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Astrophysics is the part of astronomy that deals principally with the physics of the universe, including luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium. (Source: NASA/GSFC - "Imagine the Universe!" Dictionary.)

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Electromagnetic Spectrum Last Updated on 2009-04-24 00:00:00 The electromagnetic spectrum spans the entire range of electromagnetic radiation from the lowest frequency/longest wavelength to the highest frequency/shortest wavelength. Various segments of the spectrum are useful in exploring the Universe, and enable us to "see" into the universe beyond what we are able to see from visible light. The electromagnetic spectrum at-a-glance. (Source: NASA.)   Band Wavelength   Frequency Temperature Range Quantum Energy Radio 600 m - 0.187 m 30 Hz - 1.6x106 Hz   2x10-9 - 0.6x10-5 eV Microwave 187 mm - 1 mm 1 - 300 106 Hz   0.6x10-5 - 0.1x10-2 eV Sub-millimeter 539 - 616 μm 487 - 556x106 Hz   2.0x10-3 - 2.3x10-3 eV Far Infrared 40 - 350 μm 300x106 Hz - 30x1012 Hz 11.6 - 140 K 3.1x10-2 - 0.35x10-2 eV Mid Infrared 5 - 40 μm 30 - 120x1012 Hz 140 - 740 K 3.1x10-2 - 2.5x10-1 eV Near Infrared 1 - 5 μm 120 -... More »
PBS - People & Discoveries Last Updated on 2009-04-12 23:55:28 What's the story behind electroshock therapy? Who's responsible for increasing the size of the Universe? What laboratory mishap led to the saving of countless lives? The answers to these questions, plus many more, await you in People and Discoveries, a databank consisting of 120 entries about 20th century scientists and their stories. Physics and Astronomy: Rrelating to the atom, the astronomical, and the abstract. People Jocelyn Bell Niels Bohr Albert Einstein Jim Gates Murray Gell-Mann Werner Heisenberg Edwin Hubble Henrietta Leavitt J. Robert Oppenheimer Max Planck Ernest Rutherford Erwin Schrodinger Steven Weinberg   Discoveries 1900   Planck discovers the quantum nature of energy 1905   Einstein publishes the special theory of relativity 1912   Leavitt discovers a correlation between Cepheids' period and luminosity 1913... More »
Virial Theorem Last Updated on 2009-03-19 00:00:00 In mechanics, the virial theorem provides a general equation relating the average over time of the total kinetic energy, , of a stable system, bound by potential forces, with that of the total potential energy, , where angle brackets represent the average over time of the enclosed quantity. Mathematically, the theorem states where Fk represents the force on the kth particle, which is located at position rk. The word "virial" derives from vis, the Latin word for "force" or "energy", and was given its technical definition by Clausius in 1870.[1]Fritz Zwicky was the first to use the virial theorem to deduce the existence of unseen matter, what is now called dark matter. The significance of the virial theorem is that it allows the average total kinetic energy to be calculated even for very complicated systems that defy an exact solution, such as those considered in statistical... More »
Notes from the Astronomy Underground- Astropalooza Last Updated on 2008-09-28 00:00:00   According to the tagline in Ridley Scott’s 1979 blockbuster Alien, “In space, no one can hear you scream.” It’s true that sound waves, unlike light, need a medium- some kind of substance to carry their energy across a distance. And space is a vacuum, which, save the occasional solar system, fuzzy nebula, or bizarre stellar end product, is devoid of any respectable amount of matter. No matter, no sound, right?   Well, almost. Space is not completely empty. There are about one or two hydrogen molecules per square centimeter in the sparsest of regions. It beats our clumsy, terrestrial vacuum chambers handedly, but it’s not a vacuum in the strictest connotation of the word. Sound waves can still propagate through space, but so slowly and ineffectively that it would be pointless for aerophilic humans to do anything about it. Unless of course, we had ears many millions... More »
Gravity: Canonical Quantization Last Updated on 2008-06-11 00:00:00 The second program called 'Canonical Quantum Gravity' theory has a very different parentage. Following the established mathematical techniques developed by Schroedinger and Dirac, Einstein's equation for gravity gets rewritten, and it is in this new form that it can be analyzed for a consistent set of 'operators' and 'states' following conventional quantum mechanics. This forces quantum gravity theory to come out looking like a respectable quantum theory, but in which the dynamics of 3-D geometries play a key role, and the new Hamiltonian formulation describes how a system, the complete 3-D geometry of the universe, changes from state to state. Whereas quantum mechanics depends on considering the histories of particles as they move through space-time, quantum gravity must, in addition, consider all possible geometries of space as they unfold in time. Gravity: Canonical... More »