When we look at the stars with our naked eyes, we see only a tiny part of their light. Stars, planets, galaxies, clouds of dust and gas, and other matter in space are sending out energy all the time. This energy, called electromagnetic energy, travels in pulses or waves. Like waves traveling through the ocean, they can be very long and lazy, very short and peppy, or anything in between.

Visible light is what we call the particular range of wavelengths that our eyes can see. But this light tells only a small part of the story of the stars.

To learn the rest of the story, we have had to invent new kinds of electromagnetic energy detectors and put these detectors on new kinds of telescopes. Optical telescopes are the oldest kind, but now we have invented new kinds. Each kind of telescope adds to our knowledge and understanding of the Universe.

Optical telescopes gather visible light, just like our eyes, but greatly magnified. Using special cameras that work the same way our digital cameras work, astronomers can photograph planets, stars, and galaxies. For hundreds of years we have built optical telescopes on Earth, but they work even better in space. This is because light twinkles--it wiggles or jiggles as it travels through Earth's atmosphere, so putting an optical telescope in space makes a big difference and we get much clearer photographs. The Hubble Space Telescope has given us some amazing views of the Universe that we could not see from Earth.

Radio telescopes are large dish antennas designed to collect long, lazy radio waves. Radio waves also shine right through Earth's atmosphere, so putting these telescopes on the ground works fine.

Although radio waves are long, they still move fast, at the same speed as light. Radio waves can be used for lots of different things. Our TV, radio, and cell phone signals are carried by radio waves of specially selected wavelengths. Other wavelengths can be used to look out at our Universe to learn about planets or galaxies. Yet others are used by NASA's Deep Space Network of antennas to detect the faint radio signals from spacecraft traveling to faraway destinations, like Mars. We use these radio waves to send messages to our spacecraft and receive information from their special instruments.

Infrared telescopes work best in space. Why? Objects that are even a little bit warm put out infrared energy. So it makes sense to put an infrared telescope into space where it won't just detect all the warm things on Earth. Even in space, we must make the telescope very cold using refrigerator-type technology so that it won't just detect itself! Then the telescope can look around and see the infrared light coming from the rest of the Universe. The new Spitzer Space Telescope is just such an instrument.

Ultraviolet telescopes must be space telescopes because very little ultraviolet energy gets through Earth's atmosphere. A new telescope, called GALEX (for Galaxy Evolution Explorer) is taking a survey of nearly the entire sky in ultraviolet light. Hot, young stars put out a lot of ultraviolet light, so GALEX will be able to find the places where stars are being born.

X-ray telescopes also must be placed in space, because x-rays can't penetrate Earth's atmosphere. It's a good thing, too, because x-rays are so energetic they would soon kill almost every living thing on Earth. X-rays are made by the hottest events and objects in the Universe. The Chandra X-ray telescope has helped us learn more about black holes, pulsars, quasars, and other exciting cosmic objects.

Gamma ray telescopes can operate only in space. Gamma rays cannot pass through Earth's atmosphere, thank goodness! Otherwise, Earth would most likely be a very fried and lifeless place. Gamma rays are the smallest wave and most energetic form of electromagnetic energy (that is, light) that we have been able to detect. Huge bursts of gamma rays have been detected from all parts of the sky, but astronomers do not yet know what events trigger them. The Swift Gamma Ray Burst Explorer space observatory to be launched in 2004 will study them.