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The Question

(Submitted May 26, 1997)

As a Radiographer for 14 years, I am familiar with how diagnostic X-rays are produced by man. Other, than the obvious difference of mechanical means of producing the X-rays that Dr. Roentgen discovered; or even the radiation such as that noted at Chernobyl (please bear my ignorance) are we talking a natural phenomena when you say High-Energy Astrophysics--X-rays & gamma-rays or something other than the mechanized means of production? By now, you see how little knowledge I have of what is titled "High- Energy Astrophysics"...

The Answer

Yes, the X-rays we observe are natural -- not man made. They are produced very far away in the myriad of phenomena described in Imagine the Universe! (http://imagine.gsfc.nasa.gov/). The X-rays then travel for hundreds to billions of years before they happen to hit the detectors on our X-ray telescopes.

In fact there are two basic natural methods of producing extraterrestrial X-ray -- thermal and non-thermal. Two members of our ask_astro team replied to your message -- one describing the thermal processes and the other the non-thermal processes. I have attached a brief description of each with this E-mail (see below), and as I mentioned above there is much information on this at our Web site.

Sincerely,

Jonathan Keohane and much of the Ask an Astrophysicist Team


Appendix A: Thermal X-rays (by Mike Arida)

At 37 C the human body emits infrared radiation. This is called blackbody radiation.

The hotter the object, the higher the energy of the photon's emitted.

When you heat a piece of metal in a fire till it glows read you have energized some of those photons to the red part of the visible spectrum.

The Sun, at 5,000 C emits most of its energy in the yellow/green part of the visible spectrum.

A tungsten light bulb get to be about 10,000 C and emits in the bluish/white end of the spectrum.

X-rays, being much more energetic than visible light, require a hotter source, in the 1 - 10's of millions of degree range. So one method of X-ray production is in the very hot gas expanding outward after a supernova explosion, or the gas heated as it spirals (and accelerates) into a black hole.


Appendix B: Non-Thermal X-ray (by David Palmer)

Many X-rays studied by high-energy astronomers are produced by high-energy electrons being accelerated or decelerated, either by being deflected by a magnetic field, or by hitting other particles. X-ray tubes used in radiography work the same way: a beam of electrons is fired into a metal target, and as they stop the electrons produce X-rays.

There are also gamma-rays produced by the decay of radioactive isotopes. These are produced on Earth by reactors such as Chernobyl, and in the sky by reactors such as novae and supernovae.

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