meet, they can annihilate each other and release energy. In the Standard Model of Particle Physics, antiparticles are natural analogues to all the particles.

arcminute: A unit of angle corresponding to 1/60th of a degree. The full Moon is 30 arcminutes in diameter.

arcsecond: A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arcminute. An arcsecond is approximately the size of a dime viewed from a distance of 1 mile.

array: There are two examples of arrays in common use in astronomy: (1) a group, or array, of telescopes combined to simulate a single large telescope, kilometers or even thousands of kilometers across; (2) astronomical instruments composed of detector arrays or charge-coupled devices that consist of thousands of individual detectors constructed on centimeter-sized wafers of silicon, or other materials.

astronomical unit (AU): The mean distance between Earth and the Sun.

Atacama Large Millimeter Array (ALMA): A single research instrument composed of up to 80 high-precision antennas that will enable research into optically dark regions by probing the millimeter portion of the electromagnetic spectrum, where the cold universe shines brightly. ALMA is designed to probe the first stars and galaxies and directly image the formation of planets.

baryon: A subatomic particle with mass and three constituent quarks bound together by the strong force, such as a proton or neutron. Ordinary matter as we know it consists largely of baryons.

baryon acoustic oscillations: Cosmological perturbations in the early universe excited sound waves in the photon-baryon fluid. These baryon acoustic oscillations define a standard ruler whose length is the distance that sound can travel, at a speed of ~c/√3, before decoupling.

big bang: The theory that the universe began with all matter and energy concentrated to very high density and temperature some 13 billion years ago. The present universe expanded from that era and is still expanding.

black hole: A region of space in which the gravitational pull is so strong that, classically, nothing can escape. The boundary of this region is called the black hole’s event horizon. Black holes can form when a massive star undergoes gravitational collapse.

blazar: Believed to be an active galactic nucleus (AGN) that has one of its relativistic jets pointed toward Earth, so that the emission we observe is dominated by phenomena occurring in the jet region. Among all AGNs, blazars emit over the widest range of frequencies and are detected from radio to gamma ray.

B-mode polarization: A kind of polarization of radiation that is primarily vortex-like and is theorized to be predominantly produced during the inflation period.

boson star: A star composed of self-gravitating nonbaryonic matter called bosons. All fundamental particles in nature can be divided into one of two categories: fermions or bosons. Bosons, unlike fermions, do not obey the Pauli exclusion principle. While examples of fermions include electrons, protons, neutrons, quarks, and neutrinos, particles classified as bosons include photons and gluons.

brown dwarf: A star-like object that contains less than about 0.08 the mass of the Sun and is thus too small to ignite nuclear fuels and become a normal star. Brown dwarfs emit small amounts of infrared radiation owing to the slow release of gravitational energy and may be a component of dark matter.

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