Plasma - a hot ionized gas, that is, it is composed of a mix of free electrons and free atomic nuclei

Potential Energy - stored energy that can be converted into other forms; especially gravitational energy

Proton - a subatomic particle that carries a positive charge, one of the constituents of the atomic nucleus

Radioactive Isotope - an isotope of any element which decays (or decomposes) through the spontaneous emission of subatomic particles and gamma-rays

Red Giant - a star that has greatly increased in size and has a relatively cool surface which glows red; such stars occupy the upper right hand corner of the Hertzsprung-Russell diagram

Resolution - degree to which fine details in an image can be resolved, or separated

Schwarzschild Radius - see Event Horizon

Spacetime - a system of three spatial coordinates and one temporal coordinate with respect to which the time and location of any event can be specified

Special Relativity - the physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving

Spectral Class - a classification of a star according to the characteristics of its spectrum

Spectrum - array of colors or wavelengths obtained when light is dispersed, as in passing it through a prism or grating

Star - a self-luminous sphere of gas

Stellar Spectroscopy - breaking down the electromagnetic radiation from a star in order to study the different wavelengths individually

Supergiant - an old, high-mass star greatly expanded from its original size; larger and brighter than a giant star

Supernova - catastrophic explosion of a star which can cause it to shine brighter than a galaxy for a few weeks or so