acceleration: A change in the speed or direction of some object.
atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
atomic: Having to do with atoms, the smallest possible unit that makes up a chemical element.
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.
electromagnetism: The science that has to do with the physical links between electricity and magnetism. It’s also the term for the properties of an electric current that cause it to generate a magnetic field. This term can also be applied to the physical force (the electromagnetic force) that governs interactions between charged particles and which are due to their electric charge and their release or absorption of light (photons).
force: Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
fundamental: Something that is basic or serves as the foundation for another thing or idea.
gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
inertia: The tendency of objects to resist changes in their motion.
mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
neutron: A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.
orbit: The curved path of a celestial object or spacecraft around a galaxy, star, planet or moon. One complete circuit around a celestial body.
particle: A minute amount of something.
proton: A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.
radiation: (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
radioactive: An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.
radioactive decay: A process by which an element is converted into a lighter element through the shedding of subatomic particles (and energy).
strong force: (in physics) A fundamental interaction in nature, one that binds quarks together to make other types of subatomic particles, such as protons and neutrons. This force also holds protons and neutrons together within an atom’s nucleus. Not all subatomic particles can “feel” this force. Electrons and other leptons, for instance, are immune to it.
sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
universe: The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
