Glossary
Astronomy glossary
514 definitions · page 5 of 9.
- irregular galaxy
- a galaxy without a clearly defined spiral or elliptical shape
- isotope
- forms of an element in which the atoms all have the same number of protons but different numbers of neutrons
- jet
- a narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole
- Jet Propulsion Laboratory
- the lead U.S. center for robotic exploration of the solar system, located in Pasadena, California; JPL spacecraft have visited all known planets except Pluto
- jet stream
- a high-speed, wandering wind current in the upper troposphere that blows from west to east and affects weather
- Jovian planet
- a planet with characteristics similar to Jupiter (see gas giant).
- kelvin
- a unit of temperature equal to one degree on the Celsius scale and 1.8 degrees on the Fahrenheit scale; also the absolute temperature scale defined so that 0 kelvin is absolute zero
- kelvins
- temperature units equal to degrees Celsius; 0 on the Kelvin scale is absolute zero
- Kuiper Belt
- a region in the outer solar system beyond Neptune’s orbit that contains billions of small, icy bodies; Pluto is the largest known Kuiper Belt Object
- L chondrite
- a chondrite (a stony meteorite containing small, round, silicate granules called chondrules) that has a low amount of iron
- Lagrange point
- one of five locations in space relative to two bodies where a third, less massive body can maintain a stable orbit around a common center of mass
- Large Magellanic Cloud
- an irregular galaxy that orbits the Milky Way Galaxy
- last quarter
- the phase of the moon three-quarters of the way around its orbit from new moon; the western half is illuminated
- latitude
- the angular distance north or south from the equator to a point on Earth's surface, measured on the meridian of the point
- lens
- a curved piece of glass that brings light to a focus
- lenticular galaxy
- a galaxy possessing a large bulge and small disk
- libration
- the small oscillations in the moon’s motion that allow Earth-based observers to see slightly more than half the moon’s surface
- light pollution
- light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects
- light-gathering power
- the ability of a telescope to collect light; the larger a telescope’s aperture, the greater its light-gathering power
- light-year
- the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km)
- limb
- the apparent edge of a celestial object
- limiting magnitude
- the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using
- LINER galaxy
- A low-ionization nuclear emission-line region galaxy belongs to a common class of otherwise normal galaxies that display low-ionization line emissions near their central regions
- Local Group
- the galaxy cluster containing roughly 35 galaxies to which the Milky Way Galaxy belongs
- local supercluster
- the galaxy supercluster to which the Local Group belongs; it spreads over 100 million light-years and boasts the Virgo Cluster as its dominant member
- long-period comet
- Comets that have orbital periods greater than 200 years.
- longitude
- the angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England
- luminosity
- the total amount of light that an object radiates
- lunar eclipse
- a phenomenon caused by the Earth passing between the sun and moon
- lunar month
- the period of one complete revolution of the moon around Earth, 29.5 days
- lunation
- the time between two successive new moons; approximately 29.5 days
- magnetograph
- an instrument that maps the strength, distribution, and direction of magnetic fields on the sun's disk
- magnetometer
- an instrument used to measure the strength and direction of a magnetic field
- magnetopause
- the boundary between Earth's magnetic field and the solar wind
- magnetosphere
- the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind
- magnitude
- the measurement of an object's brightness; the lower the number, the brighter the object
- main sequence
- the band of stars on a Hertzsprung-Russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores
- Maksutov telescope
- a catadioptric telescope that uses a deeply curved meniscus lens as the correcting plate
- mantle
- the portion of a planet’s interior above the core but below the crust
- mare
- a dark and relatively smooth area on the surface of the moon or a planet.
- mass
- a measure of the total amount of matter within an object
- mass loss
- the loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova
- megaparsec
- one million parsecs; equal to 3.26 million light-years
- meridian
- an imaginary circle on the celestial sphere that connects the zenith to the north (or south) celestial pole
- Messier Catalog
- A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s
- Messier objects
- A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s
- meteor
- a flash of light that occurs when a meteoroid burns up in Earth’s atmosphere; also popularly known as a shooting star
- meteor shower
- a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point
- meteor showers
- a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point
- meteor storm
- Meteor storms are rare events that occur when Earth encounters dense regions within a meteor stream. Such encounters can increase normal meteor rates by more than 1,000 meteors per minute.
- meteorite
- a rock from space that survives passage through Earth’s atmosphere and falls to the ground
- naked-eye
- something visible or accomplished without the aid of binoculars or a telescope (e.g. a naked-eye object or naked-eye observing)
- near-infrared
- light from the part of the infrared band of the electromagnetic spectrum closest to the visible range
- nebula
- a cloud of interstellar gas and dust; some nebulae represent stellar nurseries, others represent stellar graveyards
- neutrino
- a subatomic particle produced in nuclear reactions and in supernovae that very rarely interacts with matter; neutrinos have no electrical charge and travel at or very close to the speed of light
- neutron
- a subatomic particle with no electric charge that resides in an atomic nucleus; it has about the same mass as a proton
- neutron star
- the collapsed, extraordinarily dense, city-sized remnant of a high-mass star.
- new moon
- the phase in which the moon is in the same direction as the sun in Earth’s sky, so it is unilluminated and invisible
- Newtonian telescope
- a reflecting telescope in which a flat secondary mirror (called the diagonal) in the center of the tube reflects light to a focus outside the tube
- NGC
- New General Catalogue, a 19th-century compendium of deep-sky objects such as galaxies, globular clusters, and nebulae