IN STARRY SKIES

Evening Post, Volume CXV, Issue 94, 22 April 1933, Page 21

IN STARRY SKIES

MORE SECRETS REVEALED BY THE SPECTROSCOPE

(Ey "Omega Oentauri.") We have seen that white light when passed through a prism is spread out into a beautiful coloured band, but if the light .comes from, the sun the band is not absolutely continuous. It is crossed by a great number of very narrow darlc lines. • Newton was impressed by the importance of the discovery that white light could be spread into all the colours of the rainbow and then reeombine'd into white light. But ho never grasped the key to the mystery. Woolaston and Fraunhofer saw the dark lines and Fraunhofer at once realised that they carried a messago, borne on the wings of light, across the 93,000,000 miles that separate us from the sun. "With wonderful care and patience he mapped the positions of nearly 600 of these lines and he gave the clue to deciphering the message that they bring by establishing a connection between the light emitted by sodium vapour and a pair of lines in the solar spectrum. Before reading any of the messages that the spectrum brings we must realise that an atom, is not the shnplo thing we used to suppose it to bo. Its very name is misleading, meaning uncutable or indivisible Buthorford and others have actually succeeded in breaking from the atom something simpler still, but, even if they, had nob done so, the spectroscope would have revealed its complexity. Each kind of chemical atom has" specific qualities differentiating it absolutely from others, but the atom of hydrogen, helium, or any other element seems to be exactly the same on the most distant star as it is on earth. But the atom of hydrogen, the very simplest we are familiar with, gives evidence of extraordinary complexity. Keeping perfect time it can dance an astonishing number of different steps. The dances of the different elements are as distinct as the waltz, polha, mazurka or schottische. But the Polka in Sirius or Antares is the same as in the sun or on the earth, except in so far as it may be modified by temperature, pressure, and other conditions. Now the behaviour of elementary atoms in the gaseous state may bo studied by putting them in a spirit flame or an electric spark or arc. It is found that an element jn the gaseous state gives a spectrum of bright lines. The arc and spark specra differ from the flame ones, but each separate element behaves in its own characteristic way. But in the solar spectrum the lines are dark, whilst those of the flame, the are, and the" spark are all briglit.! How are we to account for this? Let! us try an experiment. Take a spirit flame with some volatile compound, of sodium in it, and looking through the

spectroscope, notice the exact position of the bright yellow lines. Then place a much hotter source of light such as an electric filament or electric are behind the spirit flame. The electric filament will give a continuous band of colour, but in tho exact position of the sodium lines there will now be a similar dark pair. These lines are so close together as generally to appear as one. What is the explanation? The sodium atoms are very particular to adhere to the true steps in their charactistie dance, but they can readily be induced to dance more vigorously. A continuous band of colour consists of light of all wave lengths, from that of violet to that of red. Now ,the spdipm. atoms can use the energy of the .right wave length to add, .to .the intensity, of their dancb. But they are dancing at a lower temperature than the atoms in the filament, so they emit less light and the sodium lines appear dark. _ The characteristic lines in the visible spectrum are not really dark.- They only appear so by contrast with the brighter light beside them. How then are we to interpret the dark lines in the' solar spectrum? The sun is a very hot body surrounded by a glowing atmosphere of gases and vapours at a lower temperature. In the body of the sun the atoms are so crowded, and are dashing about so violently, that they are not allowed to execute their characteristic vibrations. The £ght

that comes to us from them consists of a jumble of all possible vibrations. It gives a continuous spectrum. But in the atmosphere that surrounds the sun, the atoms are at a lower temperature and are not so crowded. They can execute the steps they love. In doing so they take energy, of exactly the right wave lengths, from the light that passes amongst them as it escapes from the sun: The result is that the coloured band is crossed by thousands of dark lines. And every one of those lines is telling part of the story of the behaviour of some particular kind of atom. In this way the" spectroscope reveals . the chemistry and physics of the sun. Last week we showed a photograph "of a number of these lines. Occasionally the moon' gets exactly between the earth and the sun, and it is so near to us, that when it does so it almost exactly hides the sun's disc. The glowing atmosphere then shines as a crown of glory round the moon. The light of this halo, being due to incandescent gas, appears through the spectroscope as a series of beautifully coloured bright lines. It can be seen by the lowest band in the picture that these bright lines are exactly in the same positions as the dark lines in the ordinary solar spectrum. The lines thus tell the chemical constitution and the temperature of the solar atmosphere. In the same way we can learn about the chemistry of the stars. The details in stellar spectra are generally much too delicate for satisfactory reproduction in a newspaper. But by taking only a small part of the band it will be possible to see tho widened hydrogen lines and perhaps a few others in the spectra of some pre-solar stars. Those are all very much hotter bodies than tho sun. Alpha Leonis is tho wellknown star Kegulus, in the lion's heart, which owes its name to the fact that it was supposed to be the ruler of the affairs of heaven. Alpha Lyrae or Vega was the Northern Pole star about 12,000 B.C. and will be the Pole Star again in another 12,000 years. It is sometimes called in the Northern Hemisphere, "The Glory of the Summer Heavens." Gamma Lyrae, or Sulaf, is a much fainter star which is approaching at the rate of 12 miles a second. Sirius, or Alpha Canis Majoris is .the brightest star in the heavens. It is really, a rather brilliant star emitting 30 times as much light as the sun, but it is only five hundred and fifty thousand times as far away. Now whilst this seems a great distance, only half a dozen stars are yet known to bo so near to us, and great numbers are intrinsically brighter than Sirius. The spectra we show today proves that hydrogen plays a very important p#rt in the atmosphere of these extremely hot stars. ' Being the lightest of the elements, its atoms movo at tho greatest speed. In the case of a celestial collision hydrogen leads the outrush of gas from the third body. In the. atmospheres of the hottest stars it extends much" farther than other gases. It is followed by helium and other elements, approximately in the order of their atomic weights. But the behaviour of the atoms is greatly in-

fluenced by chemical affinity and other factors. The inert gases, heluim, neon, argon, crypton and xenon, Bickerton calls "cosmic pioneers." But the spectroscope can tell us much more about celestial bodies than their chemical composition and temperature, and wo shall have to consider next the wonderful way in which it reveals their movements, not only the vibrations of the atoms, but the actual motion relative to our system of the bodies themselves.

"Russia Fights for Peace" will be the subject of discussion in the Trades Hall, Vivian 'Street, tomorrow night..,ft. .. 8 Three speakers,--one of whom will be -^lajor W. N. Pharazyn, .will describe what they .saw in the last war and what the attitude of the Soviet Union is .towards world peace. The meeting is to be held under the auspicesl of the Friends of the Soviet Union.