Angstroms and Nanometers
Astronomers still use an old unit of measurement for the wavelengths of electromagnetic radiation. The unit is the angstrom, named after the Swedish astronomer who first named these wavelengths. One nanometer is equal to 10 angstroms. Therefore, green light has a wavelength of about 5000 A, 500 nanometers, or 5 X10 -7 meters.

are so tiny that scientists use another unit, the electron volt, to describe them. This is the energy that an electron gains when it falls through a potential difference, or voltage, of one volt. It works out that one electron volt has a wavelength of about 0.0001 centimeters. X-rays range from 100 electron volts (100 eV) to thousands of electron volts. Gamma rays range from thousands of electron volts to billions of electron volts.

Using The Electromagnetic Spectrum

All objects in space are very distant and difficult for humans to visit. Only the Moon has been visited so far. Instead of visiting

stars and planets, astronomers collect electromagnetic radiation from them using a variety of tools. Radio dishes capture radio signals from space. Big telescopes on Earth gather visible and infrared light. Interplanetary spacecraft have traveled to all the planets in our solar system except Pluto and have landed on two. No spacecraft has ever brought back planetary material for study. They send back all their information by radio waves.

Virtually everything astronomers have learned about the universe beyond Earth depends on the information contained in the electromagnetic radiation that has traveled to Earth. For example, when a star explodes as in a supernova, it emits energy in all wavelengths of the electromagnetic spectrum. The most famous supernova is the stellar explosion that became visible in 1054 and produced the Crab Nebula. Electromagnetic radiation from radio to gamma rays has been detected from this object, and each section of the spectrum tells a different piece of the story.

For most of history, humans used only visible light to explore the skies. With basic tools and the human eye, we developed sophisticated methods of time keeping and

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