No telescope will show a star as anything but a point of light, and if we are to find out what the stars are really like it is necessary to use instruments based upon the principle of the spectroscope. Many people had written to ask me about this, and the programme of October 1960 was the result.
Space research continues to make progress. Of the recent artificial satellites one of the most conspicuous is the American-launched Echo, which is in the nature of an inflated balloon, and which shines as brightly as any star. At a casual glance it does, indeed, look very much like a star; but a few seconds' observation will show that it is moving perceptibly in a west-to-east direction.
It would be wrong to suggest that we have probed far into space. Perhaps we shall never do so, since astronomical distances are so immense. The Moon and planets are near neighbours of ours, but the stars are almost inconceivably remote. Even the closest of them - naturally excluding the Sun - lies at a distance of over four light- years. This means that even in giant telescopes a star appears as a mere point of light: no measurable disk is visible.
It would seem a difficult matter to obtain any precise information about the stars themselves. Were astronomers compelled to work with telescopes alone, the problem would be insuperable. During the past 200 years, however, spectroscopy has come to the fore, and has provided information which could hardly be gained in any other way. Stellar spectroscopy is admittedly a complex science, but its basic principles are certainly easy enough to understand.
All matter is composed of atoms, which form atom-groups or molecules. So far as we know, there are only ninety-two different types of atoms occurring naturally, and these make up the ninety- two fundamental elements. (A few more elements have been made artificially, but these are unstable, and probably do not occur in nature; in any case, the general argument is unaffected.) Hydrogen is the lightest element, with a nucleus and one planetary electron; uranium the heaviest, with ninety-two electrons, while the remainder form a complete series. All other substances are compounds.
To link atomic theory with stellar spectroscopy, we must go back to the work of Newton in 1666. This was the year of the Plague, when Cambridge University had been closed and Newton was living at his Lincolnshire home carrying on his own experiments. When he passed a beam of sunlight through a prism, he found that the light was spread out into a rainbow, with red at one end of the band and violet at the other. He then placed a screen with a hole to admit the light of one colour only, and passed this one colour through a second prism. This time there was no rainbow; the light was slightly refracted, but remained undispersed. Newton realized the reason for this behaviour. Sunlight is really made up of all the colours of the rainbow, and the prism splits it up, since different colours are refracted unequally - blue more than red, and so on. In the case of a single colour, as in Newton's second experiment, no such effect will be produced, since we are no longer dealing with a mixture.
For various reasons Newton never carried this work much further, and, leaving aside some inconclusive results obtained by W. H. Wollaston in 1802, we come next to the studies made by a young German optician, J. von Fraunhofer. About 1814 he began the researches for which he is best remembered. He constructed a spectroscope, and, using it together with a telescope, examined the spectrum of the Sun. In addition to the rainbow, he made out hundreds of relatively narrow dark lines crossing the coloured band; the lines were permanent, and were always to be seen in the same positions. Their intensities, too, did not change. Particularly prominent, for instance, were two lines close together in the yellow region of the spectrum.
The dark lines are still known as Fraunhofer Lines, but Fraunhofer himself never explained them fully; unfortunately he died while still a comparatively young man. It was left to Gustav Kirchhoff, Professor of Physics at Heidelberg University, to lay the foundations of modern stellar spectroscopy. Kirchhoff's classic 'laws' were announced in 1859. The first law states that an incandescent solid, or gas under high pressure, will produce a rainbow band or continuous spectrum. This accounts for the rainbow produced by the Sun. A gas under ordinary pressure, however, will not yield a continuous spectrum; instead, it will produce isolated bright lines. This is known as an emission spectrum. The essential fact, as Kirchhoff's second law pointed out, is that each element (or compound) is responsible for its own particular set of lines. The lines make up the 'trade-mark' of that particular element, and cannot be duplicated by lines due to any other element. For instance, among the lines yielded by incandescent sodium are two, close together and yellow in colour. These make up the famous double D-line.
The Fraunhofer Lines in the solar spectrum may be explained by means of Kirchhoff's third law. A simple experiment should prove helpful. If you burn salt in a flame, you will produce sodium vapour, which will give the characteristic sodium lines when observed spectroscopically. Now observe the spectrum of an electric light bulb; this will give a continuous rainbow, since the filament is an incandescent solid (Kirchhoff's first law). Next take the bulb and put it behind the flame, so that you are looking at the emission spectrum of the sodium superimposed against the continuous spectrum of the bulb. Instead of a rainbow with bright lines superimposed upon it, what you see takes the form of a rainbow crossed by dark lines. In fact, the atoms of sodium in the flame are removing part of the corresponding portion of the continuous spectrum; which is why the dark streaks are termed absorption lines.
The crux of the matter is that the positions of the lines are unaffected, and this means that they can be tracked down to the elements responsible for them. As soon as the continuous background is removed, the emission lines become brilliant once more. (Incidentally, even in an absorption spectrum they are not genuinely dark, but appear so because of their lesser brilliance compared with the background.)
Among the lines seen by Fraunhofer was the doublet in the yellow region of the spectrum. This corresponds in position and intensity to the bright yellow line produced by sodium. We have seen that nothing except sodium can produce such a line; therefore we can show that there is sodium in the Sun. The analysis can be extended, and up to the present time over sixty of the ninety-two naturally-occurring elements have been identified there.
It is worth noticing that one now-familiar element was discovered in the Sun before it was known on Earth. In 1869 the British astronomer N. Lockyer was examining the solar spectrum when he noted that one line in the orange-yellow region did not correspond with any element known to him. He suggested that it might be due to an unknown element which he named helium, from the Greek word for 'sun'. Not until a quarter of a century later was helium identified in the laboratory.
Basically, then, spectroscopy seems simple enough; but in practice it is highly complicated even with regard to the Sun, where we have so much available light that spectra of very high dispersion may be obtained. When we turn to the stars, things are much more difficult still.
Though the Sun is often termed a typical star, it is important to remember that the stars are by no means all alike. This is stressed by their obvious differences in colour. Look, for instance, at Vega in Lyra and Capella in Auriga, which are about equal in apparent brightness and lie on opposite sides of the Pole Star. Vega is decidedly bluish in hue, while Capella is yellow. The obvious - and correct - inference is that Vega has a hotter surface than Capella, which is bound to affect the spectrum. Actually Capella is much the larger of the two stars, and is three times as luminous as Vega, which also means that it is farther away from us.
Orange and orange-red stars are not uncommon. A good example is Kocab in Ursa Minor, which is of the second magnitude and lies roughly between Polaris and the 'tail' of the Great Bear. With the naked eye its colour is perceptible, and binoculars bring it out excellently. Betelgeux in Orion, Aldebaran in Taurus and Antares in Scorpio are examples of bright orange-red stars, while Rigel in Orion, and Sirius, the Dog-Star, and are almost pure white.
In 1890 E. C. Pickering, at Harvard, divided the stars into various spectral types. His system has been modified since, but has been accepted in principle, so that nowadays eleven separate types are recognized. The 'spectral alphabet5 is as follows: W, O, B, A, F, G, K, M, R, N, S. Generally speaking, types W to A include the white and bluish-white stars; F and G, the yellow; and K to S, the red and orange-red. Our Sun is an average star of type G. To refine the system further, each class is subdivided, the divisions being indicated by figures. For instance, a star of type F5, such as Procyon in Ganis Minor, will be midway between type F (or Fo) and G (or Go). Altair in Aquila, which is classed as A7, will actually be more like a star of type Fo than one of Ao.
Studies of stellar spectra reveal considerable differences, together with some enigmas. Type W stars, for example, yield emission as well as absorption lines, and seem to be surrounded by immense gaseous shells. The greenish-white O stars are extremely hot, with surface temperatures in the region of 35,000 degrees Centigrade. B-stars, bluish-white in colour, show prominent helium lines, and have high luminosity, often amounting to thousands of times that of the Sun; Alkaid in the Great Bear is a typical object of the class. A-type stars such as Sirius are dominated by hydrogen lines and have temperatures of about 11,000 degrees.
Typical F-type stars are Beta Cassiopeia; (F3); Procyon (F5) and Polaris (F8). Hydrogen is weaker than in type A, but calcium lines are very strong. When we come to type G, we must include the Sun, which is classed as a yellow dwarf, as well as Capella, which we rank as a yellow giant. After G5 we start to meet lines produced by molecules, and this is a sure indication of lower surface temperature, since great heat prevents molecules from being formed at all. Metallic and molecular lines become steadily more prominent in the spectra of cooler stars, and the results are so complex that they are not easy to interpret. Not all stars may be classified in this way. The extraordinary White Dwarfs, for instance, reveal almost nothing apart from a few broad hydrogen lines, and some less abnormal stars, such as Gamma Cassiopeiae, have to be classed as 'peculiar' from the spectral point of view.
Neither are matters as straightforward as might be thought from the details given above. We have seen that B-type stars show strong helium lines, but it is not safe to assume that helium is particularly abundant. It may simply be that the conditions prevailing at the star's surface are suitable for the helium lines to show up strongly. Moreover, direct spectroscopic analysis is limited to the outer layers of a star, and we have no first-hand information about conditions lower down. Yet a thorough knowledge of surface temperature and constitution is a good beginning, and spectroscopic results have led to the building-up of a reliable picture of what a star is really like. Even with a relatively mild star such as the Sun, the central temperature must be in the region of 12,000,000 to 15,000,000 degrees.
By itself, the telescope has limited range. It can gather a great deal of light, but - as we have noted - not even the Palomar reflector can show a star as a measurable disk. It is not difficult to understand why Auguste Comte, a French philosopher, wrote in 1825 that the chemistry of the stars was certain to remain permanently unknown to mankind. The development of spectroscopy has altered matters so drastically that for many investigations it is now the spectroscope, not the telescope on its own, which is the astronomer's chief research tool. When noting the blueness of Vega, the yellowish hue of Capella, or the lovely orange-red of Betelgeux and Aldebaran, it is interesting to bear in mind that spectral analysis has provided at least a partial answer to the age- old question: What makes up the stars ?
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