The
electromagnetic spectrum Light, or electromagneticradiation, comes in many
forms. There are radio waves, microwaves, infrared light, visible light,
ultraviolet light, X-rays and gamma rays, all of which form what is known
as the 'electromagnetic spectrum'
The electromagnetic
spectrum is subdivided into seven regions according to wavelength. Each
portion of the spectrum interacts with matter in a slightly different way
and is given a different name. From longest to shortest wavelength the
seven divisions are:
Radio (wavelengths greater
than 0.3 metres)
Earth's atmosphere hides
most electromagnetic radiation from space except visible light, certain
infrared frequencies and radio waves. For this reason, we can place radio
telescopes on Earth's surface and radio astronomy was the first
non-optical study of radiation from space. A number of the most massive
galaxies were found to be extremely powerful sources of radio waves. Radio
astronomy led to the discovery of pulsars which pulse regular radio
emissions.
Microwaves (wavelengths
between 1 millimetre and 0.3 metres)
Earth's atmosphere begins
to shield radiation from us. The most important form of microwave
radiation in astronomy is called the Cosmic Microwave Background (CMB).
Discovered in 1965, CMB comes from all parts of the Universe with the same
intensity. CMB became solid evidence for the 'Big Bang' theory, which
predicted that the shockwave of the primeval explosion would be still
detectable. ESA's Planck mission will study the CMB and thus will be
seeing the Universe as it was almost at its beginning.
Infrared (wavelengths
between 700 nanometres – 1 millimetre)
The primary source of
infrared radiation is heat. The higher the temperature, the faster the
atoms and molecules in an object move and the more infrared radiation. The
first infrared space mission was IRAS (Infrared Astronomical Satellite)
which detected about 350 000 infrared sources. Later, ESA's Infrared Space
Observatory (ISO) made important studies of the dusty regions of the
Universe. ESA's Herschel mission will build on this work.
Visible (wavelengths
between 400 – 700 nanometres)
Until 1945, most astronomy
was optical. This meant studying a very small range of wavelengths. It is
from these optical wavelengths that most people derive their picture of
the Universe, dominated by bright stars and galaxies. Visible light is
predominantly released by objects between 2000 and 10 000°C. The NASA/ESA
Hubble Space Telescope has a powerful optical telescope on board which
enables it to take stunning photographs in real colour.
Ultraviolet
(wavelengths between 10 – 400 nanometres)
As soon as observations
from above the atmosphere became possible, the classical techniques of
optical astronomy were extended into the ultraviolet. The Sun and other
hot objects are sources of ultraviolet radiation. In 1978, the
International Ultraviolet Explorer (IUE) was launched. IUE dominated
ultraviolet space astronomy for nearly two decades. It generated spectra
showing intensities at different wavelengths from selected objects in the
sky. Temperatures, motions, magnetism and chemical composition are all
discernible in the ultraviolet spectra.
X-rays (wavelengths
between 0.01 – 10 nanometres)
Most of the observable
matter in the Universe today is in a hot state, radiating short-wavelength
radiation and X-rays. Massive clouds of gas at a very high temperature
fill the spaces between galaxies. Whenever a new star is formed, a
collapsing cloud of gas reaches temperatures sufficient for nuclear
reactions to start, powering the star. Conditions in the primeval Universe
were very different - with only a few pre-existing molecules and no dust
available for cooling, only the most massive clouds could collapse. They
would make not stars, but black holes. Theorists suspect that giant black
holes may have been among the earliest objects created in the Universe and
would have produced X-rays. Two ESA missions, XMM-Newton and XEUS are
designed to observe these X-rays.
Gamma rays (wavelengths less than 0.01 nanometres)
Gamma rays from space are blocked by the Earth’s atmosphere – fortunately
for us, because this powerful radiation is lethal. Gamma-ray telescopes in
space give evidence for the processes that made the Universe habitable.
When a massive star has used up its hydrogen fuel, it ends in a supernova
explosion, emitting gamma rays. During this explosion, radioactive
elements are formed and ejected into space, decaying or combining to form
the other elements. ESA's COS-B satellite (1975-1982) created a catalogue
of gamma-ray sources. ESA's Integral spacecraft, launched in 2002, takes
this work forward, studying the phenomenon known as 'gamma-ray bursts'.
