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Skyview NASA - Astrophysics Dictionary
Category: Sciences > Astophysics
Date & country: 11/09/2007, USA
Words: 219

large scale structure
The largest spatial features in an image.

Laboratory for High Energy Astrophysics (GSFC, Code 660)

Electromagnetic radiation that is visible to the human eye.

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 × 1012 km, 5,880,000,000,000 miles or 63,240 AU

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

Lyman series
The series which describes the emission spectrum of hydrogen when electrons are jumping to the ground state. All of the lines are in the ultraviolet.

magnetic field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

magnetic monopole
A hypothetical particle which constitutes sources and sinks of the magnetic field. Magnetic monopoles have never been found, but would only cause fairly minor modifications to Maxwell's equations. They also seem to be predicted by some grand-unified theories. If magnetic monopoles do exist, they do not seem to be very common in our Universe.

magnetic pole
Either of two limited regions in a magnet at which the magnet's field is most intense.

the region of space in which a planet's magnetic field dominates that of the solar wind.

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 visible star has magnitude 6, with the scale rule such that a decrease of one unit 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.

megabits per second
A unit for measuring how fast data can be sent through a computer network equal to one million bits per second.

Used by astrophysicists to refer to all elements except hydrogen and helium, as in: 'the universe is composed of hydrogen, helium and traces of metals'.

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.

1/1000 of a bar. Standard sea-level pressure is about 1013 millibars.

molecular gas
Gas that is composed of atoms that are bound to each other as molecules. The most abundant molecule in space is molecular hydrogen (two hydrogen atoms bound to each other) followed by carbon monoxide (CO, or a carbon and oxygen atom bound together). Molecular gas may be mixed with atomic gas.

A measure of the state of motion of a body; mathematically, it is equal to the product of its mass and velocity.

A non-proprietary software tool using hypertext links to navigate and retrieve data from the Internet -- developed by the National Center for Supercomputing Activities.

A diffuse mass of interstellar dust and gas.

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. (32 k GIF)

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.

A start 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.

NASA Research Announcement

National Space Science Data Center

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.

The angle between a body's equatorial plane and orbital plane.

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

Office of Guest Investigator Programs (GSFC, Code 668)

A property of matter that prevents light from passing through it; non-transparent. The opacity or opaqueness of something depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultra-violet light, but is opaque to visual light.

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

Orbiting Solar Observatory 8

A large distance often used in astronomy, it is equal to 3.26 light years, or 3.1 × 1018 cm (see scientific notation).

The point in the orbit closest to the planet.

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

The point in the orbit closest to the Earth.

The point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion)

To cause a planet or satellite to deviate from a theoretically regular orbital motion.

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.

Principal Investigator

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 × 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 nu.

planetary nebula
A shell of gas ejected from, and expanding about, a certain kind of extremely hot star.

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.

ROSAT (and X-ray) analysis system developed within IRAF

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 Almage…

A rotating neutron star whose light appears to pulse. These pulses are the result of beams of radiation sweeping through the direction to the earth, much like a lighthouse beacon. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, x-ray, and gamma-ray energies.

A specific type of quasi-stellar source.

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.

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.

Energy radiated in the form of waves or particles; photons.

radiation belt
Regions of charged particles in a magnetosphere.

Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish. 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 by1.22 lambda/d.

red giant
A star that has low surface temperature and a diameter that is large relative to the Sun.

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 qualitatively, in all frames. That is, there is no frame that is better (or qualitatively any different) from any other. This principle, along with the constancy principle, constitute the founding principles of special relativity.

relativity, Theory of
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.

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.

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.

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 primary would break up the secondary.

Röntgen Satellite

A body that revolves around a larger body.

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 × 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 × 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.

semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru 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).

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.

of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

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. Although singularities generally predict inconsistencies in theory, singularities within black holes do not necessarily imply that general relativity is incomplete so long as singularities are always surrounded by event horizons.

small scale structure
The smallest spatial features in an image.

solar flares
Violent eruptions of gas on the Sun's surface.

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 its characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).

The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

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.

A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.

speed of light (in vacuo); c
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.

Structured Query Language -- The ANSI standard database language

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.

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. Th…

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.

Cooler (and thus darker) regions on the sun where the magnetic field loops up out of the solar surface.

The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions 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).

The Spectrum X-Gamma mission

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.

Thomson, William 1824-1907
also known as Lord Kelvin, the British physicist who developed the Kelvin scale of temperature and who supervised the laying of a trans-Atlantic cable. (11 k GIF)