The Electromagnetic Spectrum

Stars and other cosmic objects do not just emit light. They can emit lots of other type of wave. Visible light is just a very small part of the electromagnetic spectrum. We cannot see the other types of wave but we can build instruments to detect them and if we want a visible image we can get a computer to produce one from the information we give it, a process called tomography.

All electromagnetic waves travel at the same speed, 3 x 10⁸ m/s (300,000,000 m/s). They have different wavelengths and frequencies as shown below. Hot objects emit electromagnetic waves. Hotter objects tend to emit more high frequency waves than cooler objects. This is one way that we can tell the temperature of a star.

Our own Sun emits a lot more waves than we can see.

pics NASA

We can learn a huge amount about what's going on inside stars and galaxies by studying the non-visible waves they emit.

Many of these waves, however, tend to be absorbed by the Earth's atmosphere. Infra red, for example, is absorbed by water vapour and X rays are absorbed by the atmosphere. For this reason professional telescopes tend to be built as high up as possible, above even the clouds.

Telescopes in space don't have this problem and can detect the whole spectrum without being hindered by the atmosphere. The Hubble space telescope not only collects visible light but also has ultra violet and infra red detectors. The Hubble telescope was one of NASA's four Great Observatories. Four space telescopes, each designed to specialize in different wavelengths.

The Hubble Telescope	Compton Gamma Ray Observatory	Chandra X Ray Observatory	Spitzer Space telescope
Visible and Ultra Violet	Gamma and X Rays	X Rays	Infra Red