O P

A disagreement between cosmological assumptions and observation. The apparent paradox: in an infinite, static universe the night sky should blaze as the surface of a star; but the night sky is dark. Why? When German astronomer Heinrich Olber asked this simple question in 1826, he effectively touched the sky. Newton had declared the universe infinite. But the dark night sky demands a dynamic, finite universe. An expanding evolving universe resolves the paradox.

Old

A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters.

Oort Comet Cloud

The large spherical region around the Sun from which most "new" comets come; a reservoir of objects with aphelia at about 50,000 AU, or extending about a third of the way from the nearest other stars.

Opacity
A property of matter that prevents light from passing through it. The opacity or opaqueness of something depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultraviolet light, but is opaque to visible light.

Open Cluster
A comparatively loose or "open" cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of the Galaxy; sometimes referred to as a galactic cluster.

Open Universe
A model of the universe in which gravity is not strong enough to bring the universe to a halt; it expands forever. In this model the geometry of space-time is such that if you go in a straight line, you not only can never return to where you started, but even more space opens up than you would expect from Euclidean geometry.

Opposition

A superior planet is said to be "in opposition" when it is directly on the opposite side of the Earth from the Sun. This is generally the closest it comes to the Earth and the time at which it is most easily visible.

Optical Double Star
Two stars at different distances that are seen nearly lined up in projection so that they appear close together, but that are not really gravitationally associated.

Optical Telescope

A telescope in which mirrors are used to reflect an image to the observer.

Orbit

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

Orbit Period

The duration of one orbit. For instance, Earth's orbital period around the Sun is one year, and the Moon orbits Earth in 27.3 days.

Orbit Plane

A plane containing at least two masses orbiting around a common center of gravity. Earth and Sun orbit each other in an orbital plane, as do Earth and Moon. However, the Earth-Moon orbital plane crosses the Earth-Sun orbital plane at a five-degree angle.

Oscillation
A periodic motion; in the case of the Sun, a periodic or quasi-periodic expansion and contraction of the whole Sun or some portion of it.

Ovoid

Shaped like an egg.

Oxygen

An element vital to the development of life, and widespread in the universe. Oxygen makes up 20% of Earth's atmosphere.

P (51)

Pair Production
The physical process whereby a gamma-ray photon, usually through an interaction with the electromagnetic field of a nucleus, produces an electron and an anti-electron (positron). The original photon no longer exists, its energy having gone to the two resulting particles. The inverse process, pair annihilation, creates two gamma-ray photons from the mutual destruction of an electron/positron pair.

Palimpsest

A circular feature on the surface of dark icy moons such as Ganymede and Callisto lacking the relief associated with craters; Pamlimpsests are thought to be impact craters where the topographic relief of the crater has been eliminated by slow adjustment of the icy surface.

Palus

Literally "swamp"; really a small plain.

Parabola
A conic section of eccentricity 1.0; the curve of the intersection between a circular cone and a plane parallel to a straight line in the surface of the cone.

Parallax

An angle formed by measuring the position of an object from two points at opposite ends of a baseline. Astronomers measure the parallax of a star using Earth's orbital diameter as a baseline. The astronomer observes and photographs a star at a six-month interval, when Earth is at opposite ends of an imaginary baseline defined by the width of its orbit around the Sun. Similarly, you can use the distance between your eyes as a baseline. Look at a distant object, say a telephone pole, with one eye closed then the other to see a parallax shift.

Astronomers look for a parallax shift in two star images as a way of estimating a star's distance from Earth. The greater the shift with respect to background stars, the closer the star. This sort of measurement is applicable to only nearby stars, no more than a few hundred light years away. At 300 light-years, the angular shift is equivalent to a U.S. quarter seen at a distance of 300 miles, or 500 kilometers.

Parsec

One parsec is the distance to that star, when 1 astronomic unit (1 A.U.= average distance between earth and sun) is at 1 second angle( when angle A is 1 second in above diagram). Thus, if the angle is 2 seconds, it is half parsec. Distance (parsec) = 1 Astronomic Unit / angle A (second angle). It is also an average interval between the fixed stars in the galaxy system. 1000 parsec is called 1 kilo parsec, and 1 million-parsec is called mega parsec.

Particle Accelerator

A machine that, using electromagnets, can accelerate moving charged particles, giving them more energy.

Patera

Shallow crater, scalloped, complex edge.

Payload

The cargo carried into space by a launch vehicle or on an artificial satellite..

Peculiar Velocity
The velocity of a star with respect to the local standard of rest; that is, its space motion, corrected for the motion of the Sun with respect to our neighboring stars.

Penumbra

The outer filamentary region of a sunspot.

Perfect Radiator
A body that absorbs and subsequently re-emits all radiation incident upon it.

Periapsis
The point in an orbit when two objects are closest together. Special names are given to this point for commonly used systems: See also Periastron, Perihelion, and Perigee. The opposite of Apoapsis.

Periastron
The place in the orbit of a star in a binary-star system where it is closest to its companion star.

Perigee

The point of minimum orbital distance from the Earth and maximum orbital velocity around the Earth.

Perihelion

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)

Period

The time in it takes for a planet or satellite to complete a single revolution about its primary. The sidereal period is measured by using a star seen from the primary or a line joining the primary and a star as a starting and finishing point.

Period-Luminosity Relation
An empirical relation between the periods and luminosities of certain variable stars.

Perturb

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

Phase

The size of the illuminated portion of a planet or moon, as seen from Earth.

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.
The reason is that a photon has energy in proportion to its wavelength, and the constant of proportionality is the Planck constant. Below a certain frequency -- and thus below a certain energy -- the incident photons do not have enough energy to knock the photoelectrons out of the metal. Above that threshold energy, called the work function, photons will knock the photoelectrons out of the metal, in proportion to the number of photons (the intensity of the light). At higher frequencies and energies, the photoelectrons ejected obtain a kinetic energy corresponding to the difference between the photon's energy and the work function.

Photometry
The measurement of light intensities.

Photon

One of elementary particles. Mass of unmoving particle is zero.
Light is a wave but at the same time, it has a characteristic of a particle. This light as a particle is called as photon.

Photosphere

The visible surface of the Sun; the upper surface of a convecting layer of gases in the outer portion of the sun whose temperature causes it to radiate light at visible wavelengths; sunspots and faculae are observed in the photosphere.

Pi
The constant equal to the ratio of the circumference of a circle to its diameter, which is approximately 3.141593.

Plage

Bright regions seen in the solar chromospheres.

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 x 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 x nu

Planet
Any of the nine largest bodies revolving about the Sun, or any similar bodies that may orbit other stars. Unlike stars, planets do not give off their own light, but only reflect the light of their parent star.

Planetarium
An optical device for projecting on a screen or domed ceiling the stars and planets and their apparent motions in the sky.

Planetary Nebula
A shell of gas ejected from, and enlarging about, a certain kind of extremely hot star that is nearing the end of its life.

Planitia

Low plain.

Planum

Plateau or high plain.

Plank's Quantum Principle

The idea that light (or any other classical waves) can be emitted or absorbed only in discrete quanta, whose energy is proportional to their wavelength.

Plasma

A low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

Pointing
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.

Polar Axis
The axis of rotation of the Earth; also, an axis in the mounting of a telescope that is parallel to the Earth's axis.

Polarization

A special property of light; light has three properties, brightness, color and polarization.

Polar Orbit

A satellite orbit passing above or close to the Earth's poles.

Positron

The antimatter equivalent of an electron. It has the same mass as an electron, but a positive, rather than negative, charge.

Precession

The "wobble" of the Earth's rotational axis. One complete wobble takes 26,000 years. Precession changes the pole star as seen from Earth. Thuban was the pole star while the Egyptians built the Pyramids in Egypt. The motion of precession rotated the Earth's axis away from Thuban towards Polaris. 13,000 years from now, Earth's rotational axis will point at Vega.

Prime Focus

A reflecting telescope's primary mirror focal point. Astronomers place cameras at prime focus to record bright, wide field images.

Principle of Equivalence
Principle that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment.

Prominence

An eruption of hot gases above the photosphere of the Sun. Prominences are most easily visible close to the limb of the Sun, but some are also visible as bright streamers on the photosphere.

Promontorium

Cape; headland.

Proper Motion

The motion of a celestial object across our sky. The proper motions of stars visible to the unaided eye are caused by the relative motions of our solar system and stars as we orbit around the center of the Milky Way.

Proton

A positively charged particle in the nucleus of an atom. A proton's electrical charge has the same magnitude as that of an electron. A single proton, however, is 1,836 times the mass of an electron. Usually, the number of protons balances the number of electrons within an atom. In this case, the atom is electrically neutral. When the balance is tipped, the atom becomes electrically charged and is called an ion.

Protostar

A young star that ha snot yet started nuclear fusion in its core.

Pseudocrater

A generally circular crater produced by a phreatic eruption resulting from emplacement of a lava flow over wet ground.

Pulsar

A spinning neutron star with a magnetic field on the order of one trillion Gauss. This magnetic field accelerates electrons along the magnetic poles, which then radiate photons that form a beam of light projecting along the poles. If the beam shines toward the Earth, astronomers see a flickering beacon all over the electromagnetic spectrum. The Crab Nebula pulsar pulses 30 times a second. The light curve shows a strong pulse followed by a weak pulse. The strength difference between the primary and secondary pulses is probably due to the direction of the pulsar's magnetic field.

Pulsating Variable
A variable star that pulsates in size and luminosity.