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- Parameter
- a special type of variable that may be required as the input for a function. If the variable being passed to the function does not match the data type that the function expects, the program will produce an error.
- Parsec
- another measuring stick used by astronomers. One parsec equals about 3.3 lightyears.
- Particle Accelerator
- these devices accelerate charged particles to speeds near the speed of light. By colliding the particles together and studying what comes out of the collisions, scientists are able to learn about the fundamental properties of matter.
- Period
- periods apply to things that repeat, and measure how long it takes them to repeat. For instance, the period of the Earth to turn around its axis is 24 hours and 56 minutes, while the period for the Earth to travel around the sun is 365 days.
- Photometry
- the process of measuring the brightness of an astronomical object.
- Photon
- an individual quantum or particle of light. A single packet of light, characterized by its wavelength, frequency, or energy.
- Photon Epoch
- the time near the beginning of the Universe which was dominated by photons and lasted from approximately 10 seconds to 3 x 105 years.
- Photon Flux
- the number of emitted photons that are detected in a given area per unit time. For instance, the number of photons passing through a square meter-sized detector in one second would be the photon flux.
- Pixel
- the smallest image-forming unit on a computer screen or a CCD.
- Photon
- a positively charged subatomic particle usually found in the nucleus of an atom.
- Planetesimal
- objects that are large enough to gravitationally attract gas, dust and other planetesimals, thus forming larger bodies, and eventually protoplanets. Planetesimals range in size from about 1 km to the size of the Moon.
- Planck Epoch
- refers to the period from the beginning of the Universe to the Planck Time. This epoch only lasted `10^-43` seconds.
- Planck Time
- the unit of time that is determined by unit, or dimensional, analysis using fundamental constants such as the speed of light, gravitational constant and the value of (Pi). It is approximately equal to 5.4 x 10-44 seconds.
- Plasma
- a gas composed of unbound electrons and positively charged atomic nuclei.
- Positive Feedback Loop
- when an action results in the amplification of that action, it is said to be in a positive feedback loop. See Episode 101 notes for an example.
- Positron
- the anti-particle of the electron. It has all the same characteristics as an electron except for its charge, which is positive.
- Potential Energy
- the energy that is stored in objects due to is position in a force field. For example, the atoms in a molecular cloud have gravitational potential energy that is turned into heat energy, which is just the energy of the random motions of the particles as the cloud collapses towards its gravitational center.
- Primordial Black Hole
- very tiny, theoretical black holes that may have formed during the Big Bang. Originally predicted by Prof. Stephen Hawking, these types of black holes have very short lifetimes and have not yet been detected.
- Programming
- the process by which computers are given instructions to perform a particular task.
- Programming Language
- just as there are many human languages, computers also have many different languages, and each language has its own rules that a computer programmer must know and understand in order to write programs in that language.
- Protron
- protons are positively charged and are one of two types of particles found in the nuclei of atoms (the other being neutrons). The type of element (e.g. Hydrogen, Helium, etc.) is determined by the number of protons in its nucleus. For example, all hydrogen atoms have one proton in their nucleus, helium atoms have two protons, lithium atoms have three protons, and so on. A proton is made up of two Up quarks and one Down quark. Note: A hydrogen atom is made up of one proton, zero neutrons, and one electron.
- Proportion Counters
- detectors in which high-energy photons (such as X-rays) ionize gas, producing a number of charged particles that is proportional to the energy of the incoming photon. The photon’s energy can then be determined by reading out the resultant pulse of charge.
- Protoplanetary Disk
- a disk of swirling gas and dust surrounding a young star that may eventually form into the planets, asteroids and debris of a solar system.
- Protoplanets
- regions of dust and gas in a protoplanetry disk that are condensing under their own gravity but are still too unstable to be considered fully formed planets. Protoplanets are at least as large as the Moon. Collisions between protoplanets can eventually form planets.
- Pulsar
- a rapidly rotating neutron star with a strong magnetic field, which emits twin beams of energy that sweep through the observer’s field of view like beams from a lighthouse.
