CHAPTER 6 | LIGHT AND TELESCOPES 117
unhindered, giving radio astronomers an unobscured view. (Th e
same is also true, to a large extent, of infrared radiation.)
Finally, radio telescopes are important because they can
detect objects that are more luminous at radio wavelengths than
at visible wavelengths. Th is includes, for example, intensely hot
gas orbiting black holes. Some of the most violent events in the
universe are detectable at radio wavelengths.Astronomy from Space
You have learned about the observations that ground-based
telescopes can make through the two atmospheric windows in
the visible and radio parts of the electromagnetic spectrum. Most
of the rest of the electromagnetic radiation—infrared, ultraviolet,
X-ray, and gamma ray—never reaches Earth’s surface; it is
absorbed high in Earth’s atmosphere. To observe at these wave-
lengths, telescopes must go above the atmosphere.The Hubble Space Telescope
Named after Edwin Hubble, the astronomer who discovered the
expansion of the universe, the Hubble Space Telescope is the
most successful telescope ever to orbit Earth (■ Figure 6-22). It
was launched in 1990 and contains a 2.4-m (95-in.) mirror with
which it can observe visible, near-ultraviolet, and near-infrared
light. It is controlled from a research center on Earth and
observes continuously. Nevertheless, the telescope has time to
complete only a fraction of the projects proposed by astronomers
from around the world.
Most of the observations Hubble makes are at visual wave-
lengths, so its greatest advantage in being above Earth’s6-5
Advantages of a Radio Telescope
Building large radio telescopes in isolated locations is expensive,
but three factors make it all worthwhile. First, and most impor-
tant, a radio telescope can reveal where clouds of cool hydrogen
and other atoms and molecules are located. Th ese clouds are
important because, for one thing, they are the places where stars
are born. Although cool clouds of gas are completely invisible to
normal telescopes because they produce no visible light of their
own and refl ect too little to be detected in photographs, some gas
atoms and molecules do emit radio photons. Cool hydrogen, for
example, emits radio energy at the specifi c wavelength of 21 cm.
(You will see how the hydrogen produces this radiation when you
learn about the gas clouds in space in Chapter 10.) Other gas
molecules emit radio energy with their own characteristic wave-
lengths. Th e only way astronomers can detect these clouds is
with a radio telescope.
Another reason radio telescopes are important is related to
dust in space. Astronomers observing at visual wavelengths can’t
see through the dusty clouds in space. Light waves are so short
that they are scattered by the tiny dust grains and never get
through the dust to reach optical telescopes on Earth. However,
radio signals have wavelengths much longer than the diameters
of dust grains, so radio waves from far across the galaxy pass
■ Figure 6-21
(a) The largest steerable radio telescope
in the world is the GBT located in Green
Bank, West Virginia. With a diameter of
100 m, it stands higher than the Statue
of Liberty. (Mike Bailey: NRAO/AUII) (b)
The 300-m (1000-ft) radio telescope in
Arecibo, Puerto Rico, hangs from cables
over a mountain valley. The Arecibo
Observatory is part of the National
Astronomy and Ionosphere Foundation
operated by Cornell University and the
National Science Foundation. (David
Parker/Science Photo Library)ab