Glossary items by letter: l — n


A carefully ground or molded piece of glass, plastic, or other transparent material that causes light to bend and either come together or spread apart to form an image.

Lens doublet

A set of two lenses, one concave and one convex, made from different types of glass. Together the lenses correct both spherical and chromatic aberrations. A single lens alone cannot correct these aberrations.

Light Curve

A plot showing how the light output of a star (or other variable astronomical object) changes with time.


The distance that a particle of light (photon) will travel in a year — about 10 trillion kilometers (6 trillion miles). It is a useful unit for measuring distances between stars.


The solid part of a planet’s surface, composed of the crust and upper mantle. On Earth, it includes the continents and the sea floor.

Local Group

A small cluster of more than 30 galaxies, including the Andromeda galaxy, the Magellanic Clouds, and the Milky Way galaxy.

Long-Period Comet

A comet having an orbital period greater than 200 years and usually moving in a highly elliptical, eccentric orbit. Comets have orbits that take them great distances from the Sun. Most long-period comets pass through the inner solar system only once. Hale-Bopp is an example of a long-period comet.


The amount of energy radiated into space every second by a celestial object, such as a star. It is closely related to the absolute brightness of a celestial object.

Lunar Eclipse

A darkening of the Moon, as viewed from Earth, caused when our planet passes between the Sun and the Moon.

Lyman Limit

A specific wavelength (91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom (13.6 eV). Galactic space is opaque at wavelengths shorter than the Lyman limit. Subsequently, light from cosmic objects at wavelengths less than the Lyman limit is exceedingly difficult to detect.

Magellanic Clouds

The Magellanic Clouds are two dwarf irregular galaxies. Known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), the galaxies are in the Local Group. The closer LMC is 168,000 light-years from Earth. Both galaxies can be observed with the naked eye in the southern night sky.

Magnetic Field

A region of space in which magnetic forces may be detected or may affect the motion of an electrically charged particle. As with gravity, magnetism has a long-range effect and magnetic fields are associated with many astronomical objects.

Magnetic-Field Lines

Imaginary lines used to visualize a magnetic field. Magnetic field lines are related to the strength of the magnetic object’s influence and point in the same direction as a compass needle would.


A region of space above the Earth’s (or other planet’s) atmosphere where magnetic fields influence the motions of charged particles. The magnetosphere magnetically deflects or traps charged particles from space that would otherwise bombard the planet’s surface.


Enlargement in the size of an optical image. For telescopes, magnification is not as important as the ability to gather light, which depends on the diameter of the primary lens or mirror.


The process of enlarging the size of an optical image.


The interior region of a terrestrial (rocky) planet or other solid body that is below the crust and above the core.


A dark, flat, large region on the surface of the Moon. The term is also applied to the less well-defined areas on Mars. Although maria literally means “seas,” watery regions do not exist on the Moon or Mars. Marias on the Moon may be evidence of past volcanic lava flows.


The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and Phobos.

Marshall Space Flight Center (MSFC)

NASA center overseeing the research, development, and implementation of three primary areas essential to space flight: reusable space transportation systems, generation and communication of new scientific knowledge, and management of all space lab activities. Located in Huntsville, Alabama, the center aided in the design, development, and construction of the Hubble Space Telescope.


A measure of the total amount of matter contained within an object.

Matter-Antimatter Annihilation

A highly efficient energy-generation process in which equal amounts of matter and antimatter collide and destroy each other, thus producing a burst of energy.

Megaparsec (MPC)

Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between galaxies.


The closest planet to the Sun. The temperature range on Mercury’s surface is the most extreme in the solar system, ranging from about 400° C (750° F) during the day to about –200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has virtually no atmosphere, no moons, and no water.


A bright streak of light in the sky caused when a meteoroid enters the Earth’s atmosphere. The streak of light is produced from heat generated by the meteoroid traveling into the Earth’s atmosphere.


The remains of a meteoroid that plunges to the Earth’s surface. A meteorite is a stony or metallic mass of matter that did not completely vaporize when it entered the Earth’s atmosphere.


A small, solid object moving through space. A meteoroid produces a meteor when it enters the Earth’s atmosphere.


A chemical compound consisting of five atoms: one of carbon and four of hydrogen. On Earth, methane is a colorless, odorless gas and is the principal ingredient of natural gas. In the cold vacuum of space, methane is a white solid but, when hit by sunlight, it can become a gas.


A very small meteoroid with a diameter of less than a millimeter. Micrometeoroids form the bulk of the interplanetary solid matter scattered throughout the solar system.


An electromagnetic wave in the region between infrared and radio wavelengths. Microwave wavelengths fall between one millimeter and one meter.

Milky Way

The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion stars and has a diameter of 100,000 light-years.

Milky Way Galaxy

The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion stars and has a diameter of 100,000 light-years.


The building blocks of rocks. They are naturally occurring substances formed through geological processes, and often have a crystalline form. They can be single elements (such as gold or silver) or compounds (such as quartz, marble or turquoise).

Modern physics

A group of several theories developed in the early to mid-20th century that explains how small particles are affected by light, how measurements change when objects move very fast, and how gravity affects space and time.

Molecular Cloud

A relatively dense, cold region of interstellar matter where hydrogen gas is primarily in molecular form. Stars generally form in molecular clouds. Molecular clouds appear as dark blotches in the sky because they block all the light behind them.

Molecular Velocity

The average speed of the molecules in a gas of a given temperature.


A tightly knit group of two or more atoms bound together by electromagnetic forces among the atoms’ electrons and nuclei. For example, water (H2O) is two hydrogen atoms bound with one oxygen atom. Identical molecules have identical chemical properties.


A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.


The support structure for a telescope that bears the weight of the telescope and allows it to be pointed at a target. The mounting of today’s research telescopes also allows astronomers to track the object as it appears to move across the sky.

Multi-Layer Insulation (MLI)

A “skin” or blanket of insulation covering the Hubble Space Telescope, which protects the observatory from temperature extremes. This insulation protects the telescope from the cold of outer space and also reflects sunlight so that the telescope does not become too warm. The MLI on Hubble is made up of many layers of aluminized Kapton, with an outer layer of aluminized Teflon.

National Aeronautics And Space Administration (NASA)

A Federal agency created on July 29, 1958 after President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958. NASA coordinates space exploration efforts as well as traditional aeronautical research functions.


The region of the infrared spectrum that is closest to visible light. Near-infrared light has slightly longer wavelengths and slightly lower frequencies and energies than visible light.

Near Infrared Camera And Multi-Object Spectrometer (NICMOS)

An instrument that sees objects in near-infrared wavelengths, which are slightly longer than the wavelengths of visible light. (Human eyes cannot see infrared light.) NICMOS is actually three cameras in one, each with different fields of view. Many secrets about the birth of stars, solar systems, and galaxies are revealed in infrared light, which can penetrate the interstellar gas and dust that blocks visible light. In addition, light from the most distant objects in the universe “shifts” into infrared wavelengths due to the universe’s expansion. By studying objects and phenomena in this spectral region, astronomers probe our universe’s past, present, and future; and learn how galaxies, stars, and planetary systems form. Astronauts installed NICMOS aboard the Hubble Space Telescope in February 1997 during the Second Servicing Mission.


A cloud of gas and dust located between stars and/or surrounding stars. Nebulae are often places where stars form.

Nebular Theory

The idea that our solar system originated in a contracting, rotating cloud of gas that flattened to form a disk as it contracted. According to this theory, the Sun formed at the center of the disk and the planets formed in concentric bands of the disk.


The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the largest of which is Triton. Neptune has a ring system, just like all the giant gaseous outer planets.


A neutral, weakly interacting elementary particle having a very tiny mass. Stars like the Sun produce more than 200 trillion trillion trillion neutrinos every second. Neutrinos from the Sun interact so weakly with other matter that they pass straight through the Earth as if it weren’t there.

Neutrino Detector

A device designed to detect neutrinos.


A neutral (no electric charge) elementary particle having slightly more mass than a proton and residing in the nucleus of all atoms other than hydrogen.

Neutron Star

An extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense: they are only 10 kilometers or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.

New Outer Blanket Layer (NOBL)

Covers that protect Hubble’s damaged external blankets and help to maintain the telescope’s normal operating temperatures. The covers are made of specially coated stainless steel foil, which is trimmed to fit each particular equipment bay door.

Newtonian reflector

A type of reflecting telescope whose eyepiece is located along the side of the telescope. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light at a right angle through the side of the telescope to the eyepiece.

Non-Thermal Radiation

Radiation that is not produced from heat energy — for example, radiation released when a very fast-moving charged particle (such as an electron) interacts with a magnetic force field. Because the electron’s velocity in this case is not related to the gas temperature, this process has nothing to do with heat.

North Celestial Pole (NCP)

A direction determined by the projection of the Earth’s North Pole onto the celestial sphere. It corresponds to a declination of +90 degrees. The North Star, Polaris, sits roughly at the NCP.


A binary star system (consisting of a white dwarf and a companion star) that rapidly brightens, then slowly fades back to normal.

Nuclear Transformation

The process by which an atomic nucleus is transformed into another type of atomic nucleus. For example, by removing an alpha particle from the nucleus, the element radium is transformed into the element radon.


The core of a comet, made up of ice, dirt, and rock.

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