P.3 Observing
the universe
Distant stars and galaxies are too far away for us to reach.
We cannot go to them to study them. So everything we know about distant stars
and galaxies comes from analysing the radiation they produce.
Telescopes are devices used to observe the universe. There
are many different types and some are even sited in space.
Optical telescopes
Optical telescopes observe visible light from space. Small
ones allow amateurs to view the night sky relatively cheaply but there are very
large optical telescopes sited around the world for professional astronomers to
use.
Optical telescopes on the ground have some disadvantages:
they can only be used at night
they cannot be used if the weather is poor or cloudy.
Other telescopes
Radio telescopes detect radio waves coming from space.
Although they are usually very large and expensive, these telescopes have an
advantage over optical telescopes. They can be used in bad weather because the
radio waves are not blocked by clouds as they pass through the atmosphere.
Radio telescopes can also be used in the daytime as well as at night.
X-rays are partly blocked by the Earth's atmosphere and so
X-ray telescopes need to be at high altitude or flown in balloons.
Space telescopes
Objects in the universe emit other electromagnetic radiation
such as infrared, X-rays and gamma rays. These are all blocked by the Earth's
atmosphere, but can be detected by telescopes placed in orbit round the Earth.
Telescopes in space can observe the whole sky and they can
operate both night and day. However, they are difficult and expensive to launch
and maintain. If anything goes wrong, only astronauts can fix them.
A large number of observatories have been launched into
orbit, and most of them have greatly enhanced our knowledge of the
extraterrestrial universe.
Performing astronomy from Earth's surface is limited by the
filtering and distortion of electromagnetic radiation (scintillation or
twinkling) due to the atmosphere. Some terrestrial telescopes (such as the Very
Large Telescope) can reduce atmospheric effects with adaptive optics. A
telescope orbiting Earth outside the atmosphere is subject neither to twinkling
nor to light pollution from artificial light sources on Earth.
Space-based astronomy is even more important for frequency
ranges which are outside of the optical window and the radio window, the only
two wavelength ranges of the electromagnetic spectrum that are not severely attenuated
by the atmosphere. For example, X-ray astronomy is nearly impossible when done
from Earth, and has reached its current importance in astronomy only due to
orbiting X-ray telescopes such as the Chandra observatory and the XMM-Newton
observatory. Infrared and ultraviolet are also greatly blocked.
Space observatories can generally be divided into two
classes: missions which map the entire sky (surveys), and observatories which
make observations of chosen parts of the sky.
Many space observatories have already completed their
missions, while others continue operating, and still others are planned for the
future. Satellites have been launched and operated by NASA, ESA, Japanese Space
Agency and the Soviet space program later succeeded by Roskosmos of Russia.
