The Realm of the Galaxies

The Realm of the Galaxies

The Clouds of Magellan

Prominent among the stars and of southern skies are two large misty patches which look like detached fragments of the Milky Way. Portuguese navigators of the 15the century called them the “Clouds of the Cape”. They were first described with reasonable accuracy in 1521 by Antonio Pigafetta, chronicler of Magellan’s voyage round the world. When photographed with large telescopes the two clouds of Magellan, as they now are called, are seen to be magnificent objects. With their great population of stars and gas of shining gas, they are no less than galaxies, similar in several respects to our own Milky Way System.

The Clouds of Magellan lie eel outside our Galaxy at a distance of about 180,000 light-years. They extend far beyond their naked-eye limits, have diameters of the order of 40,000 light years and 25,000 light-years, and are companion galaxies to our own. The large Cloud contains objects similar to those found in our Galaxy and in particular a magnificent bright nebula associated with the star 30 Doradus. But its spiral arms are fragmentary and difficult to trace, The Small Cloud has no definite structure and is comparatively free from interstellar dust and bright nebulosity.

The Large Cloud of Magellan. Source of the photo: NASA

Cepheids

Both clouds contain large numbers of variable stars called Cepheids after their prototype Delta Cephei. In 1912, from a study of photographs of the Lesser Cloud, Miss Henrietta Leavitt of the Harvard Observatory discovered that these stars have an important property – the longer the period of their light variations, the brighter they appear. For practical purposes they can be assumed to be all equally distant from us. Hence the relationship is one between period of light change and luminosity. This provides astronomers with an important measurement tool for determining stellar distances. If a group of stars contain a Cepheid, the determination of the Cepheid’s period leads directly to its luminosity and this, compared with the apparent brightness, establishes its distance. The method has been applied with great success to globular star clusters, the Clouds of Magellan, and other neighbouring galaxies.

Messier 31

The only other galaxy visible to the unaided eye is Messier 31, the great Galaxy in Andromeda. It was first mentioned in 1612 by Simon Marius, a German astronomer, who compared its appearance in the telescope to that of a candle flame seen through a piece of horn. Astronomers in the 19th century were uncertain whether it was a mass of shining gas or a system of stars. But all doubts were removed in the early 1920th. Photographs taken with the 100-inch telescope of the Mount Wilson Observatory not only gave indications of separate star clouds, bright nebulae, and open star clusters, but also showed some of the brighter stars as individual objects. In 1923 Edwin Hubble, using the 100-inch telescope, detected the first of many Cepheids in this galaxy and was thus able to put the determination of its distance on a sound basis.

The light from Messier 31 takes about 2.2 million years to reach us – as interval of time far longer than the entire history of Man. If the sun was placed in Messier 31 it would be beyond detection even with the full photographic power of the 200-inch telescope on Palomar Mountain. The overall diameter is roughly 180,000 light-years, or almost double that of our Galaxy. If we represented our Galaxy by a disk one inch in diameter, Messier 31 would be another disk nearly two inches across separated from the first by a distance of 22 inches.

Messier 31, the Great Galaxy in Andromeda. Photograph: Adam Evans

Another Spiral Galaxy

Messier 31 is a spiral galaxy similar in content to the Galaxy. It even has its own system of about 200 globular clusters and also two small elliptical companion galaxies. It appears oval in shape because its plane is tilted by about 15 degrees to the line of sight and this tends to make its spiral arms difficult to trace. The latter are outlined by clouds of interstellar dust and contain numerous star clusters and patches of bright nebulosity. The entire object is rotating about its center, but, since one rotation takes many million years, the general appearance remains unchanged from one century to the next. Photographs of the central region taken with plates sensitized to red light show that the nucleus is an enormous assemblage of highly luminous reddish star and closely resembles a gigantic globular cluster.

Messier 33

Another nearby galaxy is Messier 33 in the constellation of Triangulum. This, too, has a spiral structure, exceptionally well shown since we happen to view it almost face on. Its distance is reckoned to be two million light-years, and its diameter about 50 thousand light-years.

Messier 31, the Milky Way Galaxy, the Clouds of Magellan, and Messier 33 are the largest members of at least 17 galaxies known as the Local cluster. All 17 objects are contained in a volume of space some three-million light-years across. Most of the other members are lightweight systems, several of them being faint and sparse, and resembling large galactic star cluster. On the other hand, Messier 31 and the Galaxy are themselves lightweight systems compared with giant elliptical galaxies like Messier 87 in Virgo. Likewise the local cluster is a small affair compared to many other clusters, some of which contain several hundred galaxies.

NGC 4565, a spiral galaxy in Coma Berenices, seen edge on. Photograph: Ken Crawford