The tendency of a moving object to shorten in the direction of its motion, as viewed by an observer who is stationary relative to the object.
Lepton:
A fermion that is unaffected by the strong force. Leptons include electrons, muons, taus, and neutrinos.
Leptoquark:
According to theory, a massive boson that enabled leptons and quarks to exchange identites. Leptoquarks would have occurred only during a brief period that began 10^-43 second after the Big Bang.
Light:
The visible part of the electromagnetic spectrum; may also refer to the entire electromagnetic spectrum, as in the "speed of light", the speed of all forms of electromagnetic radiation.
Light-year:
An astronomical distance unit equal to the distance light travels in a vacuum in one year, almost six trillion miles.
Liquid metallic hydrogen:
An exotic, highly compressed form of liquid hydrogen that readily conducts electricity.
Local group:
The cluster of thirty or more galaxies to which the Milky Way belongs.
Longitude:
On a celestial body, the
Lorentz transformations:
Equations that relate the measures of length, time, and mass from one uniformly moving frame of reference to another.
Lunar occultation:
The Moon's passage in front of an object, which allows precise measures of the object's size, location, and features.
Luminosity:
An object's total energy output, usually measured in ergs per second.
Lunar Occultation:
The Moon's passage in front of an object, which allows precise measures of the object's size, location, and features.
Lyman-alpha line:
A strong spectral line produced by hydrogen at a wavelength of 1,216 angstroms.
The large and small magellanic clouds, which are the two galaxies closest to Earth, visible from the southern Hemisphere.
Magellanic Stream:
A trail of hydrogen gas extending from the Magellanic Clouds toward the Milky Way's south pole.
Magnetic Field:
A region within which the effects of magnetism are observable. the strength of the field is indicated by the force exerted on moving charged particles within it.
Magnetic Field Line:
An indication of the directon and strength of a magnetic field at a particular location.
Magnetic pole:
One of two locations on the surface of a neutron star or other body where its magnetic field lines converge, as opposed to the poles defined by the body's axis of rotation.
Magnetometer:
A device for measuring the strength and direction of a magnetic field.
Magnetopause:
the well-defined boundary between a planet's magnetosheath and its magnetosphere.
Magnetosheath:
The region around a planet in which its magnetic field is the dominant magnetic influence; the magnetosphere is bounded by the magnetosphere.
Magnetosphere:
The region around a planet in which its magnetic field is the dominant magnetic influence; the magnetosphere is bounded by the magnetopause.
Magnitude:
A designation of an object's brightness or luminosity relative to that of other objects. Apparent magnitude refers to observed brightness; absolute magnitude refers to an object's hypothetical brightness at a standard distance of about 32.6 light-year
s from the observer.
Main sequence:
A diagonal region in the Hertzsprung-Russell diagram that includes 90 percent of all stars.
Mantle:
The layer of a planet between the outer core and the crust.
Maser:
A source of radiation that produces intense radio beams; hudroxyl molecules and water molecules have been found to act as masers.
Mass:
A measure of the total amount of material in an object, determined either by its gravity or by its tendency to resist acceleration.
Mass Increase:
The tendency of a moving object to become more massive, as perceived by an observer who is staitoinary relative to the object.
Mass-Luminosity (M-L) Relation:
A formula relating the mass of a star to its luminosity; in general, the larger the mass, the brighter the star.
Mass spectrometer:
A device used to determine a substance's chemical composition by measuring the varied masses of its components.
Matter:
The category of all fermion particles, as opposed to antiparticles; may also refer generically to both matter and antimatter.
Meander:
A loop-shaped curve formed by water action, as in a river channel.
Merger:
A collision between two or more galaxies that results in their stars combining to form a single galaxy.
Meridian:
An imaginary north-south line in the sky directly over any point on a planet's surface.
Messier Catalog:
An eighteenth-century catalog compiled by French Astronomer charles Messier, to keep track of nebulae; includes several galaxies, designated by their posion in the catalog. For Example, M31, the Andromeda galaxy, is the thirty-first entry.
Metastable state:
An intermediate energy state in an atom, higher than the lowest, or ground, state but lower than excited states.
Meteoroid:
A small metallic or rocky body found in space. A meteoroid entering a planet's atmosphere is called a meteor. Meteors often burn up in the atmosphere; thos that reach the surface are called meteorites.
Methylidyne:
A two-atom molecule of carbon and hydrogen; the first molecule discovered in interstellar space.
Microwave:
A radio wave of very high frequency and short wavelength.
Milky Way:
The Earth's galaxy, a giant spiral of at least 100 billion stars. The Sun is two-thirds of the way out from the Milky Way's center.
Molecular Cloud:
A concentration of interstaellar gas and dust sometimes hundreds of light-years in diameter. Such clouds are the most massive objects in the Milky Way.
Molecule:
The smallest unit of a compound that retains the properties of that substance. A molecule may consist of a single atom or of two or more atoms bonded together.
Momentum:
A measure of an object's inertia; an object's less stable.
Moon:
One of a planet's natural sattelites, generally no smaller than ten miles in diameter. There are more than fifty known moons in the Solar System, including Earth's.
A cloud of interstellar gas or dust or a mixture of both. Nebulae may glow with their own or reflected light, or they may obscure light from other sources, producing dark patches in the night sky.
Neutrino:
One of a class of neutral leptons with little or no mass.
Neutron:
A neutral particle, made up of three quarks, with a mass similar to that of a proton; normally found in an atom's nucleus.
Neutron Star:
A very dense body composed of tightly packed neutrons; one possible product of a supernova explosion. Neutron stars are observed as pulsars.
New General Catalogue (NGC):
A catalog of star clusters, galaxies, and nebulae published in 1888. Many galaxies are named for their posion in the catalog; for example, NGC 5178 is the 5,178th entry.
N-galaxy:
An active elliptical galaxy with a small, very brught nucleus that varies in luminosity.
Noise:
Meaningless random changes in radiation that tend to obscure a specific signal.
Non-Euclidean geometry:
see Geometry.
Nonthermal emission:
Electromagnetic radiation produced by any of several processes, including synchrotron radiation. The characteristic pattern of a nonthermal emission is that it increases in intensity as frequency increases.
Nova:
A star that exhibits a sudden, temporary increase in brightness thousands of times it s normal appearance.
Nuclear Fusion:
see Fusion.
Nucleation:
The start of the process of condensation, during which a core particle acts as a nucleus around which molecules collect.
Nucleosynthesis:
The combining of protons and neutrons to form the atomic nuclei of chemical elements.
Nucleus:
The massive center of an atom, composed of protons and neutrons and orbited by an electrons.
The greatest distance from which light now being received on Earth could have originated.
Occultation:
An event in which one celestial body passes in front of another, partially or totally obscuring it.
Open Cluster:
A loose grouping of stars, ranging in number from a few dozen to few thousand, found in the plane of a galaxy; contrasted with globular cluster.
Open Universe:
A universe that is on average less dense than the critical density and so continues to expand. According to relativity theory, the geometry of such a universe is saddle-shaped.
Oppostion:
the alignment of two celestial bodies on opposite sides of the sky as seen from Earth. The oppositionof a superior planet occurs when Earth passes between it and the Sun; a perfect oppositon occurs when a planet is also at its closest approach to Eart
h, at its best point for observation.
Orbit:
The path of an object revolving around another object.
Orbital decay:
The outcome when a body moves too slowly to sustain ts orbit; the orbit becomes smaller and smaller until the orbiting and orbited bodies collide.
O Stars:
A spectral class of massive, young, blue stars, the most luminous tyupe, found primarily in the spiral arms of the galaxy. Because they emit high levels of radiation, O stars ionize the hydrogen in surrounding clouds.
Ovoids:
Moundlike structures on Venus, possibly caused by crustal expansion, measuring half a mile high and 100 to 350 miles in diameter; also called coronae.
A star's apparent motion on the celestial sphere over a six-month period. Measured in seconds of arc, it is used to determine a star's distance; the greater the parallax, the nearer the star.
Parsec:
An astronomical unit of distance equal to approximately 3.2 light years.
Particle:
A fundamental component of matter, antimatter, or force, such as a proton, neutron, lepton, or boson.
Particle Accelerator:
A device, often several miles long, used to accelerate subatomic particles to high velocities and fire them at other particles or at targets. The results of collisions suggest the particles' properties.
Particle Physics:
The experimental study of subatomic particles, often using particle accelerators.
Penumbra:
The area of lighter shadow surrounding the central shadow cast by illuminated body. see Umbra.
Perihelion:
The point in its orbit where a planet is closest to the sun.
Period-Luminoisty Law:
A relation linking a Cepheid variable's absolute magnitude to the length of its period, or cycle of brightening and dimming (the longer the period, the brighter the star); a critical tool for estimating intragalctic distances. By comparing Cepheid's
absolute magnitude with its apparent magnitude, astronomers can calculate the star's distance.
Permafrost:
Ground that is permanently frozen unless artificially heated.
Phase
One of the recurring appearances of a celestial body viewed from Earth. As an object such as the Moon or Venus moves along its orbit, the amount of its surface visible from Earth increases (waxes) and decreases (wanes) at a regular, periodic rate.
Phase Transition:
A complete change from one physical state to another. Freezing is a phase transition from a liquid to a solid state.
Photoelectric effect:
The fact that light at certain frequencies can knock electrons out of a charged metal sheet.
Photometer:
A device that mesures an object's brightness, or apparent magnitude, by detecting its emitted photons.
Photometry:
the measurement of an object's brightness, or apparent magnitude, by means of an instrument that counts the number of photons detected in a time period.
Photon:
A force carrier, or boson, that conveys electromagnetic force, or radiation, and is associatd with a specific frequency.
Photon sphere:
A region around a black hole that captures light traveling at particular angles; bounded on the inside by the static limit.
Photopolarimeter:
A specialized telescope that mesures changes in the amount of light coming from a source, enarbling scientist to infer density, structure, and other properties of the source.
Pixel (also picture element)
In a digital image, a single dot numerically assigned an intensity that can be represented as real or false color.
Planck Era:
Theoretically, the very brief period after the Big Bang and up to the Planck Time. Conditions during the Planck Era violate the rules of conventional quantum mechanics and general relativity and cannot be adequately described by current physics.
Planck's constant:
A number whose value is important to the equations of quantum mechanics; equal to the ratio of a photon's energy to its frequency.
Planck Time:
In theory, an instant 10^-43 second after the Big Bang after which the universe would have followed conventional physical laws.
Planet:
A large, nonstellar body that orbits a star and shines only with reflected light.
Planetary Nebula:
A shell of gas, ejected from a red giant star; appears planetlike in a low-powered telescope.
Planetesimal:
In theory, a small orbiting body that actively accretes mass from random collisions and will eventually become a full-scale planet.
Plasma:
A gaslike association of ionized particles that responds collectively to electric magnetic fields. Because plasma particles do not interact the way particles of ordinary gas do, plasma is considered a fourth state of matter, along with solid, liquid,
and gas.
Polar Caps:
The distinctive surface areas near teh poles of Earth and Mars. On Mars, the polar caps are mainly frozen carbon dioxide, perhaps including some water ice. The Martian polar caps shrink and expand with the seasons.
Polar Hood:
A dense cloud cover sometimes seen over the polar caps on Mars.
Polar-ring galaxy:
a peculiar galaxy with an elongated central body encircled by a pole-to-pole ring of stars at right angles to the plane of the central galaxy.
Polarization:
The tendency of some electromagnetic waves to vibrate preferentially in a single plane rather that uniformly in all directions perpendicular to their motion. Polarization is produced by the source of the radiation and by the medium through which it tr
avels.
Pole:
Ono of two points on a planet's surface where its rotational axis ends; a planet's magnetic poles may or may not coincide with those formed by the rotational axis.
Polycyclic aromatic hydrocarbon (PAH):
A complex, carbon-based molecule with a ring structure, found in soot on Earth. PAHs are thought by some scientists to be present in large quantities in interstellar clouds.
Positron:
An antiparticle to the electron, carrying a positive electric charge.
Potential Energy:
An object's stored energy; for example, the latent force of a body poised to fall.
Probe:
An automated, unmanned spacecraft used to collect data and transmit it to Earth.
Proper motion:
The movement of an object across the clestial sphere, reflecting its actual motion relative to the Sun. In contrast, stellar parallax is a yearly cycle relative to the Earth that does not produce a net change of position.
Protogalaxy:
A roughly spherical hydrogen cloud from which a galaxy forms; about thirty times the size of a mature galaxy.
Proton:
A positively charged particle made up of three dquarks, with about 2,000 times the mass of an electron; normally found in an atom's nucleus.
Proton-proton reaction:
A fusion reaction that predominates in stars of two and a half solar masses or less, in which hydrogen nuclei, consisting of single protons, fuse to form deuterium and, ultimately, helium.
Protostar:
A large gaseous sphere, held together by its own gravitational attraction, that shrinks and ompresses to become a star.
Proto-Sun:
In theory, the gaseous matter at the center of the Solar nebula, held together by its own gravitational attraction, that shrank and compressed to become the Sun about 4.5 billion years.
Pulsar:
A radiating source from which bursts of energy are received at precisely spaced intervals of several seconds or less. Pulsars are thought to be rapidly rotating neutron stars with very strong magnetic fields.
Pyrite:
A metallic yellow mineral compound of iron and sulfur.
