Absorption (dark) lines: Colors missing in a continuous spectrum because of the absorption of those colors by atoms.
Atomic spectra: The range of characteristic frequencies of electromagnetic radiation that are readily absorbed and emitted by an atom. The atomic spectrum is an effect of the quantized orbits of electrons around the atom.
Cation: An ion with a positive charge.
Circumstellar: Confined to the vicinity of a star.
Doppler effect: The change in the wavelength of waves from a source reaching an observer when the source and the observer are moving with respect to each other along the line of sight; the wavelength increases (red shift) or decreases (blue shift) according to whether the motion is away from or toward the observer.
Electronic transition: The transfer of an electron from one energy level to another, accompanied by emission or absorption of electromagnetic radiation.
Extinction: The dimming of light when it passes through some medium, such as the earth's atmosphere or interstellar material.
Interstellar medium: All the gas and dust found between stars.
Polycyclic aromatic hydrocarbon (PAH): A large molecule containing carbon atoms arranged in a honeycomb lattice of hexagonal groups, with hydrogen atoms attached. These large molecules behave differently from small dust particles. In particular, they can reach very high temperatures immediately following the absorption of light waves, which leads to emission at shorter infrared wavelengths.
Prebiotic: Prior to the development of life.
Reddening: The preferential scattering of blue photons, compared to longer wavelengths, as light passes through interstellar dust. Stars appear redder than they would in the absence of dust.
Rotational energy states: Discrete energy levels exhibited in a molecule due to its freedom to rotate about various axes in space, with consequent rotational transitions.
Solar mass: The amount of mass in the Sun, about 2 x 1030 kg.
Spectrum: The array of colors or wavelengths obtained when light is dispersed, as by a prism; the amount of energy given off by an object at every different wavelength.
Vibrational energy states: Discrete energy levels exhibited in molecules due to the quantization of internuclear distances, with consequent vibrational transitions.