Wolk, Scott J. High Energy Astrophysics Division, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts.
- X-ray Observatories
- Einstein Observatory
- X-ray timing
- Recent missions (launched in 2000–2010)
- Prospective missions
- Types of Sources
- Galactic sources
- Extragalactic sources
- Links to Primary Literature
- Additional Readings
The study of x-ray emission from extrasolar sources (solar x-ray astronomy is a separate field), including virtually all types of astronomical objects from comets through stars and from galaxies to quasars. X-rays lie between ultraviolet light and gamma rays (γ rays) in the electromagnetic spectrum. This corresponds to a range from 20 nanometers (2 × 10−8 m) down to 10 picometers (10−11 m), or about 60 eV to 125 keV, with low energy being considered "soft" and high energy being considered "hard." This space science requires balloons, rockets, or space satellites to carry experiments above most (or all) of the Earth's atmosphere, which would otherwise absorb the radiation. Observations in the soft band (below about 10 keV) must be carried out above the atmosphere, while hard x-ray observations can be made at high altitudes achievable by balloons. There is a limit to the observable spectrum at low energies because of absorption by neutral interstellar hydrogen gas. At high energies (short wavelengths), space remains transparent through the entire x-ray spectrum. See also: Electromagnetic radiation; Rocket astronomy; X-rays
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