Page:Popular Science Monthly Volume 90.djvu/287

 Is Space All Aglow?

The new startling theory advanced by Professor Barnard to explain non-luminous bodies brought out in relief against the sky

���The photograph on the left is a luminous neb- ula which is a mass of gas. But w h a t is the curious irregular hole among the stars in the photograph on the right? These black holes are supposed to be openings in the rich regions of the stars. Professor Barnard is inclined to the belief that the supposed holes are feebly luminous bodies, and that they were once bright like the nebula shown at the left, but have' lost their light and become blackened with age

���THERE is coming to the front the con- sideration of the dark spots in the sky. Professor Barnard of the Yerkes Observatory has been studying this side of the question lately, and the results that he has already obtained lead him to speculations that are both new and interest- ing.

The nebulae of the sky have generally been considered to be intensely hot. These huge masses of luminous gas would cool aixi contract until, after millions of years, the more compact stars were formed. The stars might cool further until no light could be seen coming from them; but it was not generally believed that the gases of the nebulae themselves could cool and still remain in the gaseous state. One of the first results of Professor Barnard's work, however, was to lead him to believe differently. Let us follow him through his reasoning.

He says that in photographing the sky, large dark markings have often been noticed in the photographs. At first blush they seem merely huge openings in the rich region of stars through which one looks out into the blackness of the space beyond. Although there are undoubtedly such va- cancies, the more one becomes familiar with others of them, the less this explana- tion appeals.

To suggest the true explanation, Profes- sor Barnard has prepared the pair of photo- graphs appearing on this page. These, he

��explains, have been made exactly to the same scale, and a striking resemblance is seen between the two objects that stand out in them. But one is an ordinary luminous nebula, and the other is a dark — what? His observations induce him to believe that it, too, is a nebula, but one in which the great mass of gas has finally cooled and lost its light. The gas of the nebula, like many others that Professor Barnard has studied, is still dense enough to take definite outline and to stand out against the luminous background behind. But right at this point we would be led to still another conclusion. There are dark spots having very definite outlines to be seen in the heavens, where, there is every evidence to believe, there are neither luminous stars nor luminous nebulae to light up a background for them. What then, is the cause for the luminous back- ground in such cases? There seems to be but one possible explanation, and that is that space itself is luminous. Space itself might be filled with the feeblest luminosity, so feeble, indeed, that at the tremendous distances to the fixed stars it is not even perceptible to sight. Then, as space is sup- posed to be of infinite extent, this luminosity will increase in apparent density until, finally, it would become dense enough to affect a sensitive photographic plate. Only in this way would it be possible to explain how non-luminous objects could be brought out in relief in these parts of the sky.

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