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P - Q - R

Select the letter that (probably) leads off the word in question. Please suggest new words or corrections.



  • perihelion: The point nearest the sun in the orbit of a planet or other celestial body.

  • Phobos: One of two small moons orbiting Mars discovered in 1877 by American astronomer Asaph Hall.

  • photosphere: The visible outer layer of a star, especially of the Sun.

  • planet: A large, round object, larger than an asteroid, that is formed inside an accretion disk. Unlike a star, a planet does not produce its own energy via nuclear reactions. There are nine planets in our solar system. Ex. Earth.

  • planetary nebula: An expanding gas cloud that was created as the result of gas being expelled from a sunlike star in the later stages of its life. Ex. M57 - The Ring Nebula.

  • Pluto: The ninth and final known planet from the Sun. Pluto is the smallest planet in diameter. It is rocky and orbited by one moon named Charon.

  • Prime Meridian: The longitude on Earth from which the 24 hour UTC clock is centered. It runs through Greenwich, England, among other places. Also see "Universal Coordinated Time".

  • pulsar: A very fast-spinning and highly dense object that emits light, radio and/or X-rays in very tight beams, similar to a lighthouse.



  • quasar: A small, extremely bright object at the center of galaxies who's origin is believed to result from the expenditure of large amounts of energy from the surroundings of a giant black hole.



  • red giant: A large, red sunlike star in its latter stages of life.

  • redshift: An measurement of the increase in the wavelength of light often due to the Doppler effect. It is used as a means of more accurately predicting the distances to cosmic objects. Objects with higher observed redshifts are farther away than those with lower redshifts.

  • right ascension: The coordinate system in the sky that corresponds to longitude on Earth. It is measured eastward from the vernal equinox. Also see "vernal equinox".


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