In the night sky: Orion
In the night sky: Orion

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In the night sky: Orion

3.3 The electromagnetic spectrum

An image of the various kinds of electromagnetic radiation.
Figure 12 Various kinds of electromagnetic radiation can be distinguished by their wavelength or frequency. In case of visible light, different wavelengths are perceived as different colours but there is no fundamental distinction between the parts of the spectrum

In this section, you will consider different kinds of electromagnetic radiation, and what information comes from the different parts of the spectrum.

Most astronomical observations are made by the detection of radiation (often used as an abbreviation for electromagnetic radiation). The light we receive from the Sun is actually all the visible colours summed together.

Think of a rainbow. A rainbow is formed when light passing through drops of rain is refracted (or bent) in such a way that the rays of light are spread out according to their wavelength. This is called a spectrum. The colours of the rainbow are the visible parts of the electromagnetic spectrum – the part of the spectrum to which human eyes are sensitive. These colours are just a tiny part of the electromagnetic spectrum, which includes X-rays, microwaves and radio waves.

One way to describe the different components of the electromagnetic spectrum is in terms of waves. A wave may be defined as a regularly repeating disturbance that transports energy from one place to another. For instance, the regular crashing of an ocean wave on a beach.

The distance between one part of the wave profile and the next identical part of the wave profile is known as the wavelength of the wave. Two adjacent crests of the wave are a convenient pair of locations to use for this definition, although any pair of similar points will do. This is shown in the lower right-part of the figure at the start of the section.

How does this change our view of Orion, most particularly the Orion Nebula? In the next section, you will see what the Orion Nebula looks like when radiation from other parts of the electromagnetic spectrum is examined.


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