ESO Astronomical Glossary - P


Cerro Paranal is the name of the mountain where ESO's Very Large Telescope (VLT) is located. It is situated in Northern Chile, in the Atacama desert, at 120 km from the city of Antofagasta at an altitude of 2600 m.

A parsec is a unit of distance commonly used in astronomy and cosmology, the parsec is equal to about 3.262 light years, or 3.09 x 1016 metres. The name 'parsec' arises from the definition of the unit, the distance at which an object is seen to move by 1 arcsecond because of the Earth's orbit around the Sun (an effect called 'parallax').

Photometry is the measurement of light intensities from astronomical objects. It is a highly important tool in astronomy.

Photons are the carriers of individual 'packets' of electromagnetic radiation.

A planet is a large celestial body, consisting of rocks and gas, that orbits a star and does not shine on its own but reflects the star's light. As an additional condition in our solar system, the body has to be large enough to clear a free path on its orbit. There are eight planets orbiting the Sun in the solar system.

Planetary nebula
Planetary nebulae are a type of emission nebula that are created when an average-sized star, with a mass of less than eight times the mass of the Sun, ends its life. With the star finally unable to sustain the nuclear fusion reactions in its core, it sheds its layers of gas, leaving the bare hot core (the white dwarf) exposed. The ejected gas forms a nebula or cloud round the dwarf, and both will slowly cool and fade over many tens of thousands of years.

A plasma is an extremely hot, ionised gas. It is composed of free-floating ions (positively charged) and free electrons (negatively charged). A plasma behaves differently from a neutral gas and conducts electrical currents. Stars are composed of plasma.

Pluto is a dwarf planet in our Solar System discovered in 1930. It has one large satellite, Charon, and several smaller ones. Pluto orbits the Sun in a highly elliptical orbit. Until 2006, Pluto was known as the 9th planet.

Polarimetry is the study of the polarisation properties of light from astronomical sources.

Polarisation is a state in which the directions of the electric or magnetic field in an electromagnetic wave change in a regular pattern. The polarisation of light from astronomical sources can yield information that cannot be obtained with imaging or spectroscopy, in particular related to the properties of magnetic fields. The study of the polarisation of light is called polarimetry.

Pre-Main Sequence
Pre-Main Sequence (PMS) refers to stars that are no longer protostars, having stopped their accretion process and dispersed their surrounding gas and dust envelopes, but have not yet reached the Main Sequence. Their source of energy is gravitational contraction rather than hydrogen fusion.

Proper Motion
Proper motion is the actual motion of a star across the sky. This motion is due to the orbit of the star in the Milky Way Galaxy. It is measured in arcseconds per year.

Protoplanetary disc
A protoplanetary disc is a disc of gas and dust that surrounds a newborn star; planets form from collisions of particles inside the disc.

A protostar is a cloud of hot, dense gas and dust that is gravitationally collapsing to form a star.

A pulsar is a rapidly spinning neutron star that emits energy in pulses.