Visible light reveals only part of the Universe.  How other wavelengths fill out the picture - from gamma-rays to radio.
 

Outline:

Visible light is just part of a whole spectrum of radiation.  Properly called the electromagnetic spectrum, it stretches from the longest wavelengths of radio, through infrared, visible light, ultraviolet and X-rays, to the shortest wavelengths of gamma-rays.  All radiation is invisible - we only see light because it reflects off particles in the atmosphere.

To "see" radiation other than visible light, a new kind of astronomy is needed .  Radio waves are caught in huge dish-like antennae - detecting beams emitted by pulsars and revealing the causes of galactic turmoil.  Infrared observations unveil vast, warm clouds of cosmic gas and dust - the birthplaces of stars.  Like ultraviolet and radio waves, relatively little infrared  penetrates Earth's atmospheric shield.

The best "invisible" observations are from Earth orbit - like the Infrared Space Observatory and the Hubble Space Telescope which can sense not only visible light but infrared and ultraviolet radiation.  Ultraviolet unmasks hot young stars and the churning surface of our Sun.  At shorter wavelengths, X-rays pinpoint the hottest and most energetic areas of the Sun.  They also reveal the violence of the cosmos - some of it associated with black holes.

Gamma-rays are the most energetic radiation, evident in the hottest and most violent parts of the Universe.  "Gamma Ray Bursters" - blasts of intense energy from the farthest reaches of the cosmos - remain a mystery.  A closing montage of our Galaxy, the Milky Way, at different wavelengths.
 
 

Sub-chapters:

Radiation Spectrum

*  The electromagnetic spectrum - radiation at many wavelengths - stretches from the shortest wavelength gamma-rays to the longest radio waves.

*  Visible light is only a tiny fraction of all possible wavelengths in the electromagnetic spectrum.  In fact, all radiation is invisible.  We only see visible light because it is reflected off particles in the atmosphere.

*  Sensing invisible radiation - infrared as heat, ultraviolet as suntan.

*  If we observe the Universe only in visible light, we miss the big picture.

*  An orchestra plays - but only a small range of notes are audible.  It's like viewing the Universe in visible light alone.  Hearing the full range of the orchestra is like observing all the radiation from the Universe.
 

Radio and Infrared

*  Radio waves are the longest in the radiation spectrum.  Captured in huge radio dishes, the the information is translated into images.

*  Radio waves reveal the turmoil of Centaurus A - the aftermath of a galactic collision.  Also detected:  the radiation beams of a pulsar, a tiny superdense star.

*  Infrared radiation is absorbed by water vapour in the air.  Earthbound observation is limited to arid mountain tops and frozen wastes of Antarctica.

*  But like ultraviolet and the longest radio waves, little infrared penetrates Earth's atmospheric shield, so the Infrared Space Observatory (ISO) must work from orbit.
 

ISO's Universe

*  The coldest laboratory in the Universe, ISO picks out warm clouds of gas and dust - the birthplaces of stars.  It reveals the spiral structure of the Andromeda Galaxy and penetrates the shells of dust that surround a cometary nucleus.

*  ISO also detects abundant water - vital for life - towards the centre of the Milky Way.
 

Ultraviolet Views

*  The Hubble Space Telescope can view in visible light, infrared and ultraviolet.

*  Observations in ultraviolet reveal hot young stars, a cloud of hydrogen enveloping Halley's Comet, the churning surface of our Sun.
 

The X-Ray Sky

*  X-ray astronomy unveils the hottest and most energetic regions of the Sun, together with its corona and seering atmosphere.

*  The orbiting ROSAT observatory, which targets cosmic hot-spots, discovers 60,000 X-ray sources, a black hole at the centre of the galaxy M87 and the pulsing and energetic relic of an exploded star.
 

Gamma-Ray Blasts

*  The Compton Observatory, launched from Space Shuttle, observes gamma-rays - the hottest, most energetic radiation of all.  Compton detects jets of gas shooting from a supermassive black hole and mysterious gamma-ray bursters - blasts of energy from deep space whose origin is unknown.

* In Our Galaxy:

- Gamma-ray observations reveal cosmic rays colliding with hot gas.

- Galactic hot-spots show up in X-rays.

- Infrared spotlights clouds of dust.

- The Galactic Centre emits radio waves.

- Finally, in visible light, the majesty of the Milky Way.
 
 

Background:

The Nature of Light

After thousands of experiments and centuries of study, scientists still find it difficult to answer the question: What Is Light?  The problem is that light is invisible.  Before it can be seen, light must be reflected from something.  When it enters Earth's atmosphere, light from the Sun is reflected and scattered by tiny particles of dust or water droplets.  It is this interation that colours our clouds and makes the sky blue.

This effect can be seen at the cinema.  The beam of light that passes from the projector to the screen glints through millions of tiny dust particles floating in the air.  Each particle is reflecting the light.  They enable us to see the beam.

The ancient Greeks carried out the first experiments on the nature of light.  Euclid, in the third century BC, knew about the reflection of light.  A century later, Ptolemy investigated the refraction of light as it passed from one transparent substance to another.

In the 17th century, Isaac Newton attempted to explain the properties of light.  He suggested that a light beam consisted of a stream of tiny particles, which he called corpuscles.  Newton theorised that light travelled in straight lines.  He believed that the reflection of light by a mirror took place as corpuscles bounced off the surface of the mirror.

Newton thought corpuscles were attracted to certain transparent substances and moved faster in them.  This was his explanation of refraction - when light passes from air into water it is bent.  His notion was that the amount of bending depended upon how much faster corpuscles travelled in water or glass than through air.   Newton thought there were different kinds of corpuscle for each colour of light.

A little after Newton, the Dutch physicist Christian Huygens had a new idea.  Huygens correctly reasoned that light travelled as waves.  Drop a pebble into water and waves emanate from the point of impact.  Huygens believed that light travelled that way - but that the waves were very small.

Today we understand that the distance between the "tops" of adjacent light waves is a few ten-thousandths of a millimetre.  This tiny measurement is called the wavelength of the light.  Such waves rise and fall several hundred million million times every second.  This is called the frequency of light waves.

Although Huygens had explained reflection, refraction and several other properties of light, it took more than a century for his ideas to win support.  The important difference between the ideas of Newton and Huygens was that Huygens' theory needed light to travel more slowly in glass or water than in air.  Newton predicted the opposite.  The matter was settled in 1850 when the French physicist Jean-Bernard-Leon Foucault showed that light travelled faster in air than in water.  So Huygens was right and Newton was wrong.  Finally, in the 1860s, the Scottish physicist James Clerk Maxwell showed that light waves were just one form of electromagnetic radiation.

But light still had mysteries.  The wave theory did not explain all the properties of light.  In 1900, the German physicist Max Planck demonstrated that light travels in separate "packets" of energy known as "quanta" - as do other forms of radiation.  Then, in 1904, Albert Einstein suggested that electromagnetic radiation sometimes behaves as though it consists of particles.  He called them photons.  Thus, modern physicists were forced to recognise that, in some ways, electromagnetic radiation behaves as a wave - but, in others, it behaves as though it consists of particles.  No longer are the different explanations of Newton and Huygens regarded as rival theories.
 

Electromagnetic Waves and the Electromagnetic Spectrum

From the early 19th century, it was known that visible light travels as a transverse wave where the vibrations move in a direction perpendicular to the advancing wave front.  Physicists assumed that the wave needed something to travel through.  So they dreamt up an invisible medium called "luminiferous ether".

Then, in the 1860s, James Clerk Maxwell laid out the theory of electromagnetic waves.  Such waves, which have both electric and magnetic components, were produced, he postulated, by the oscillation or acceleration of an electric charge.  Maxwell declared that light was an electromagnetic phenomenon - and did not need ether as a medium.  Nevertheless, the ether notion took some time to disappear because it fitted with the Newtonian idea of an absolute space-time frame for the Universe.

Ether was seriously discredited in 1881 by the American scientists Albert Michelson and Edward Morley.  Their work - vital in the development of the theory of relatively - led to the realisation that the speed of electromagnetic waves in a vacuum is always the same, regardless of the velocity of the source or of the observer.

Indeed, all electromagnetic waves share two characteristics.  Firstly, they need no material medium for transmission - they can move through a vacuum.  Thus, light and radio waves are able to travel to Earth through the vacuum of space from the Sun and stars.  Secondly, in a vacuum, regardless of their frequency and wavelength, electromagnetic waves all travel at a wave velocity of 299,792 kilometres per second.  Any wave which has these two properties is an electromagnetic wave.

Electromagnetic waves form a spectrum - the electromagnetic spectrum - extending from waves of extremely high frequency and short wavelength to those of extremely low frequency and long wavelength.  Visible light is a small part of the spectrum.  In order of decreasing frequency, the spectrum consists of gamma rays, hard and soft X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.

All components of the electromagnetic spectrum show typical properties of wave motion, including diffraction and interference.  The wavelengths range from billionths of a centimetre to many kilometres.  The length and frequency of electromagnetic waves are important in determining their heating effect, visibility, penetration and other characteristics.
 

The Infrared Space Observatory

Launched in 1995, the Infrared Space Observatory (ISO) proved both a cool customer and a great success.  An international project of the European Space Agency, ISO carried four instruments:  ISOCAM - an infrared camera, SWS - a short wavelength spectrometer, LWS - a long wavelength spectrometer, and ISOPHOT - a photometer and polarimeter.  Infrared radiation was routed to these intruments by a reflecting telescope with a 60-centimetre aperture.

Most importantly, the telescope was cooled by 2,200 litres of liquid helium in continuous circulation.  With a boiling point at below minus 270 degrees Celsius, the helium absorbed heat and vented it from the craft.  This kept ISO super-cool - preventing its own infrared emissions from swamping those it was trying to detect from remote cosmic sources.  ISO would last as long as its coolant.

ISO was 5.3 metres tall and, at launch, weighed 2.3 tonnes.  Its observations were strictly programmed and executed.  Data collected from space was immediately transmitted to ISO's ground station.  The craft's elliptical orbit of Earth allowed long observing runs.  While observing, ISO was stabilised by dual star-trackers that also provided "pointing" information.

On April 8, 1998, ISO's liquid helium ran out - more than ten months after the official "expiry" date.  This extra observing time allowed ISO to view more than 26,000 cosmic infrared sources.  In part, ISO's longevity was due to the daily loss of helium coolant being 17 per cent less than expected.  Among other triumphs, ISO's "pointing" accuracy was ten times better than detailed in the initial specifications and stability was five times better than the acceptable level.  ISO viewed the cosmos for between 90 and 95 per cent of the observing time available.  Cool.
 
 

Links for Further Information:

Good illustration of the electromagnetic spectrum showing the location and wavelength range of each type of radiation.
http://noradcorp.com/spectrum.htm

Deep Space Network radio astronomy page.  Impressive information on radio astronomy, along with links to other useful sites and images.
http://dsnra.jpl.nasa.gov/

ISO home page - comprehensive site featuring news, mission and spacecraft data, scientific discoveries, links, press releases and an extensive image gallery.
http://isowww.estec.esa.nl/

Extreme Ultra-Violet Explorer page, containing information about EUVE, articles, history, daily operations, status information, image gallery and links.
http://www.cea.berkeley.edu/~pubinfo/html/EUVE.html

ROSAT home page - news, history of the spacecraft, mission information, data analysis, image gallery and related sites.
http://heasarc.gsfc.nasa.gov/docs/rosat/rosogof.html

Compton Gamma Ray Observatory page - general information plus data concerning CGRO's imaging instruments and links.
http://erbscobe.gsfc.nasa.gov/CGROHomePage.html
 
 

Questions and Activities for the Curious:

1.  How is the colour of light and its wavelength related?

2.  Draw a diagram to show the principal regions of the electromagnetic spectrum - from the shortest wavelengths to the longest.

3.  Why it is important for astronomers to observe the same astronomical objects at a variety of wavelengths?

4.  Why are infrared telescopes sited on mountain tops and in Antarctica?

5.  Which gases in the Earth's atmosphere absorb ultraviolet radiation

6.  Why is it necessary to study the birthplaces of stars at infrared wavelengths?

7.  Which regions of the Sun are best shown in X-ray images?

8.  Is light really invisible?  It is - and a simple experiment proves the point.  Take a cardboard container about the size of shoebox.  Make two cardboard tubes about 10 centimetres long and three centimetre in diameter.  Cut circular holes precisely at the centre of each end of the box, both holes three centimetres in diameter.

Paint the inside of the box and the insides of the tubes with black watercolour.  Push the tubes into the holes at each end of the box.  Fix them in place with tape and make sure there are no holes elsewhere.  Firmly glue down the lid of the box.  Finally, cut a window in the side of the box and cover the window with a piece of transparent plastic.

Take the box into a darkened room and put it on a table.  Shine a torch into one of the tubes and hold a piece of white card a short distance away from the opposite tube.  You will see that the torchlight reaches the piece of white card.  But, if you look through the window of the box,  you will see - nothing!  The beam of light from the torch is invisible.  Outer space is like the inside of the box:  total darkness.