Last week Dr. Charles Greeley Abbot, head of the Smithsonian Institution and world-famed authority on solar radiation, declared that if William Shakespeare had lived in the 20th Century he would have become an astronomer. One thing Astronomer Shakespeare would have had to get straight from the start is the different classes of nebulae. Astronomers apply nebula not only to whole galaxies of stars far beyond the Milky Way. but also to patches of dark or faintly luminous matter within the Milky Way, which is the home galaxy of earthlings. They may be distinguished by calling the local patches galactic nebulae, the great outer star clusters extragalactic nebulae. About 100 of the nebulae within the Milky Way are planetary: that is, they have a star at the core and are presumed to shine by light reflected from it.
Many a blue nebula has been discovered, shining by light from a hot blue star. If the reflection theory is correct, red nebulae should also exist, with comparatively cool red stars as illuminators. Last August Russian-born, dimple-chinned Director Otto Struve of Yerkes Observatory announced discovery of the first known red nebula. It fans out from the red super giant star Antares to a distance of about two quadrillion miles.
Last week The Telescope, published by Harvard Observatory, pointed out that conclusions as to the size of the particles in this red nebula could be drawn from its color. If the nebular material were microscopically fine, the light of Antares would be reddened, just as the sun is reddened when it shines through a long slope of the hazy atmosphere at evening. Also, the light from the nebula would be made bluer by selective scattering of the same kind which makes the earth's sky blue.* Actually, the color of the star and that of the nebula are almost identical. Therefore the nebular fragments must be big enough to reflect the light intact—as big as sand grains, peas, marbles, melons or mountains.
By the same reasoning, Drs. Walter Baade and Rudolf Minkowski of Mt. Wilson Observatory have come to quite a different conclusion as to the size of the particles in the vast dust cloud of the Orion Nebula. In this case the particles scatter the blue and yellow-green components of the light of stars in the cloud, letting only long infra-red rays filter through. Hence the stars appear much redder than normal. The wave lengths Baade & Minkowski recorded convinced them that the dust grains in the nebula were about .000004 inch in diameter.
Interstellar Gas. Until this year the only gases known to occur in interstellar space were sodium and calcium. Ordinarily these metallic elements must be strongly heated before they vaporize, but in the utter cold of space, close to absolute zero, they exist in exiguous quantities as free molecules and therefore as gases. In the ultraviolet range of the spectrum of the stars Chi 2 Orionis and Chi Aurigae, Astronomers Walter S. Adams and Theodore Dunham Jr. of Mt. Wilson Observatory found four lines (one of them almost blotted out by the interference of Earth's atmosphere) which they identified as originating from the element titanium. The peculiar sharpness of the titanium lines indicated that the light had picked them up not from the stars but in its travel across the not quite empty gulfs of the galaxy. Thus titanium takes its place as the third metallic gas in interstellar space.