Glossary – K

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Kelvin
the standard unit that scientists use to measure temperature. The temperature at which water freezes on the surface of Earth is 273 Kelvin, and the temperature that water boils is 373 Kelvins.
Kepler’s 3rd Law of Planetary Motion
the square of the period (P) of an object’s orbit is proportional to the cube of the semi-major axis (a) of that orbit. Mathematically, this can be represented as P^2 ∝ a^3. Kepler discovered this law by studying the motions of the planets around the Sun, but it is a fundamental law that can be derived from Newton’s Law of Gravitation.
KeV
Kilo Electron Volts. The electron volt (eV) is a measurement of energy; it is the energy gained by an electron (or proton) dropping through a potential of one volt. This is a tiny amount of energy, appropriate for describing the energies of atomic and subatomic particles. To give an example, the energy of visible light photons is about 1 eV. Adding the prefix “kilo” means we are talking about one thousand electron volts (keV). It takes about 6 x 1015 keV to equal 1 joule.
Kilogram
the standard unit for measuring mass.
Kilometer
a unit for measuring length. The average length between Earth’s center and its surface is about 6,000 Kilometers.
Kilovoltage Peak
a common X-ray imaging setting; refers to the maximum value of the applied voltage during production of X-rays.
Knot
a small region of gas in a nebula that has a higher density than the gas around it.