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Rutheford-Bohr atom originating in 1911 by Ernest Rutheford and Niels Bohr, the erroneous the­ory of electrons circling in a regular orbit around a nucleus, as planets around the Sun.

scintillation a flash of light emitted by a phosphor when it absorbs a photon or ionizing particle.

slow neutron also known as a thermal neutron, a slow-moving neutron capable of fission.

spin a particle's rotation in a consistent direction about an axis.

stable referring to a particle that does not decay or disintegrate into another element.

string as proposed in string theory, a vibrating, one- dimensional stringlike particle with length but no thickness, postulated as the basic unit of all matter. Strings interact by splitting and joining.

string theory any one of several theories that states that the basic unit of matter or reality in the universe is not the point of a particle but a vibrat­ing, one-dimensional stringlike particle that splits and joins with other strings. The basic string theory requires that spacetime have 26 dimensions, while an advanced theory, the superstring theory, requires 10 such dimensions. String theory provides a unifying structure to explain the behavior of natural forces and elementary particles; however, the theory cannot be proven. (The term string theory increasingly refers to superstring theory.)

strong force one of the four fundamental forces of nature, a force that binds quarks, antiquarks, and gluons and makes hadrons. Also known as nuclear force or strong interaction.

strontium 90 a dangerous radioactive isotope, found in fallout following a nuclear explosion.

subatomic referring to the interior of an atom or that which is smaller than an atom.

supercollider a giant, high-speed particle accelerator.

superstring theory see string theory.

tau the heaviest lepton.

thermonuclear reaction an occurrence of fusion created by extremely high temperatures. One of a chain of such reactions that occur continuously on the Sun and stars.

tracer a radioisotope with a short half-life placed inside a substance that is injected into the body and tracked. High levels of the radioisotope in an organ may indicate cancer.

weak force one of the four fundamental forces of nature, it is involved in the interchange of energy, mass, and charge within the nuclei between leptons and quarks and their antiparticles. The weak force is responsible for the change of one particle into another. Also known as weak interaction.

weakon any one of three large particles, the neutral Z particle, or the positive and negative W particles, involved in the weak force.

WIMP weakly interactive massive particle; a hypo­thetical subatomic particle with a large mass that interacts weakly through gravity, a possible form of dark matter in the universe.

space

accelerating universe the expansion of the universe, with galaxies receding into the far reaches of space at greater and greater speeds. This is the most widely accepted model of the universe.

accretion disk a massive, rotating disk of gas and dust surrounding a newborn star or a black hole.

active galactic nucleus an extremely bright and energized region at the center of some galaxies, thought to be a giant accretion disk surrounding a black hole.

albedo the amount of light reflected off a celestial object.

aphelion a planet or comet's point of orbit farthest from the Sun.

asterism a group of stars not belonging to any of the 88 recognized constellations.

asteroid a very large rocky mass, ranging in size from a half mile to 600 miles across, that travels through space. Also known as a minor planet.

asteroid belt a ring of asteroids orbiting the Sun between Mars and Jupiter.

astral of or pertaining to the stars.

astrobiology the science concerning life or the potential for life on other planets.

astrology a pseudoscience that claims to be able to predict one's destiny according to the position of celestial bodies when one was born; widely discred­ited by science.

astronomical unit mean distance of the Earth from the Sum—92,900,000 miles—used to express the dis­tances of other celestial bodies.

astronomy the study and observation of the universe.

astrophysics the science concerning the physical properties of the universe, such as light, chemical makeup, gravitational forces, etc.

aurora the colored bands or streamers of light that appear across the night sky when charged particles from the Sun interact with Earth's magnetic field. In the Northern Hemisphere, the aurora is known as the northern lights, or aurora borealis. In the South­ern Hemisphere, it is the southern lights, or aurora australis.