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M - N - O

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



  • Mars: The fourth planet from the Sun. It's reddish in appearance and shows evidence of liquid water on its surface in the distant past .

  • Mercury: A small, rocky planet closest to the Sun.

  • Messier, Charles (1730-1817): An 18th century comet hunter probably best known for compiling a list of 110 celestial objects known as the "Messier Catalog" that is still in use today.

  • meteor: A natural solar-system object that enters Earth's atmosphere and is subsequently destroyed in a flash of light before reaching the surface. Several regular meteor showers occur throughout the year including the Leonids and Perseids to name two.

  • meteorite: A meteoroid that has landed on the surface of Earth.

  • meteoroid: A small rock in space consisting of rock and/or iron that is probably a chip of an asteroid.

  • Moon, The: The large rocky body orbiting the Earth around every ~28 days.



  • nebula: A large cloud of gas and dust in space, much smaller than a galaxy. They can actually be a star producing region. They can emit, reflect, and/or absorb light and radiation from nearby stellar radiation. Ex. M42 - The Orion Nebula.

  • neutron star: An object only tens of miles across, but greater in mass than the Sun.

  • Neptune: The eighth planet from the Sun. It is a gas giant.

  • New General Catalog: Also known as the "NGC", this catalog was compiled by John Louis Emile Dreyer around 1887. It contains a variety of objects with a detailed legend for descriptions of each of them.

  • nova: see "supernova"



  • occultation: The process by which one celestial body passes in front of another, blocking it from the view of an observer as seen from Earth. Similar to an "eclipse".

  • open cluster: Gravitationally bound concentrations of stars believed to originate from large cosmic gas and dust clouds. Smaller than a globular cluster and usually have a short lifespan as a cluster. Also see "star cluster" and "globular cluster". Ex. M45 - the Pleiades star cluster.

  • opposition: The positioning of two celestial bodies relative to a third (usually the Sun, Earth, and a third celestial body) such that they are 180 degrees apart in Right Ascension. Note that the bodies need not actually lie on a straight line since they may lie at different Declinations.

  • orbit: The path and process by which one object revolves or moves around another, held together by their mutual gravitation.

  • Oort Cloud: A theoretical region that surrounds the solar system at about 50 to 100 astronomical units; the Oort Cloud is presumably a remnant of the solar system's accretion disk and it is thought to contain trillions comets.


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