The Milky Way

The Milky Way

On a clear, moonless night, the Milky Way looks like a path of pearly light among the stars. Many early peoples interpreted it as such and called it variously a celestial girdle, a road to heaven, and the celestial counterpart of the river Nile. It was first traced in detail and described in scientific terms by Ptolemy, the great Alexandrian astronomer of the second century A.D. Ptolemy commented on its patchy appearance, and described how it reached it brightest levels in the region of Sagittarius and narrowed down almost to nothing in the constellations of Auriga and Turus.

Stars, Dust, Gas

The telescope shows that the Milky Way is composed of myriads of stars, all so faint that they cannot be seen as single objects by the unaided eye. It also shows that the patchiness is produced by the way the stars are distributed. In the bright parts they are thickly scattered, whereas in the dark areas they are relatively few and far between. Photographs of the regions of Cygnus through to Sagittarius show places where the stars are so numerous and apparently so close together that their images pile into one another to form great star clouds.

Orion Nebula. Image in public domain

They also show dark irregular patches, due to clouds of interstellar dust that cut off the light of the more distant stars. The stars we do see in these dark wastes are mostly foreground objects, or stars that lie between us and the dust clouds.

Dust clouds that show fairly definite outlines are called dark nebulae. A good example is the Coalsack, a roughly oval-shaped dark patch near the Southern Cross. It appears permanent mainly because it is comparatively near to us (about 500 light-years) and therefore has hardly any foreground stars projected on it.

In many parts of the Milky Way the stars appear to be immersed in heavy folds and wreaths of bright nebulous material. The material consists of extremely rarefied gases whose light, like some of that from the tails of comets, is due to luminescence. The gases absorb the short-wave energy of the hot stars embedded in them and re-emit it at longer wavelengths. Objects of this kind are called bright nebulae or, more specifically, gaseous nebulae.

The Milky Way System – The Great Nebula In Orion

A particularly fine gaseous nebula is the Great Nebula in Orion. It can be seen quite easily without optical aid on a clear, moonless night, but there is no record that it was ever noticed before the invention of the telescope. The brightest parts, about 900 light-years away, are concentrated on a group of stars known as Theta Orionis. The group forms part of a cluster of intensely hot stars whose total radiation causes the nebula to shine over immense distances. They are thought to be comparatively young, with ages of the order of 10,000 to 40,000 years, and to have had their origin in the great gas cloud. The ragged appearance of the nebula is due largely to intervening dust clouds. In itself it is a vast spherical mass of luminescent gas which extends over practically the whole constellation of Orion.

Another fine gaseous nebula is associated with Eta Carinae, a nova-type star in southern skies. Sometimes called the “Keyhole Nebula” because of its shape, it can be traced over an area at least 25 times the apparent area of the full moon, but much of it lies hidden behind clouds of interstellar dust.

An Optical Effect

The Milky Way, as William Herschel discovered, is an optical effect. To appreciate the meaning of this, suppose the stars were evenly scattered throughout space and formed a great ball with the sun at the centre. The entire night sky would then glow with the light of innumerable faint stars. Now imagine that the hall is greatly flattened so that it becomes more like a watch in shape. From our central position we would see more stars in the direction of the rim than in directions at right angles to the rim. This would bring about the effect of a misty band of light, but the Milky Way so produced would be uniform in width and brightest in the middle of its course.

A model much nearer the actual situation is one in which the stars form a greatly flattened system shaed like a fried egg. The sun is located close to the central plane but about two-thirds of the way from the centre to the edge. The system, called the Milky Way System, or Galaxy, has no definite edges or boundaries, and its stars are concentrated in and around its central plane and also towards the centre. At the centre is the nucleus, an almost ball-like mass of highly-luminous giant stars. It lies in the direction of Sagittarius, but we cannot see it from the earth since it is hidden by obscuring clouds of dust.

Spiral Arms

We know how the Galaxy comes to have a flattened shape. The entire system, roughly 100,000 light-years across, is rotating. We also know, mainly by observations with radio telescopes, that the Galaxy has a spiral structure. The sun and its neighbours form a small knot in an immense spiral arm of stars, gas, and dust, one of two fairly continuous and almost circular arms. In their overall contours these arms curve away gently from the nucleus and, after making two or three turns, trail off into intergalactic space. Most of the stars in the Galaxy, along with the interstellar gas and dust, are confined to the galactic disk.

Two Populations of Stars

Must of the stars in the Galaxy, along with the interstellar gas and dust, are confined to the galactic disk. They travel round the centre in circular orbits similar to that of the sun, form a stellar population known as Popualtion I, and in general are believed to be comparatively young. The remainder, together with the globular clusters, are scattered around the disk to form a thinly populated spherical halo or system.

They travel round the centre in elliptical orbits, often steeply inclined to the plane of the disk. Known as Population II they are thought to represent older stars formed before the Galaxy acquired its spiral structure. Finally, both populations are immersed in extremely tenuous and ionized gas, permeated by lines of magnetic force and in itself a source of radio emission. Several other spiral galaxies have been found to be weak sources of continuous radio emission, so the Galaxy is not alone in this respect.