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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.)


Beams of particles, usually coming from an active galactic nucleus or a pulsar. Unlike a jet airplane, when the stream of gas is in one direction, astrophysical jets come in pairs with each jet aiming in opposite directions.


kelvin (after Lord Kelvin, 1824 - 1907)
The fundamental SI unit of thermodynamic temperature defined as 1/273.16 of the thermodynamic temperature of the triple point of water. More practically speaking, the Kelvin temperature scale measures an object's temperature above absolute zero, the theoretical coldest possible temperature. On the Kelvin scale the freezing point of water is 273 ( = 0o C = 32o F) [ K = 273 + C = 273 + 5/9 * (F-32)]. The Kelvin temperature scale is often used in sciences such as astronomy.

Kepler, Johannes (1571 - 1630)
German astronomer and mathematician. Considered a founder of modern astronomy, he formulated the famous three laws of planetary motion. They comprise a quantitative formulation of Copernicus's theory that the planets revolve around the Sun.

Kepler's laws (J. Kepler)
Kepler's first law
A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's second law
A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.

Kepler's third law
The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.

kilogram (kg)
The fundamental SI unit of mass. The kilogram is the only SI unit still maintained by a physical artifact (a platinum-iridium bar) kept in the International Bureau of Weights and Measures at Sevres, France. One kilogram is equivalent to 1,000 grams or about 2.2 pounds; the mass of a liter of water.

Refers to the calculation or description of the underlying mechanics of motion of an astronomical object. For example, in radioastronomy, spectral line graphs are used to determine the kinematics or relative motions of material at the center of a galaxy or surrounding a star as it is born.

Kirchhoff's law of radiation (G.R. Kirchhoff)
The emissivity of a body is equal to its absorbance at the same temperature.

Kirchhoff's laws (G.R. Kirchhoff)
Kirchhoff's first law
An incandescent solid or gas under high pressure will produce a continuous spectrum.

Kirchhoff's second law
A low-density gas will radiate an emission-line spectrum with an underlying emission continuum.

Kirchhoff's third law
Continuous radiation viewed through a low-density gas will produce an absorption-line spectrum.


A representation of the luminosity of an object in terms of Solar luminosity. The average luminosity of the Sun is about 4x1033 erg/sec. Astronomers often express units for other objects in terms of solar units, which makes the resulting numbers smaller and easier to deal with.

Lagrange, Joseph (1736 - 1813)
A French mathematician of the eighteenth century. His work Mecanique Analytique (Analytical Mechanics; 1788) was a mathematical masterpiece. It contained clear, symmetrical notation and covered almost every area of pure mathematics. Lagrange developed the calculus of variations, established the theory of differential equations, and provided many new solutions and theorems in number theory. His classic Theorie des fonctions analytiques laid some of the foundations of group theory. Lagrange also invented the method of solving differential equations known as variation of parameters. Show me a picture of Joseph Lagrange !

Lagrange points
Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance. There are five, labeled L1 through L5. L1, L2, and L3 lie along the centerline between the centers of mass between the two masses; L1 is on the inward side of the secondary, L2 is on the outward side of the secondary; and L3 is on the outward side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary.

L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than about 1/24.96 the mass of the primary. These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It's a device that produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher levels so that when they return to lower energy levels they emit energy.

Laboratory for High Energy Astrophysics, 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.

The common term for electromagnetic radiation, usually referring to that portion visible to the human eye. However, other bands of the e-m spectrum are also often referred to as different forms of light.

light curve
A graph showing how the radiation from an object varies over time.

light year
A unit of length used in astronomy which equals the distance light travels in a year. At the rate of 300,000 kilometers per second (671 million miles per hour), 1 light-year is equivalent to 9.46053 x 1012 km, 5,880,000,000,000 miles or 63,240 AU (see scientific notation).

The outer edge of the apparent disk of a celestial body.

LISA (Laser Interferometer Space Antenna)
A NASA mission which will detect gravitational waves. LISA will consist of three satellites that use laser interferometry to monitor their positions relative to each other. Gravitational waves passing by the satellites cause small changes in the distances between the satellites.

The rate at which a star or other object emits energy, usually in the form of electromagnetic radiation.


A representation of the mass of an object in terms of Solar mass. The average mass of the Sun is about 2x1033 grams. Astronomers often express units for other objects in terms of solar units, since it makes the resulting numbers smaller and easier to deal with.

magnetic field
A description of the strength of the magnetic force exerted by an object. Bar magnets have "di-polar" fields, as the force is exerted from the two ends of the bar. In simple terms, the earth, the sun, stars, pulsars all have dipolar magnetic fields.

magnetic pole
Either of two limited regions in a magnet at which the magnet's field is most intense. The two regions have opposing polarities, which we label "north" and "south", after the two poles on the Earth.

The region of space in which the magnetic field of an object (e.g., a star or planet) dominates the radiation pressure of the stellar wind to which it is exposed.

The portion of a planetary magnetosphere which is pushed in the direction of the solar wind.

The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest star visible without a telescope has magnitude 6. A decrease of one magnitude represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.

A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

A word used for any kind of stuff which contains mass.

A unit of energy used to describe nuclear warheads. The same amount energy as 1 million tons of TNT.

1 mega-ton = 4 x 1022 ergs = 4 x 1015 joules.

Messier, Charles (1730 - 1817)
The 18th century French astronomer who compiled a list of approximately 100 fuzzy, diffuse looking objects which appeared at fixed positions in the sky. Being a comet-hunter, Messier compiled this list of objects which he knew were not comets. His list is now well known to professional and amateur astronomers as containing the brightest and most striking nebulae, star clusters, and galaxies in the sky.

meter; m
The fundamental SI unit of length, defined as the length of the path traveled by light in vacuum during a period of 1/299 792 458 s. A unit of length equal to about 39 inches. A kilometer is equal to 1000 meters.

metric system
See SI.

Microquasars are stellar mass black holes, that display characteristics of the supermassive black holes found at the centers of some galaxies. For instance, they have radio jets - something not every black hole has.

Electromagnetic radiation which has a longer wavelength (between 1 mm and 30 cm) than visible light. Microwaves can be used to study the Universe, communicate with satellites in Earth orbit, and cook popcorn.


The National Aeronautics and Space Administration, founded in 1958 as the successor to the National Advisory Committee for Aeronautics.

nebula (pl. nebulae)
A diffuse mass of interstellar dust and gas. A reflection nebula shines by light reflected from nearby stars. An emission nebula shines by emitting light as electrons recombine with protons to form hydrogen. The electrons were made free by the ultraviolet light of a nearby star shining on a cloud of hydrogen gas. A planetary nebula results from the explosion of a solar-like type star.

A fundamental particle produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting.

A particle with approximately the mass of a proton, but zero charge, commonly found in the nucleus of atoms .

neutron star
The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the Sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugar cube of neutron-star stuff on Earth would weigh as much as all of humanity!" Neutron stars can be observed as pulsars.

Newton, Isaac 1642 - 1727
English cleric and scientist; discovered the classical laws of motion and gravity; the bit with the apple is probably apocryphal. Show me a picture of Isaac Newton !

Newton's law of universal gravitation (Sir I. Newton)
Two bodies attract each other with equal and opposite forces; the magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centers of mass of the two bodies.

Newton's laws of motion (Sir I. Newton)
Newton's first law of motion
A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton's second law of motion
For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's third law of motion
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction

The random fluctuations that are always associated with a measurement that is repeated many times over. Noise appears in astronomical images as fluctuations in the image background. These fluctuations do not represent any real sources of light in the sky, but rather are caused by the imperfections of the telescope. If the noise is too high, it may obscure the dimmest objects within the field of view.

nova (plural: novae)
A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times before fading back to its original luminosity.

nuclear fusion
A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence "Energy = Mass times the Speed of Light squared". This is the source of the Sun's energy.


Orbiting Astronomical Observatory 3 - NASA ultraviolet/X-ray mission, also known as Copernicus.
    * Tell me more aboutOAO 3. (

The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.

A property of matter that prevents light from passing through it. The opacity or opaqueness of something depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultraviolet light, but is opaque to visible light.

The path of an object that is moving around a second object or point.

Orbiting Solar Observatory 3

    * Tell me more about OSO 3 (

Orbiting Solar Observatory 8
    * Tell me more about OSO 8 (


pair production
The physical process whereby a gamma-ray photon, usually through an interaction with the electromagnetic field of a nucleus, produces an electron and an anti-electron (positron). The original photon no longer exists, its energy having gone to the two resulting particles. The inverse process, pair annihilation, creates two gamma-ray photons from the mutual destruction of an electron/positron pair.
The apparent motion of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.

The distance to an object which has a parallax of one arc second. It is equal to 3.26 light years, or 3.1 x 1018 cm (see scientific notation). A kiloparsec (kpc) is equal to 1000 parsecs. A megaparsec (Mpc) is equal to a million (106) parsecs.

The point in an orbit when two objects are closest together. Special names are given to this point for commonly used systems: see periastron, perihelion, and perigee. The opposite of apoapsis.

The point of closest approach of two stars, as in a binary star orbit. Opposite of apastron.

The point in its orbit where an Earth satellite is closest to the Earth. Opposite of apogee.

The point in its orbit where a planet is closest to the Sun. Opposite of aphelion.

The transfer of a photon's energy to an atom, molecule or nucleus.

photoelectric effect
An effect explained by A. Einstein which demonstrates that light seems to be made up of particles, or photons. Light can excite electrons (called photoelectrons in this context) to be ejected from a metal. Light with a frequency below a certain threshold, at any intensity, will not cause any photoelectrons to be emitted from the metal. Above that frequency, photoelectrons are emitted in proportion to the intensity of incident light.
The reason is that a photon has energy in proportion to its wavelength, and the constant of proportionality is the Planck constant. Below a certain frequency -- and thus below a certain energy -- the incident photons do not have enough energy to knock the photoelectrons out of the metal. Above that threshold energy, called the work function, photons will knock the photoelectrons out of the metal, in proportion to the number of photons (the intensity of the light). At higher frequencies and energies, the photoelectrons ejected obtain a kinetic energy corresponding to the difference between the photon's energy and the work function.

The smallest (quantum) unit of light/electromagnetic energy. Photons are generally regarded as particles with zero mass and no electric charge.

The constant equal to the ratio of the circumference of a circle to its diameter, which is approximately 3.141593.

Planck constant; h
The fundamental constant equal to the ratio of the energy of a quantum of energy to its frequency. It is the quantum of action. It has the value 6.626196 x 10-34 J s (see scientific notation).

Planck equation
The quantum mechanical equation relating the energy of a photon E to its frequency nu:

E = h x nu

planetary nebula
A shell of gas ejected from stars like our Sun at the end of their lifetime. This gas continues to expand out from the remaining white dwarf.

A low-density gas in which the individual atoms are ionized (and therefore charged), even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

The direction in the sky to which the telescope is pointed. Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.

A special property of light; light has three properties, brightness, color and polarization. Polarization is a condition in which the planes of vibration of the various rays in a light beam are at least partially aligned.

The antiparticle to the electron. The positron has most of the same characteristics as an electron except it is positively charged.

A particle with a positive charge commonly found in the nucleus of atoms.

Very dense regions (or cores) of molecular clouds where stars are in the process of forming.

Ptolemy (ca. 100-ca. 170)
A.k.a. Claudius Ptolemaeus. Ptolemy believed the planets and Sun to orbit the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit). He authored a book called Mathematical Syntaxis (widely known as the Almagest). The Almagest included a star catalog containing 48 constellations, using the names we still use today. Show me a picture of Ptolemy !

A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.

Pioneer Venus Orbiter
    * Tell me more about PVO (

[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 ]

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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