Shall we set down astronomy among the subjects of study?
I think so, to know something about the seasons, the months and the years is of use for military purposes, as well as for agriculture and for navigation. It amuses me to see how afraid you are, lest the common herd of people should accuse you of recommending useless studies.
Plato, in The Republic
Astronomy is about using observational data to test hypotheses about the nature and behaviour of very distant objects, such as stars and galaxies. That immediately sets it apart from experimental disciplines. It is simply impossible to make stars and do experiments with them, even if one could get funding to do it. Nature provides us with a laboratory of a sort, but it also decides what goes on there. We just have to hope that we can observe something that provides us with a way of testing whether our ideas are right. Fortunately, the laboratory we have is enormous and it has a lot going on within it. We observe, measure, catalogue and model (but not necessarily in that order). Eventually patterns emerge, as do rare but decisive exceptions. Models are gradually refined to account for the observations and, hopefully, we end up with some measure of understanding.
As an example of this process, consider how stars work. To the ancients, stars were remote and intangible. The general perception was that they were made of very different material to earthly things and were therefore completely beyond comprehension. Stars are still remote and still intangible, but we now have an almost complete understanding of what they are made of, how they work, and how
long they live. We even have a good idea of how stars form, although the details of this process are still far from clear. However, none of this knowledge was gained by taking samples of 'star-stuff', or even following the life-history of individual stars. It takes billions of years for stars to burn their nuclear fuel, and no astronomer has time to watch a star for that long.
The history of stellar evolution theory is long and fascinating, but probably the most important initial breakthrough was the development of a laboratory technique called spectroscopy, pioneered by Robert Wilhelm Bunsen (of burner fame) and Gustav Robert Kirchhoff. This approach involves taking the light emitted by a hot source, such as a flame or an electrical discharge through a gas, and splitting it up using a prism or a diffraction grating. This produces a spectrum showing the familiar pattern of the colours of the rainbow, from red through to blue and violet. White light contains an even blend of all these colours.
What early spectroscopists noticed was that different materials produced light of very specific colours, represented by the appearance of very sharp lines in their spectra. At the time nobody knew why these lines were so sharp—the answer eventually came from quantum theory—but it was realized very early on that the pattern of lines emitted by a particular material in this way was effectively a fingerprint of that material. It was also realized that dark lines could appear in a spectrum, if one were to shine white light through cold matter. These dark lines appear in the same position as the bright lines given off by the same type of material when it is hot. Josef von Fraunhofer had earlier recorded the existence of hundreds of such dark lines when he took a spectrum of the Sun, but it was Kirchhoff who realized that these lines could be identified with the lines produced by the familiar chemical materials he had been playing with in laboratory experiments. Suddenly it became obvious that the Sun was not made of unknown celestial matter, but ordinary stuff. This really was an enormous breakthrough, as it changed the relationship between astronomy and the other sciences forever.
I sometimes get asked to talk to school students thinking about doing an undergraduate degree, and the most common question I get asked on these occasions is 'what is the difference between astronomy and astrophysics?' The distinction is somewhat blurred these days, but there is no doubt that the subject of astrophysics began with
Kirchhoff s realization that stars were made of stuff that could be described by the same laws as terrestrial material. This made it possible to apply the laws of physics to stars and other astronomical objects. Prior to that astronomy had largely consisted of recording the positions and motions of celestial bodies (astrometry), prediction of eclipses and providing navigational tables. But I digress.
The crucial step towards an understanding of stellar evolution was that painstaking observational studies revealed correlations between properties of stars, particularly their temperature and their brightness. There are stars of all different colours, from red to blue. The different colours indicate different temperatures with red being cooler than blue. But stars also differ in brightness from one to another. Independently two astronomers discovered correlations between the colour and brightness of stars. Ejnar Hertzsprung, a Dane working at Potsdam Observatory in Germany first published this correlation but did so in obscure journals. Later on, in 1913, the American Henry Norris Russell of Princeton came to the same conclusion. Their result is encapsulated in one of the most famous diagrams in all science: the Hertzsprung—Russell diagram.
This diagram is usually presented using funny astronomers' units ('magnitude' and 'spectral classification'), but basically it shows brightness up the vertical axis and colour (or temperature) along the horizontal axis. The band lying from top left to bottom right is called the Main Sequence, and its existence was an important spur to theoretical ideas of stellar structure. Our nearest star, the Sun, also lies
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