Glossary
Astronomy glossary
514 definitions · page 6 of 9.
- NGC objects
- deep-sky objects such as galaxies, globular clusters, and nebulae included in the New General Catalogue
- North Celestial Pole
- the point in the sky to which Earth's Geographical North Pole points
- nova
- an explosion on the surface of a white dwarf that is accreting matter from a companion star, which causes the system to temporarily brighten by a factor of several hundred to several thousand
- nuclear fusion
- the process by which two atomic nuclei combine to form a heavier atomic nucleus; this is the energy source that causes most stars to shine
- nucleosynthesis
- the creation of heavy elements from lighter ones by nuclear fusion
- nucleus
- the central region of an atom, comet, or galaxy
- O-type star
- a hot, massive blue star that emits strongly at ultraviolet wavelengths and has a surface temperature between about 28,000 to 40,000 kelvins
- OB association
- a loose grouping of O and B stars, which are the most luminous, most massive, and shortest-lived stars
- objective
- a telescope’s primary lens or mirror that gathers light and brings it to a focus
- obliquity
- the angle between a planet’s equator and the plane of its orbit
- occultation
- the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view
- omega
- 1. the ratio of the density of the universe to the critical density 2. the 24th letter of the Greek alphabet
- Omega Centauri
- a massive globular cluster in the southern constellation Centaurus located about 17,000 light-years from Earth; also known as NGC 5139
- Omega nebula
- One of the Milky Way’s numerous stellar nurseries, the Omega Nebula is about 5,000 light-years from Earth and can be seen in the constellation of Sagittarius the Archer. It is also known as the Swan Nebula, M17, NGC 6618, the Horseshoe Nebula, and the…
- Oort Cloud
- a cloud of cometary nuclei that surrounds the sun at a distance of many thousands of astronomical units
- open cluster
- a system containing a few dozen to a few thousand stars that formed from the same stellar nursery
- opposition
- the moment when a planet farther from the sun than Earth appears opposite the sun in the sky; it is the best time to observe a planet
- optical double
- Two stars at different distances that lie along nearly the same line of sight and thus appear close together
- optics
- the study of light and its properties; or lenses and mirrors
- orbit
- the path an object follows around a more massive object or common center of mass; usually elliptical in shape
- orbital period
- the length of time it takes one body to orbit another
- outgassing
- the release of gas from a rocky body
- PAHs
- Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from…
- parallax
- the apparent shift of a relatively nearby object against a fixed background due to the motion of the observer; astronomers observe the parallax of stars to measure their distances
- parsec
- the distance an object would have to be from Earth so that its parallax when viewed from two points separated by 1 AU would be one arcsecond; equal to 3.26 light-years.
- patera
- an irregular, saucer-shaped volcanic structure
- penumbra
- 1. the region of a shadow from which part of the light source remains visible2. the lighter region of a sunspot surrounding its dark center (umbra)
- penumbral eclipse
- an event that occurs when the moon passes into the outer ring of Earth's shadow (penumbra), causing a slight shading in the moon's appearance
- periastron
- the point in an object's orbit at which it is closest to the star it orbits
- perigee
- the point in a satellite’s orbit when it is closest to Earth
- perihelion
- the point in an object’s orbit when it’s closest to the sun
- period
- the time interval for a regular event to take place
- periodic comet
- a comet that has been seen to orbit the sun more than once (Comet Halley was the first recognized as periodic)
- phase
- the regular cycle of changes in the appearance of a moon or planet
- photometer
- a detector that measures the amount of light coming from an object
- photometry
- the measurement of light intensities
- photons
- individual “particles/waves†of light
- photosphere
- the visible surface of the sun
- photovoltaic
- conversion of light energy into electricity
- pixel
- short for “picture element,†the individual light detectors on a CCD chip
- Planck scale
- the smallest units of measurement scientists use to describe the universe; a Planck unit of length is 10^-33 centimeters
- planet
- a large rocky or gaseous body that orbits a star
- planetary nebula
- a glowing shell of gas ejected by a dying, low-mass star
- planetary nebulae
- glowing shells of gas ejected by dying, low-mass stars
- planetesimals
- asteroid-size bodies in a young planetary system that collide to form larger bodies
- planisphere
- a two-dimensional map of the sky with an adjustable overlay to show the part of the sky visible at any time of the night or year
- plasma
- a gas that has been heated to a state where it contains ions and free-floating electrons; also known as ionized gas
- plasmasphere
- a region of cold, high-density plasma above the ionosphere
- plate tectonics
- a theory that describes how Earth’s crust is broken into plates and how those plates move across Earth’s surface
- polar cap
- an icy region at the north or south pole of a planet
- polarization
- a state in which the directions of the electric or magnetic field in an electromagnetic wave changes in a regular pattern; light from celestial objects is often polarized
- polycyclic aromatic hydrocarbons
- Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from…
- position angle
- the direction in the sky of one celestial object from another, measured eastward from due north
- power
- the ability of a telescope or binoculars to increase the apparent size of a distant object
- Poynting-Robertson effect
- a drag on interplanetary particles caused by their interaction with solar radiation, which causes the particles to lose orbital momentum and spiral into the sun
- precession
- the slow, periodic change in the direction an object’s rotational axis caused by the gravitational influence of another body
- primary lens
- a telescope's main lens which gathers light and brings it to a focus
- primary mirror
- a telescope's main mirror which gathers light and brings it to a focus
- prime meridian
- the line of longitude that runs through Greenwich, England
- prism
- a wedge-shaped piece of glass that breaks white light into its constituent colors