David Le Page african frontiers It’s all about detective work, says one of the astronomers involved in the building of the Southern African Large Telescope (Salt). As they labour on a Karoo mountain top building Salt over the next five years, South Africa’s intergalactic Hercule Poirots know they are essentially building a great big magnifying glass, one that will be able to see far further across the universe than that orbiting hero of modern astronomy, the Hubble space telescope. But it will not be an instrument that will necessarily provide neatly cut-and-dried answers about the objects it brings into view.
Salt, for which the ground-breaking ceremony was held in September near the Karoo town of Sutherland, is largely designed to be a spectroscopic telescope. A conventional telescope gives you a picture of part of the sky, but a spectroscopic instrument analyses the different light sources in that picture. Salt will be able to see stars and galaxies billions of light years away. But most of the time it will be “tasting” and “smelling”, only occasionally producing pretty pictures. Essentially, this means that Salt will be looking at a three-dimensional rather than a two-dimensional universe. A conventional telescope image may show an extraordinary level of detail, but it often does not show how far away a specific object is: is it a dim star nearby or a bright star far away? By analysing the minute detail contained within their spectrograms, Salt will resolve many more details of particular stars and galaxies.
A spectrogram shows the relative strength of the range of different frequencies of light emitted by a star or other object. This is possible to gauge, once the light from that object has been split into all its component frequencies, just as any beam of white light can be split into its different colours by a prism.
Different frequencies of light are produced by different elements. So by measuring the various frequency levels in the spectra of stars, astronomers can determine how much hydrogen, helium and other elements they contain.
The balance of different elements changes over time as they transmute into each other through nuclear fusion, so their relative quantities give strong clues to the age of stars.
In a sense, Salt will be like a great wine taster’s nose aimed at the heavens: by “tasting” the light, the origin, age and composition of the light source can be determined.
But establishing facts in astronomy often depends on painstaking deductions made on the basis of other deductions. You can assume the butler did it if you’re sure he had the only key to the wine cellar, but a duplicate key will spoil the theory. For example, Salt will almost certainly be trained on Cepheid variables – a particular kind of star that varies in apparent brightness. Cepheids are one of the most important ways of measuring the distances to the comparatively nearby galaxies in which they appear. Fairly young and moderately hot, Cepheids go through cycles of varying brightness between 1,5 and 50 days long. In 1912 Harvard’s Henrietta Leavitt discovered that there is a reliable relationship between the period of this cycle and the brightness of the star. By determining the period of a Cepheid, you can determine its actual brightness or magnitude. Then, by measuring how bright it appears to be, you can judge how far away it is.
But although this basic principle is well established, there remains some debate about the exact figures to use when judging distances of Cepheids, which further study may resolve. So establishing the exact distance to certain galaxies depends in part on properly understanding Cepheids. The useful nature of Cepheids was discovered in the Large and Small Magellanic Clouds, irregularly shaped smaller galaxies which are only visible from the southern hemisphere and are, in fact, the nearest neighbours to our Milky Way galaxy. The Magellanic Clouds are interesting for the study of star formation and for being home to the enormous Tarantula Nebula. In fact, the fixed angle at which Salt will peer into the heavens was adjusted specifically to allow it to include the Magellanic Clouds in its field of vision. Sometimes Salt will be looking not at the stars but at the stuff in between them, the vast clouds of interstellar dust and gas that result from the supernova explosions of sufficiently massive old stars. These explosions blow into space the elements that were cooked up inside them over billions of years. All the components of Earth and our bodies: iron, carbon, nitrogen, sulphur, oxygen and all the other elements originally formed in long-dead stars billions of years ago.
Spectroscopy has helped astronomers discover that these clouds often contain complex organic molecules, molecules considered to be the most basic “stuff of life” such as water, carbon dioxide, ammonia, methane and other hydro-carbons.
Sometimes this is done by seeing which frequencies from stars behind the clouds are absorbed, a process perhaps akin to determining the murderer on the basis of everyone else’s solid alibis. You work out what’s in the clouds by seeing what’s missing in the spectra of the stars. The search for planets in orbit about other stars is one of the most exciting tasks to which Salt will be applied. Already, about 40 planets have been discovered, or more precisely, inferred to exist around other stars. Once again spectroscopy is crucial to their discovery. When a planet orbits around a star its gravitational influence creates very slight wobbles in the motion of the star. Depending on their orientation this can create the impression that sometimes the star is moving slightly more towards us, and sometimes slightly more away from us. Just as the pitch of a car moving towards one changes as it goes past – the Doppler effect – the relative motion of stars affects the apparent frequency of the light escaping them. A year ago a planet was discovered this way in orbit about 51 Pegasi, just 41 light years from Earth. Finding other planets is particularly interesting for showing that other environments exist which might support life. But with Salt, astronomers will be able to turn from 51 Pegasi to look at galaxies so far away across the universe that their light is literally billions of years old, far older than the Earth itself. In the last few years a new technique for examining such galaxies has been discovered, called gravitational lensing. Lenses magnify light by bending and concentrating it, and gravity also bends light. Thus the gravity of a galaxy can actually magnify the light of galaxies far, far behind it. It’s then possible to establish the distance of those galaxies by measuring their speed, again through the “Doppler effect” of light. That’s called the red-shift – the faster a galaxy is moving away from us, the more its light is shifted towards the red end of the spectrum. And the faster it’s moving, the further away it is. At such distances, astronomy becomes palaeo- astronomy, giving us clues to the nature of the universe and to great and deceptively simple questions: how big is it, how heavy is it, and what is it made of? All the time trying to ensure that the maid wasn’t the butler in drag. E-mail the editor: [email protected]