THE DARK PATCHES OF THE NIGHT SKY

Wanganui Chronicle, 5 March 1947, Page 7

THE DARK PATCHES OF THE NIGHT SKY

BY

'SKY PILOT'

Healthy curiosity has undoubtedly enriched human experience in multitudinous ways and is no less true in the study of the heavens than in other realms. As in the case of casual observers so also with astronomers many have wondered what reJly the dark patches of the sky suclr as the Coal Sack, near the Southern Cross, and in the Milky Way, and the great rifts from Cygnus, the Northern Cross to the Southern Cross dividing the Milky Way into two great irregular divisions and giving the appearance of lanes really arc.

Early astronomers thought that these great areas were simply holes in the sky through which we looked out into the vastness of space. Others again thought that this interstellar medium represented debris blown from the stars themselves by the violent pressure of radiation. It was thought that Novae were particularly active in this way. A study of the Sun, however, shows that most of the gaseous material ejected falls back into it again. Further there is such a great abundance of this cosmic material in the Universe that it is quite beyond this source altogether. Early in the present century the research work of that most able astronomer, Barnard, has fortunately revealed what it believed to be the true nature of these areas. He believed them to be “lanes” or rifts. They were discovered to be great clouds of tremendous areas in most cases of a very tenuous nature and composed of gases similar to our own mingled with cosmic dust, and since they were not close enough to bright stars to shine by their light they appear as great dark masses. The chief part of the cloud is gas although there is a tremendous amount of the finest of dust in them, of a non-metallic nature. “If we could employ a cosmic steam shovel to reach out into space,” says Goldberg and Aller, “and scoop up a gob of the interstellar stuff, we would probably find that the sample contained most of the common elements that are found on the earth and in the stars.” And they add in reference to the rarefied nature of the clouds. “To say that the interstellar material is spread very thinly is a gross understatement. A good approximation to the density of the stuff can be achieved by pulverising an ordinary marble and spreading it as evenly, as possible throughout a volume of a sphere 1000 miles in diameter.” VAST SKY AREAS OBSCURED These great clouds abscure vast areas of the sky, darkening out millions of suns which lie beyond. Stars which maybe seen apparently within them to the naked eye lie between the great dust cloud and ourselves. Just as rain clouds obscure the Sun and the stars then so do these cosmic dust clouds obscure from our view possibly vast rich areas of sky. They are a long way away. The dark nebulae in the Coal Sack, the rifts in Tau-

ru s and Auriga are about 500 Light years from the Sun whilst the dark cl’.ud in Ophiucus is about SOO Light years away and there are others well beyond this. Those whose illuminated part is in Orion is from 1000 to 1800 light years away. This dusty material has raised many problems as maybe expected concerning stars. For instance it has given stars appearance of a reddish colour when in reality they are bright hot stars of a bluish colour. Astronomers have been trying for sometime to solve the problem of how much of the dimness of stars is due to very great distances and how much is due to dust lying in the path between them and the earth. The reader will readily understand how dust gives a reddening effect as is seen at sunset and sunrise, and at a time when there is a great deal of smoke about.

When there are very white hot stars near the cloud it is made to glow, in tliat its gases are excited by the ultraviolet light as if they were hot gases. In these modern days are familiar with the fact that ultraviolet light can cause a substance to become bright without becoming hot as is illustrated by fluorescent lights. This helps us to appreciate how the Nebulae are caused to giow say as in the case of the great Orion Nebula. The dark nebulae are often fringed by these bright nebulae and appear as in the case of the famous Horse’s Head Nebula in Orion to run into one another quite naturally proving that the source of illumination from bright stars is not great enough to light the whole of the Nebula or dust cloud therefore leaving us with the great dark patches. Barnard’s discoveries really revolutionized the astronomer’s appreciation of the Universe. He has left u s with a conception not of voids but of compactness and great

v-'stness. He discovered more than 180 of these dark areas. Here we show a picture of the famous Nebula in Orion, one of the great ’ sights" of the sky. It is part of a tremendous area of cosmic dust cloud and gas and the part whiclt has been made to glow by reason of stars within it suclt as those in the Trapezium so familiar to observers of this wonderful Nebula. We have already said that it is 1000 to 1800 Light years away, a light year being 6 billion miles. So vast is this diffuse nebula that it takes light 10 years to cross it travelling at the rate of 186,000 miies per second. It is rotating in a period of several hundred thousand years and its mass is 10,000 times that of the Sun while its density is one millionth-billionth that of the air we breathe.

This grand Nebula is now spendidly placed for observation and is a fine sight through our local telescope. Watciiers of the sky will have noticed that is is fast moving over the sky and in a few weeks will have passed from our view. This fine celestial object | then should be viewed and appreciat- ; ed as soon as possible.