Astro Glossary

  • Parallax/Parsec

    A star's parallax is the angle it appears to move against the background of more-distant objects as Earth moves from one side of the Sun to the other. Astronomers use that angle to measure the star's distance. The technique provides highly accurate distances out to a few hundred light-years, and less-accurate ones out to thousands or tens of thousands of light-years. Astronomers generally express distances to stars and galaxies in parsecs, which is short for parallax-seconds, or the parallax in seconds of arc. One parsec equals 3.26 light-years. When describing a star's absolute magnitude, or its true brightness, astronomers use a standard distance of 10 parsecs, or 32.6 light-years. 

  • Parker Solar Probe

    Parker Solar Probe is designed to study the Sun and its hot outer atmosphere, known as the corona, from closer range than any other spacecraft to date. Launched August 12, 2018, its primary mission will consist of 24 orbits around the Sun, shaped by seven close encounters with Venus. At its closest, in 2025, it will pass less than four million miles (6.3 million km) from the Sun's surface. It will move faster than any other spacecraft in history, reaching a maximum of 430,000 miles per hour (690,000 kph) at its closest approach. Heavy shielding will protect the spacecraft from the Sun's intense heat and radiation. The craft is named for Eugene Parker, a solar physicist. He hypothesized the existence of the solar wind in the 1950s.

  • Particle Physics

    The branch of physics that deals with the most fundamental building blocks of matter. Much of the research is conducted in particle accelerators, which send streams of protons or other particles on collision courses with each other. The impacts produce torrents of particles and energy, many of which cannot be studied in any other way. Particle physicists also work with astrophysicists to probe conditions shortly after the Big Bang, as well as in some of the most powerful events in the universe.

  • Pegasus, the Flying Horse

    A large pattern of stars marked by a great square.

  • Perseus

    A constellation that arcs high overhead in fall and early winter.

  • Phoenix Mars Lander

    A craft sent to Mars’ frozen plains north of the planet’s Arctic Circle to look at conditions that are conducive for life, including the presence of liquid water, organic molecules, energy sources, and chemicals that make up our life form. Phoenix will study the conditions by digging perhaps two or three feet into the frozen soil. Phoenix will also operate a weather station. Its readings will provide new information on the interplay between the planet’s polar ice caps, its frozen ground, and its atmosphere.

  • Pioneer Probes

    Pioneer 10 and 11 blazed the trail to the outer solar system during the 1970s. Pioneer 10 became the first spacecraft to visit any planet beyond the asteroid belt when it flew past Jupiter on December 3, 1973. Pioneer 10 found that Jupiter’s interior is hotter than previously thought, discovered that the planet’s radiation belts are strong enough to kill a human being, and discovered that Jupiter’s moon Io is embedded in a giant cloud of hydrogen that encircles Jupiter. Pioneer 11 flew past Jupiter a year later, then in 1979 became the first craft to encounter Saturn.

  • Pisces, the Fish

    A constellation of the zodiac.

  • Piscis Austrinus, the Southern Fish

  • Planet Nine

    Astronomers have theorized that a Neptune-sized planet orbits the Sun in the outer solar system, far beyond the realm of the eight known major planets. The planet was posited based on the alignments of several members of the Kuiper Belt, which are big chunks of ice and rock left over from the birth of the planets. The possible Planet Nine would orbit the Sun once every 10,000 to 20,000 years, at a distance equal to 400 to 800 times the distance from Earth to the Sun. It could have been ejected into its current orbit by a close encounter with Jupiter, the solar system's largest planet.

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