy of the system.
blackbody radiation
Blackbody radiation is produced by an object which is a perfect absorber of heat. Perfect absorbers must also be perfect radiators. For a blackbody at a temperature T, the intensity of radiation emitted I at a particular energy E is given by Plank's law:
I(E,T) = 2 E3[h2c2(eE/kT - 1)]-1

where h is Planck's constant, k is Boltzmann's constant, and c is the the speed of light.
blackbody temperature
The temperature of an object if it is re-radiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not re-radiate all the excess heat and the leftover will go toward increasing its temperature.
blueshift
An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them. See also Doppler effect.
bolometric luminosity
The total energy radiated by an object at all wavelengths, usually given in joules per second (identical to watts).
Boltzmann constant; k (L. Boltzmann)
A constant which describes the relationship between temperature and kinetic energy for molecules in an ideal gas. It is equal to 1.380622 x 10-23 J/K (see scientific notation).
Brahe, Tycho (1546 - 1601)
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations of Mars in the last quarter of the 16th century formed the basis for Johannes Kepler's laws of planetary motion. Brahe lost his nose in a dual in 1566 with Manderup Parsberg (a fellow student and nobleman) at Rostock over who was the better mathematician. He died in 1601, not of a burst bladder as legend suggests, but from high levels of mercury in his blood (which he may have taken as medication after falling ill from the infamous meal). Show me a picture of Tycho Brahe !
bremsstrahlung
"Braking radiation", the main way very fast charged particles lose energy when traveling through matter. Radiation is emitted when charged particles are accelerated. In this case, the acceleration is caused by the electromagnetic fields of the atomic nuclei of the medium.
C
calibration
A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.
cataclysmic variable (CV)
Binary star systems with one white dwarf star and one normal star, in close orbit about each other. Material from the normal star falls onto the white dwarf, creating a burst of X-rays.
Tell me more about Cataclysmic Variables.
Cepheid Variable
A type of variable star which exhibits a regular pattern of changing brightness as a function of time. The period of the pulsation pattern is directly related to the star's intrinsic brightness. Thus, Cepheid variables are a powerful tool for determining distances in modern astronomy.
Tell me more about Cepheid Variables.
CGRO
The Compton Gamma Ray Observatory
Tell me more about CGRO.
Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July 1999, and named after S. Chandrasekhar. It was previously named the Advanced X-ray Astrophysics Facility (AXAF).
Chandrasekhar, S. (1910 - 1995)
Indian astrophysicist reknowned for creating theoretical models of white dwarf stars, among other achievements. His equations explained the underlying physics behind the creation of white dwarfs, neutron stars and other compact objects.
Chandrasekhar limit
A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star.
cluster of galaxies
A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.
collecting area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.
Compton effect (A.H. Compton; 1923)
An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.
Tell me more about Dr. Compton and the Compton Effect.
Tell me how gamma-ray astronomers use the Compton effect.
Copernicus
NASA ultraviolet/X-ray mission, also known as OAO-3
Tell me more about the Copernicus mission.
Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun (the "heliocentric" theory). This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the center of the universe, and all objects, including the sun, circle it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his model. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century B.C., a fact known to Copernicus but long ignored by others prior to him.). Show me a picture of Nicholas Copernicus !
corona (plural: coronae)
The uppermost level of a star's atmosphere. In the sun, the corona is characterized by low densities and high temperatures (> 1,000,000 degrees K).
Tell me about X-rays from the Sun's corona.
Tell me about X-rays from other stellar coronae.
COS-B
A satellite launched in August 1975 to study extraterrestrial sources of gamma-ray emission.
Tell me more about COS-B.
cosmic background radiation; primal glow
The background of radiation mostly in the frequency range 3 x 108 to 3 x 1011 Hz (see scientific notation) discovered in space in 1965. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself.
cosmic rays
Atomic nuclei (mostly protons) and electrons that are observed to strike the Earth's atmosphere with exceedingly high energies.
cosmological constant; Lambda
A constant term (labeled Lambda) which Einstein added to his general theory of relativity in the mistaken belief that the Universe was neither expanding nor contracting. The cosmological constant was found to be unnecessary once observations indicated the Universe was expanding. Had Einstein believed what his equations were telling him, he could have claimed the expansion of the Universe as perhaps the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of my life".
cosmological distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of spacetime could become apparent. Possible cosmological effects include time dilation and redshift.
cosmological redshift
An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself. Compare Doppler effect.
cosmology
The astrophysical study of the history, structure, and dynamics of the universe.
CXO
The Chandra X-ray Observatory. CXO was launched by the Space Shuttle in July 1999, and named for S. Chandrasekhar.
Tell me more about CXO.
D
dark matter
Name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now, has escaped all detections. There are many theories on what dark matter could be. Not one, at the moment is convincing enough and the question is still a mystery.
de Broglie wavelength (L. de Broglie; 1924)
The quantum mechanical "wavelength" associated with a particle, named after the scientist who discovered it. In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.
Declination
A coordinate which, along with Right Ascension, may be used to locate any position in the sky. Declination is analogous to latitude for locating positions on the Earth, and ranges from +90 degrees to -90 degrees.
deconvolution
An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky. For example, the optical analog would be to remove the spikes and halos which often appear on images of bright stars because of light scattered by the telescope's internal supports.
density
The ratio between the mass of an object and its volume. In the metric system, density is measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0 gm/cm3; iron is 7.9 gm/cm3; lead is 11.3 gm/cm3.
Dewar
A container (akin to a thermos bottle) that keeps cold material cold. In astronomy, these are often used for liquid nitrogen (at 77K), but can also be used for solid neon (17K) or liquid helium (4.2K). Some astronomical detectors work better at cold temperatures.
disk
(a) A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. (b) The apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope.
Doppler effect (C.J. Doppler)
The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much the frequency changes depends on how fast the object is moving toward or away from the receiver. Compare cosmological redshift.
dust
Not the dust one finds around the house (which is typically fine bits of fabric, dirt, and dead skin cells). Rather, irregularly shaped grains of carbon and/or silicates measuring a fraction of a micron across which are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.
dust tail
A stream of dust particles emitted from the nucleus of a comet. It is the most visible part of a comet.


http://imagine.gsfc.nasa.gov/docs/dict_ad.html#A





Q
quasar
An enormously bright object at the edge of our universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.
quasi-stellar source (QSS)
Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of very large redshift that is a strong source of radio waves; presumed to be extragalactic and highly luminous.
R
radial velocity
The speed at which an object is moving away or toward an observer. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky.
radian; rad
The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57o.
radiation
Energy emitted in the form of waves (light) or particles (photons).
radiation belt
Regions of charged particles in a magnetosphere.
radio
Electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth; the atmosphere of the Earth is transparent to radio waves with wavelengths from a few millimeters to about twenty meters.
Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish the location of objects which are near each other. It begins with the assumption that the central ring of one image should fall on the first dark ring of another image; for an objective lens with diameter d and employing light with a wavelength lambda (usually taken to be 560 nm), the resolving power is approximately given by
1.22 x lambda/d
Rayleigh-Taylor instabilities
Rayleigh-Taylor instabilities occur when a heavy (more dense) fluid is pushed against a light fluid -- like trying to balance water on top of air by filling a glass 1/2 full and carefully turning it over. Rayleigh-Taylor instabilities are important in many astronomical objects, because the two fluids trade places by sticking "fingers" into each other. These "fingers" can drag the magnetic field lines along with them, thus both enhancing and aligning the magnetic field. This result is evident in the example of a supernova remnant in the diagram below, from Chevalier (1977):

red giant
A star that has low surface temperature and a diameter that is large relative to the Sun.
redshift
An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer. See also Doppler effect.
reflection law
For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
relativity principle
The principle, employed by Einstein's relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the founding principles of special relativity.
relativity, theory of
Theories of motion developed by Albert Einstein, for which he is justifiably famous. Relativity More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than Newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.)
resolution (spatial)
In astronomy, the ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the resolution of the telescope.
resolution (spectral or frequency)
Similar to spatial resolution except that it applies to frequency, spectral resolution is the ability of the telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope.
resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
retrograde
The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite sense from the great majority of solar system bodies.
revolution
The movement of one celestial body which is in orbit around another. It is often measured as the "orbital period."
Right Ascension
A coordinate which, along with declination, may be used to locate any position in the sky. Right ascension is analogous to longitude for locating positions on the Earth.
Ritter, Johann Wilhelm (1776 - 1810)
Ritter is credited with discovering and investigating the ultraviolet region of the electromagnetic spectrum.
Roche limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At less than this distance the tidal forces of the larger object would break up the smaller object.
Roche lobe
The volume around a star in a binary system in which, if you were to release a particle, it would fall back onto the surface of that star. A particle released above the Roche lobe of either star will, in general, occupy the `circumbinary' region that surrounds both stars. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point. If a star in a close binary system evolves to the point at which it `fills' its Roche lobe, theoretical calculations predict that material from this star will overflow both onto the companion star (via the L1 point) and into the environment around the binary system.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
Tell me more about Wilhelm Röntgen
ROSAT
Röntgen Satellite
Tell me more about ROSAT
rotation
The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."
S
SAS-2
The second Small Astronomy Satellite: a NASA satellite launched November 1972 with a mission dedicated to gamma-ray astronomy.
Tell me more about SAS-2
SAS-3
The third Small Astronomy Satellite: a NASA satellite launched May 1975 to determine the location of bright X-ray sources and search for X-ray novae and other transient phenomena.
Tell me more about SAS-3
satellite
A body that revolves around a larger body. For example, the moon is a satellite of the earth.
Schwarzschild black hole
A black hole described by solutions to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916. The solutions assume the black hole is not rotating, and that the size of its event horizon is determined solely by its mass.
Schwarzschild radius
The radius r of the event horizon for a Schwarzschild black hole.
scientific notation
A compact format for writing very large or very small numbers, most often used in scientific fields. The notation separates a number into two parts: a decimal fraction, usually between 1 and 10, and a power of ten. Thus 1.23 x 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 x 10-8 means 5.67 divided by 10 to the eighth power or 0.0000000567.
second; s
The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. A nanosecond is equal to one-billionth (10-9) of a second.
semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by
rp = a(1-e) and ra = a(1+e).
sensitivity
A measure of how bright objects need to be in order for that telescope to detect these objects. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.
Seyfert galaxy
A spiral galaxy whose nucleus shows bright emission lines; one of a class of galaxies first described by C. Seyfert.
shock wave
A strong compression wave where there is a sudden change in gas velocity, density, pressure and temperature.
singularity
In astronomy, a term often used to refer to the center of a black hole, where the curvature of spacetime is maximal. At the singularity, the gravitational tides diverge; no solid object can even theoretically survive hitting the singularity. Mathematically, a singularity is a condition when equations do not give a valid value, and can sometimes be avoided by using a different coordinate system.
soft x-ray
Low energy x-rays, often from about 0.1 keV to 10 keV. The dividing line between soft and hard x-rays is not well defined and can depend on the context.
solar flares
Violent eruptions of gas on the Sun's surface.
solar mass
A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 Msun = 2 x 1033 grams.
special relativity
The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence. See also general relativity.
spectral line
Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).
spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.
A Dispersive Spectrometer is like a prism. It scatters light of different energies to different places. We measure the energy by noting where the X-rays go. A Non-Dispersive Spectrometer measures the energy directly.
spectroscopy
The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry that goes on in stars and in interstellar clouds.
spectrum (plural: spectra)
A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.
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speed of light (in vacuum)
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,282 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.
star
A large ball of gas that creates and emits its own radiation.
star cluster
A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is equal to 5.6697 x 10-8 Watts per square meter per degree Kelvin to the fourth power (see scientific notation).
Stefan-Boltzmann law (Stefan, L. Boltzmann)
The radiated power P (rate of emission of electromagnetic energy) of a hot body is proportional to the radiating surface area, A, and the fourth power of the thermodynamic temperature, T. The constant of proportionality is the Stefan-Boltzmann constant.
stellar classification
Stars are given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.
stellar wind
The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel.
steradian; sr
The supplementary SI unit of solid angle defined as the solid central angle of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.
supernova (plural: supernovae)
(a)The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).
(b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to the Chandrasekhar limit. These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.
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Tell me about gamma rays from supernovae
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sunspots
Cooler (and thus darker) regions on the sun where the magnetic field loops up out of the solar surface.
Suzaku
A Japanese X-ray satellite observatory for which NASA provided X-ray mirrors and an X-ray Spectrometer using a calorimeter design. Suzaku (formerly known as Astro-E2) was successfully launched in July 2005.
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SXG
The Spectrum X-Gamma mission
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Swift
Swift is a NASA mid-sized mission whose primary goal is to study gamma-ray bursts and address the mysteries surrounding their nature, origin, and causes. Swift launched November 20, 2004.
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synchronous rotation
Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.
synchrotron radiation
Electromagnetic radiation given off when very high energy electrons encounter magnetic fields.
Systéme Internationale d'Unités (SI)
The coherent and rationalized system of units, derived from the MKS system (which itself is derived from the metric system), in common use in physics today. The fundamental SI unit of length is the meter, of time is the second, and of mass is the kilogram.
T
Tenma
The second Japanese X-ray mission, also known as Astro-B.
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Thomson, William 1824 - 1907
Also known as Lord Kelvin, the British physicist who developed the Kelvin temperature scale and who supervised the laying of a trans-Atlantic cable. Show me a picture of Lord Kelvin!
time dilation
The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both special and general relativity, and is quite pronounced for speeds approaching the speed of light, and in regions of high gravity.
U
Uhuru
NASA's first Small Astronomy Satellite, also known as SAS-1. Uhuru was launched from Kenya on 12 December, 1970; The seventh anniversary of Kenya's independence. The satellite was named "Uhuru" (Swahili for "freedom") in honor of its launch date.
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ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.
universal constant of gravitation; G
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity. It is equal to 6.67428 x 10-11 m3 / kg-sec2, a value recommended in 2006 by the Committee on Data for Science and Technology. (Also see scientific notation.)
Universe
Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.
V
Vela 5B
US Atomic Energy Commission (now the Department of Energy) satellite with an all-sky X-ray monitor
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The Venera satellite series
The Venera satellites were a series of probes (fly-bys and landers) sent by the Soviet Union to the planet Venus. Several Venera satellites carried high-energy astrophysics detectors.
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visible
Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)
W
wave-particle duality
The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles) sometimes act like a wave, and sometimes act like a particle, depending on the experiment you are performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.
wavelength
The distance between adjacent peaks in a series of periodic waves. Also see electromagnetic spectrum.
white dwarf
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!
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Wien's displacement law
For a blackbody, the product of the wavelength corresponding to the maximum radiancy and the thermodynamic temperature is a constant. As a result, as the temperature rises, the maximum of the radiant energy shifts toward the shorter wavelength (higher frequency and energy) end of the spectrum.
WMAP (Wilkinson Microwave Anisotropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. From its initial results published in Feb 2003, astronomers pinpointed the age of the universe, its geometry, and when the first stars appeared.
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WWW
The World Wide Web -- a loose linkage of Internet sites which provide data and other services from around the world.
X
X-ray
Electromagnetic radiation of very short wavelength and very high-energy; X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.
XSELECT
A software tools used by astrophysicists in conjunction with the FTOOLS software to analyze certain types of astronomical data.
XTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)
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Y
Z
Z
The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to the velocity of the object. In general, with v = velocity of the object, c is the speed of light, lambda is the rest wavelength, and delta-lambda is the observed change in the wavelength, z is given by
z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of light, then
z = (delta-lambda)/lamda = v/c
Objects at the furthest reaches of the known universe have values of z = 5 or slightly greater.