August 21 1964 - Colour in the Universe

During the television coverage of the ig6y lawn tennis championships at Wimbledon, the BBC started putting out programmes in colour. Of course, tests had been made much earlier, and colour television had begun in some countries ; but Wimbledon marked the start of a new era so far as Britain is concerned.

I can see that in the future, this change-over will cause some complica­tions in The Sky at Night programme. Colours in the sky are not strong, except with rare phenomena, and how they will show upon a television screen I do not know. However, it may be interesting to look back to a programme I devoted to this topic as long ago as the summer of 1964, when colour television still seemed to lie a long way in the future.

What is the colour of the night sky? Most people would probably answer this question by saying 'Black, with white stars' - but in fact the situation is not nearly so simple. There is plenty of colour in the universe, provided that the observer knows where to look for it. The planets, for instance, have their own distinctive hues, though only in the case of Mars is the colour really conspicuous to the naked eye.

Saturn, the outermost of the planets known in ancient times, is now visible in the south-east after sunset, and remains above the horizon all night. It has a dull, leaden aspect, and the astrolo­gers of past ages regarded its influence as baleful. It appears in the guise of a moderately bright star, lying well below the prominent Square of Pegasus; there should be no difficulty in finding it, since the only possible confusion is with the first-magnitude star Fomalhaut in Piscis Austrinus (the Southern Fish). Fomalhaut, however, is lower down and not so bright.*

Saturn, like the other three giant planets, is made up of gas, and this gas is known to be largely hydrogen. This means that the details on its surface are constantly changing, and it is im­possible to draw up permanent maps, though the main cloud belts have persisted since the start of serious telescopic observation. Spots are rare, but now and then something really spectacular is seen - the last occasion being in 1933, when W. T. Hay dis­covered a brilliant white spot near the planet's equator. Of course, Saturn is less easy to study than Jupiter, partly because it is smaller and partly because it is farther away. Its equatorial dia­meter is just over 75,000 miles, while at the date of opposition 24 August 1964) its distance from us was no less than 816,000,000 miles.

The general colour of Saturn's disk is yellow, as any small :telescope will show. During 1964 there were interesting changes there. For much of July the whole equatorial zone of the planet appeared to be tinged with brown or deeper yellow; I detected :his aspect on 9 July, and it has since seen confirmed by Ameri­can observers, while in Britain it has been recorded by A. W. Heath, Director of the Saturn Section of the British Astronomical Association. By mid-August the unusual hue was less pronounced, though it had not quite disappeared. The cause of this sort of phenomenon is not known.

The real glory of Saturn lies in its ring-system. The rings, made up of large numbers of small particles moving round the planet in the manner of dwarf moons, are of great extent, but are also very thin; their thickness can hardly be more than ten miles. The appearance of Saturn therefore changes markedly according to the angle at which the rings are placed with respect to the Earth. The rings are creamy, and are actually more brilliant than Saturn itself.

Jupiter, too, is predominantly yellow, but since 1959 its appearance has been unusual. Generally there are two distinct cloud belts, one to either side of the planet's equator, together - with other belts in higher latitudes. In 1959 the whole equatorial region turned an extraordinary orange or brownish hue; this persisted, with variations, until 1963, when the two equatorial belts had run together to form a continuous 'wedge of colour' right across the disk. It now seems that things are reverting to normal, and by August 1964 the separate belts were again identi­fiable. The remarkable feature known as the Great Red Spot has also been very much in evidence - and it really has been decidedly reddish in hue, though it has lost the brick-red colour which, from all accounts, it showed for a few years following 1878. It must be admitted that, so far, we have no real idea of the nature of the Great Red Spot, and neither do we know why Jupiter exhibits these peculiar changes in colour.

A small telescope will suffice to show the yellowness of Jupiter, together with the main belts and the four large satellites. Venus, which is a fine object in the eastern sky before dawn, is even more brilliant than Jupiter, but is less spectacular telescopically. It is a very different sort of world, since it is slightly smaller than the Earth, and is closer to the Sun than we are. Like the Moon, it shows phases, and at present it is 'gibbous', i.e., between half and full. Unfortunately no details are visible on its disk, since Venus is permanently covered with a dense, obscuring atmosphere which, according to recent results obtained from balloon-borne instruments sent up from the United States, con­tains a considerable amount of water vapour. Venus is slightly yellowish, though the naked-eye observer will probably call it pure white.

Through a powerful telescope, Mars is brilliantly coloured. Most of the surface is reddish-ochre, and is thought to be coated with some sort of mineral, possibly felsite or limonite. The polar caps, which are probably made up of some icy or frosty deposit, are glittering white, while the dark patches are said to have a greenish hue at times. H. Strughold has paid great attention to Mars, and has called it 'the Green and Red Planet', but to my eyes the dark markings generally seem grey. At any rate, it seems possible that they are due to living organisms, even though doubts have arisen lately.

Of the remaining planets, Mercury is somewhat pinkish, Uranus green, and Neptune bluish, while it has been claimed that Pluto is on the yellow side of white. However, these colours are not striking, and only experienced observers will be able to detect them. Mercury is always elusive; Uranus is on the fringe of naked-eye visibility, and both Neptune and Pluto are much too faint to be seen without optical aid.

In discussing the colours of the stars, it is natural to begin with our own particular star - the Sun - which, of course, is yellow. Its surface temperature is 6,000 degrees Centigrade, and it is in every way unremarkable. There are many similar stars in the Galaxy, and no doubt plenty of other stars are attended by in­habited planets. The only sensible way to observe the Sun telescopically is to project its image on to a white screen; to look straight at the Sun, even with a very small telescope, is extremely dangerous even when a dark filter is used. By projection, however, any sunspots which may be present are well seen. These spots appear dark, but this is an effect of contrast; if they could be seen shining by themselves they would be very brilliant, but they are less luminous than the rest of the Sun's surface because they are some 2,000 degrees cooler. During the summer of 1964 sun- spots were scarce, and there were long periods when the disk was completely blank. However, this was only to be expected; the Sun exhibits a roughly regular 'cycle' of eleven years, and from now on it is likely that spot-groups will become more frequent again. The next period of maximum activity is expected around 1969.

Stars which are hotter than the Sun will be white or bluish, while cooler stars will appear red. Binoculars are powerful enough to show these various colours well, while there are some stars whose hues are evident without any optical aid. A good example of this is Vega, in the small but interesting constellation of Lyra, the Lyre or Harp. Vega is extremely bright. It cannot match Venus or Jupiter, but of the so-called 'fixed stars' visible from Britain only Sirius is its obvious superior. Moreover, Vega is almost directly overhead during summer evenings, so that there should be no trouble in identifying it. It is definitely blue; it is fifty times as luminous as the Sun, and its distance from us is twenty-seven light-years, so that we are now seeing it as it used to be twenty-seven years ago.

The blueness indicates high temperature, and in fact the sur­face temperature of Vega is about twice as hot as the Sun. There is an interesting contrast with Arcturus, which now lies in the west, more or less in line with the curve of the 'tail' of the Great Bear. Arcturus shines about as brilliantly as Vega, and is actually twice as luminous, but its surface temperature is lower; only about 4,000 degrees. This means that its colour is a glorious orange. Vega is one of three first-magnitude stars making up what is unofficially termed the 'Summer Triangle'. The other two members are Deneb in Gygnus (the Swan), which is yellowish and Altair in Aquila (the Eagle), which is pure white. Deneb, apparently the faintest of the three, has been found to be a re­markably luminous supergiant, equal to at least 10,000 Suns and lying at a tremendous distance from us.

The best example of the Red Giant star is, undoubtedly, Betelgeux in Orion. Orion is a winter constellation, but now rises before dawn, and the contrast between its two leading stars is very marked; the redness of Betelgeux is as obvious as the pure white light of Rigel. Another Red Giant is Aldebaran in Taurus (the Bull), which lies in line with the three stars of Orion's belt, and is associated with the V-shaped cluster of stars known as the Hyades.

With the naked eye, only a few of the brightest stars are strongly coloured; with fainter objects, optical aid is needed before the hues can be well seen. A good example of this is Herschel's 'Garnet Star', Mu Cephei, not very far from Polaris. To the naked eye it appears unremarkable, but any small telescope makes it look like a tiny glowing coal. The surface temperature in this case is only about 3,000 degrees.

Observers who have access to binoculars or small telescopes will find it interesting to look from star to star and note the various colours; it will not take more than a few minutes to find that the stars are utterly unlike each other. There are lovely colours, too, among double stars, of which Albireo in Gygnus is an outstanding example. Albireo is the faintest of the five stars making up the •.veil-known cross of Gygnus, and to the naked eye it seems white, but a telescope will show that it is made up of a golden-yellow primary together with a much fainter companion which some people term green and others blue. And there are bright red stars, such as Antares in Scorpio (the Scorpion) which have green com­panions. The greenness of the small attendant of Antares is accentuated by contrast, but this does not make it any the less beautiful.

Not all double stars show contrasting colours. Look, for instance, at Epsilon Lyra:, which lies close to Vega. Keen-sighted observers will note that it is made up of two; a 3-inch refractor will show that each component is again divided, so that Epsilon Lyra; is a double-double or quadruple system. All four of its suns are white.

Very faint stellar objects show colours which cannot be de­tected except by photography. There is a good example of this in Messier 57, the Ring Nebula in Lyra, which also lies near Vega. The Ring is a 'planetary nebula', but the name is a bad one, since planetary nebulae are neither planets nor nebulae. Messier 57 consist of a faint central star surrounded by a tremendous en­velope of tenuous gas, so that it looks rather like a very dim, luminous bicycle-tyre. Moderate telescopes will show it clearly, and it is easy to find, but no colour will be detected. A famous photograph taken some years ago with the largest telescope in the world, the Palomar 200-inch reflector, showed that the Ring has a bluish central area with the outer parts yellowish and red.

Star-colours are important because they provide a key to the surface temperatures, though detailed analysis is carried out with the aid of spectroscopic equipment rather than by visual estima­tion. Yet the colours are spectacular, too, so that the casual ob­server who uses binoculars to look at the orange Arcturus, the glittering blue Vega, and the dull yellow planet Saturn can hardly fail to be impressed. We live in a coloured universe; the skies are anything but drab.