AstroGlossary - P
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parallax
The apparent shift in position of an object relative to background objects when observed from two different locations. An everyday demonstration of parallax is easy to try: hold a finger in front of your face, and without moving it, wink one eye and then the other. When you see the position of your finger change with respect to the background, you are seeing parallax. Astronomers measure the parallax of a stars close to the Earth using Earth's orbital diameter as a baseline. The astronomer observes a star at a six-month interval, when the Earth is at opposite ends of an imaginary baseline defined by the width of its orbit around the Sun. Astronomers look for a parallax shift in the two images of the star, and if they find one, they can use simple geometry to estimate the star's distance from Earth. parsec
A unit of distance equal to 3.26 light-years. The name means "PARallax-SECond," and it refers to a way to measure the distances to other stars. The most accurate way to measure the distances to close stars is to use basic geometry. Astronomers measure the position of a star in the sky at six-month intervals, when Earth is on opposite sides of the Sun. If the star is close, then it will appear to shift a bit compared to the background stars. It's the same effect you see if you hold your finger in front of your face and look at it with first one eye, then the other: the finger appears to move against the background of objects. This effect is called parallax. If a star has a parallax of one second -- in other words, it appears to shift back and forth across the sky by exactly one second of arc (1/3600 of a degree), then its distance is one parsec. penumbra
1) A partial shadow cast by Earth or Moon that surrounds the umbra, or total shadow. During a total lunar eclipse, the Moon passes through Earth's penumbra, then into the umbra where the Moon "disappears". Similarly, during a total solar eclipse, Earth passes through the Moon's penumbra, and then into its umbra. 2) The region immediately surrounding the darkest portion, or umbra, of a sunspot. This region is darker and cooler than the surrounding photosphere, but lighter and hotter than the center of the sunspot. perigee
The point of minimum orbital distance from the Earth and maximum orbital velocity around the Earth. See also: perihelion
The point of minimum orbital distance from the Sun and maximum orbital velocity around the Sun. See also: photon
A discrete energy unit of light. Photons behave like waves or particles, depending on the experiment and observer. photosphere
An outer layer of a star where most light shines into space. The temperature of the photosphere is called the effective temperature of the star. Astronomers analyze the composition of the photosphere with a spectrograph and telescope to learn about the interior of the star. planet
An object that is larger than a moon but smaller than a brown dwarf and that orbits a star. The Earth is a planet, and it is the third of a series of planets in our Solar System. In order of increasing distance from the Sun, the planets in our Solar System are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. While Pluto is conventionally listed as one of the planets, it is debatable whether its small size and odd orbit allow it to be classified as such. Planets have also been discovered orbiting around other stars; these planets are called extrasolar planets. planetary nebula
A bright, colorful shell of gas and dust surrounding a star in the last stages of life. A planetary nebula is created when the star puffs off its outer atmosphere. The nebula usually looks like a doughnut, sometimes with the small, hot, rapidly evolving star visible in the center. The Ring Nebula (M57) in the constellation Lyra is an example. See also: plasma
A phase of matter in the form of hot, ionized gas. Plasma is created when energetic photons strip electrons from atoms, resulting in a hot soup of free electrons and ionized atoms. The Sun and other stars are made mostly of hydrogen plasma. Pluto
A solar system object that orbits the Sun at an average distance of 39.5 times the distance between Earth and Sun. Pluto has a mass 0.002 times the mass Earth and a radius 0.18 times the radius of Earth. It is a small body made of water ice, and temperatures there do not rise above –223 degrees Centigrade. Pluto has three moons. Discovered in 1930, it originally was classified as a planet, but was demoted in 2006. precession
The "wobble" of the Earth's rotational axis, which sweeps out an imaginary cone, much like a spinning top, once every 26,000 years. Precession changes the pole star as seen from Earth. Thuban, the brightest star in the constellation Draco, was the pole star while the Egyptians built the Pyramids in Egypt. Since that time, the motion of precession has rotated the Earth's axis away from Thuban and towards Polaris, the current pole star. In 13,000 years, Earth's rotational axis will point towards Vega, the new pole star. prime focus
A reflecting telescope's primary mirror focal point. Astronomers place cameras at prime focus to record bright, wide field images. prism
A three-dimensional shape with identical polygon bases and parallelogram sides. A familiar shape is the triangular glass prism used to refract light into a spectrum of color. proper motion
The slow motion of a celestial object across the sky, relative to other objects, due to the actual velocity of the object, for example, in its orbit around the center of the galaxy. While all celestial objects appear to move across the sky from hour to hour because of the rotation of the Earth, a few stars shift their position relative to the others when observed year after year. This relative shift in their position is their proper motion. Stars that exhibit proper motion tend to be relatively nearby and are whizzing through the solar neighborhood. Even though they are moving at many thousands of miles per hour (hundreds of km/s) relative to us, they are still far enough away that they only appear to move a few arc seconds per year or less. 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. See also: pulsar
A spinning neutron star with a magnetic field on the order of one trillion Gauss. This magnetic field accelerates electrically charged particles along the magnetic poles, forming a beam of energy that shoots into space from the poles. If the beam shines toward Earth, astronomers see a flickering beacon. The most famous pulsar, the Crab Nebula, pulses 30 times a second.
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