calendars. Telescopes were invented in the 17th century. Astronomers then mapped the sky in greater detailPPstill with visible light. They learned about the temperature, constituents, distribution, and the motions of stars.

In the 20th century, scientists began to explore the other regions of the spectrum. Each region provided new evidence about the universe. Radio waves tell scientists about many things: the distribution of gases in our Milky Way Galaxy, the power in the great jets of material spewing from the centers of some other galaxies, and details about magnetic fields in space. The first

radio astronomers unexpectedly found cool hydrogen gas distributed throughout the Milky Way. Hydrogen atoms are the building blocks for all matter. The remnant radiation from the Big Bang, the beginning of the universe, shows up in the microwave spectrum.

Infrared studies (also radio studies) tell us about molecules in space. For example, an infrared search reveals huge clouds of formaldehyde in space, each more than a million times more massive than the Sun. Some ultraviolet light comes from powerful galaxies very far away. Astronomers have yet to understand the highly energetic

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