Space exploration began with the launch of Sputnik and Astrophysics was born as the application of physics to the phenomena observed by Astronomy, which etymologically means laws of the stars.
There are 2759 entries in this glossary.| Term | Definition |
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| Neutral Hydrogen H I Or H0 |
Un-ionized atomic hydrogen gas. It is an important component of the interstellar medium, accounting for perhaps half its mass, even though its density - typically 50 atoms per cubic centimetre - is very low.
The temperature of neutral hydrogen ranges between 25 and 250 K, which is too cold for it to emit visible radiation. However, its radio emission at a wavelength of 21 centimetres has made it possible to map the distribution of neutral hydrogen in the spiral arms of our own Galaxy and other nearby galaxies.
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