atmosphere: The envelope of gases surrounding Earth, another planet or a moon.
black hole: A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
cell: (in biology) The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
cosmos: (adj. cosmic) A term that refers to the universe and everything within it.
decay: (for radioactive materials) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.
electromagnetic radiation: Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.
fission: The spontaneous splitting of a large unit into smaller self-sustaining parts. (in physics) A process in which large atomic nuclei break apart to form two or more lighter nuclei. The excess mass of the parent nuclei (compared to the resulting smaller ones) is converted into energy.
gamma rays: High-energy radiation often generated by processes in…
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