The Invisible Universe

Until the middle of the 20th century, astronomers could only study celestial objects through the radiation they emitted at visible wavelengths. But visible light is only a tiny portion of the spectrum of electromagnetic waves, ranging from extremely long-wavelength radio waves to short-wavelength gamma rays. Short wavelengths in particular are very damaging to living tissue, but fortunately Earth's atmosphere blocks out nearly all wavelengths except for visible light and some radio waves. Astronomers only discovered that objects like the Sun emitted energy at these

other wavelengths in 1946, when they used rockets to launch the first detectors into the upper atmosphere.

Since then, the arrival of the Space Age has created new fields of invisible astronomy. Orbiting observatories have now mapped the universe at wavelengths from gamma rays and X-rays, where some of the most violent events in the universe can be detected, through ultraviolet light, where the hottest stars emit most of their radiation, and into the infrared (heat) waves primarily emitted by cool objects too dim to be seen in visible light.

THE HOT UNiVERSE

Most maps of the sky at nonvisible wavelengths can only be made from orbit. This X-ray chart of the whole sky, produced by the European ROSAT satellite, reveals hot gas bubbles in the solar neighborhood in yellow and green, and intense radiation sources such as supernova remnants in blue.

LOOKiNG FOR LiFE

The 1,000-ft (305-m) radio telescope in Arecibo, Puerto Rico, beamed our first deliberate message to the stars in 1974. It has also discovered extrasolar planets and a binary pulsar.

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ACTiVE GALAXiES

Radio galaxies are a type of active galaxy (see p.77) that forms when jets of high-energy particles from the galaxy's central region collide with material in intergalactic space. They are among the most intense radio sources in the sky.

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Telescopes Mastery

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