Astro Glossary

Planetary Formation

Planetary Nebulae

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.

Planetary Rings

Planets

Pleiades Cluster, M45

A star cluster in the constellation Taurus about 440 light-years away, also known as the seven sisters.

Pluto

The prototype “dwarf planet,” Pluto 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 of 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 minus-223 degrees Centigrade. Pluto has five known moons. Discovered in 1930, it originally was classified as a planet, but was demoted in 2006.

Pluto's Moons

As of summer 2012, astronomers had discovered five moons orbiting Pluto. The largest, Charon, discovered in 1978, is roughly half the diameter of Pluto itself. Of the other moons, two have been named by the International Astronomical Union — Nix and Hydra. The other two, discovered in 2011 and 2012, have received the provisional designations P4 and P5, pending approval of formal names. Nix, Hydra, P4, and P5 are all quite small, with none larger than about 100 miles (160 km) in diameter, with the smallest only about 10 percent of that size.

Polaris, the North Star

A Cepheid variable star that marks the north celestial pole, so all the other stars seem to rotate around it.

Pole Star

Any star that aligns with Earth's polar axis. In the northern hemisphere, the pole star currently is Polaris, also known as the North Star. Thanks to precession, however, Earth wobbles on its axis, which causes the north pole to aim at different stars over a cycle of about 26,000 years. The next pole star will be Gamma Cephei, one of the leading lights of the constellation Cepheus. Other future pole stars include Vega, the brightest star of Lyra, and Thuban, in Draco, which was used to help align the Great Pyramids of Giza. In the southern hemisphere, the pole is marked by Polaris Australis, also known as Sigma Octantis. It is much fainter than Polaris, however, so it's not as useful a marker.

Pollux

A yellow giant star 35 light-years from Earth. One of the twins of Gemini.