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Astronomical spectroscopy

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.

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

Much of it has been pieced together using the techniques of astronomical spectroscopy and the purpose of this pamphlet is to give some idea of how it is done and how richly rewarding the researches have been.

What is a spectrum?

How is it that we know so much about the chemical compositions, temperatures, pressures and motions of stars and galaxies which are so very distant that we would never dream of trying to travel to them? 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 about their make--up.

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.

Colour spectrum
A glass prism splits white light into a spectrum of its consituent colours, or wavelengths.(Image by Duncan Kopernicki)

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 he may use a triangular glass prism, or more commonly nowadays a device called a diffraction grating, to disperse the light into a spectrum.

The whole apparatus for doing this job is called a spectroscope (if you look through it), or a spectrograph (if the spectrum is recorded photographically or by some means other than the eye). All modern spectrographs use diffraction gratings; the end result however, is rather similar to that produced by a prism whose action may be more familiar.

What does a spectrum tell us?

Isaac Newton was the first to realize 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, while he was engaged in those experiments which were to lead to his construction of the first reflecting telescope. This realization was to have extremely far--reaching consequences for the whole of physics and for our understanding of the Universe in particular.

It may seem paradoxical, but the path to the understanding of the very big leads through the world of the very small. We have to understand the workings and structure of atoms and molecules before we can meaningfully turn our spectrograph on the stars and galaxies.

The great revolution in physics, which took place during

When white light is passed through a cold gas, atoms in the gas absorb certain wavelengths of the light, producing dark bands in the spectrum. The pattern of bands can be used to identify the elements that make up the gas.(Image by Duncan Kopernicki.)
the first few decades of this century, led to a thorough understanding of the way in which atoms and molecules can absorb and emit light and other radiations. It was known long before that different chemical elements emitted their own characteristic coloured radiations or lines when heated in their gaseous state, but it was the understanding of the relationship between those lines and the structure of the atom or molecule that proved to be so important for the development of astrophysics.

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; this same element is also responsible for the yellow colour produced when, for example, salted water (common salt is sodium chloride) used in cooking is allowed to boil over into the flame of a gas ring.

If we look at an astronomical spectrum, and see the lines characteristic of a particular element, then we can immediately say that element is present either in the star or galaxy itself or, in some special cases, 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.

The obvious next question to ask is: how much of each element is present in a particular star? In fact this is not a very easy question to answer but it can be done, and indeed has been done for several hundreds of the brightest stars in the sky and for quite a number of other astronomical objects besides.

The picture that has emerged from these studies is fascinating. 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. Probably the most important site of this transmutation of the elements is a supernova explosion - the incredibly violent death throes of a star which is too massive to shrink quietly into obscurity. The famous Crab Nebula in Taurus is the result of such an explosion which was witnessed as a daytime star by Chinese observers in the year 1054 AD.

Most astronomers now believe that when our Milky Way system formed more than ten thousand million years ago, it consisted entirely, or almost entirely, of a mixture of hydrogen and helium, and that the present concentration of heavier elements has been built up from these raw materials over the intervening time.

The spectrograph makes a major contribution to the study of the motions of astronomical objects too. If we want to see how fast a star moves across the sky (i.e. at right angles to our line of sight), we can measure its angular or proper motion, but in order to convert this into an actual velocity we have to know its distance, and astronomical distances are notoriously difficult to measure.

Now to measure its motion along the line of sight, its radial velocity, we simply use the Doppler principle. 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 which depends on the speed. All we have to do is to measure the displacement of the stellar lines from the corresponding lines produced in a lamp mounted in the spectrograph. This procedure has the very important advantage that we do not have to know the distance to the star at all: the radial velocity comes out the same however far away it is. 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 radial velocities have been measured for many thousands of stars in our Galaxy and this is leading to an understanding of its formation and evolution. We know for instance, that the Sun makes an almost circular orbit around the centre of the Galaxy once every 250 million years but that some very much older stars move in different, non--circular orbits.

Measurement of the radial 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.

Modern spectrographs

Modern astronomical spectrographs can be very large complex instruments. The spectrograph designed for the William Herschel 4.2 metre telescope on La Palma is about the size of a small car but the optical components have to be kept in position to an accuracy of one thousandth of a millimetre. The spectrographs on La Palma are used for a wide variety of research programmes initiated by astronomers from the RGO and from University departments, often in collaboration with radio, infrared and X--ray astronomers.

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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