Astronomical spectroscopy

How the humble prism helped unlock the secrets of what the Universe is made of and where it is heading.

Many of the atoms from which our bodies are made were once, thousands of millions of years ago, deep in the interior of one of many stars which have long since either blown themselves to pieces, or perhaps faded quietly into obscurity, gently shedding part of themselves in the process.

But how have astronomers and physicists uncovered this fascinating story just by looking at the sky and working in the laboratory?

What is a spectrum?

How is it that we know so much about the chemical compositions, temperatures, pressures and motions of distant stars and galaxies? In order to answer this question we have first to ask how we know that these bodies exist at all.

Well, quite simply, we know they exist because we can see them; that is, they are emitting energy in the form of light waves and also infrared, ultraviolet and often radio waves and X-rays as well. This energy travels over those vast distances and provides us with an extremely rich source of information.

A spectrum is the result of splitting up this light into its constituent colours and it is by studying spectra that astrophysicists have been able to make their most important discoveries.

How is a spectrum produced?

The most familiar spectrum in nature is that splendid spectacle, the rainbow, which is produced when light from the Sun bounces around inside each of millions of raindrops and gets sorted out into its constituent colours in the process. When a chemist, physicist or astronomer wishes to examine a source of light they may use a triangular glass prism, or more commonly nowadays a device called a diffraction grating, to disperse the light into a spectrum.

What does a spectrum tell us?

Isaac Newton was the first to realise that the colours produced when white light is passed through a prism are a property of the light itself, rather than something introduced by the glass. He came to this conclusion in about 1666. This realisation was to have extremely far-reaching consequences for the whole of physics and for our understanding of the Universe in particular.

Probably the most familiar of the characteristic radiations from a common element is the yellow-orange light emitted by sodium vapour. Almost all of the light from a sodium vapour street lamp comes out in two very close lines in the yellow-orange part of the spectrum.

What stars are made of

If we look at an astronomical spectrum, and see the lines characteristic of a particular element, then we can immediately say whether that element is present either in the star or galaxy itself or even in the space between a star and our telescope. This is important and exciting enough in itself but so powerful are the techniques of spectroscopy, we can do much more than just detect the presence of a chemical element or molecule.

We can go further and tell how much of each element is in any star and even the Universe itself. We know that hydrogen is by far the most common element in the Universe and that hydrogen can be used as a raw material for manufacturing all the heavier elements. This process, the alchemists' dream, is going on quite quietly in the deep interiors of almost all stars including our own Sun.

Intergalactic speed cameras

The spectrograph makes a major contribution to the study of the motions of astronomical objects too. Doppler discovered that if a source of light is moving towards, or away from us, the colours or wavelengths of its spectrum lines change by an amount proportional to the speed. This means that we can measure the velocities of galaxies and quasars that are so very far away that any proper motion would be immeasurably small.

The big bang

Measurement of the velocities of other galaxies have told us that the whole Universe is expanding, with the most distant objects we can observe moving away from us at a substantial fraction of the speed of light. By combining this result with observations of the density of galaxies in space at different ages of the Universe we can see that everything started in a very small volume and expanded following what is called the big bang. Spectroscopy in the micro-wave region of the spectrum has shown us the red-shifted radiation which was emitted at the time of the big bang.

We have illustrated here only a few of the uses of the astronomical spectrograph but they do show how spectroscopy has become one of the most powerful tools of modern astronomical research.

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