from The Century Dictionary and Cyclopedia
- n. Luminosity due to the spontaneous vibratory motions of the particles of the luminous body. All bodies heated above red heat are self-luminous, whereas in most cases of luminescence the light emitted is due to excitation from without. The glowing of radium, and of other radioactive substances, is an instance of self-luminosity and it is in describing this phenomenon that the term is usually employed.
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But there is another traditional meaning or quality to emptiness and that is of endless possibility and self-luminosity.
By thus proving the self-luminosity of the stars as contrasted with the darkness of the planets, Galileo settled the age-old controversy once discussed by al-Biruni:
Now, while the general principle involved in the nebular hypothesis is true, yet the conception according to Laplace is not verified by fact, as we learn that Uranus and Neptune are still in a state of self-luminosity, while their density is the smallest of all the planets.