18 Feb 2016

Journey to the Centre of the Milky Way

There can be few people who have not admired the Milky Way at some time or another. City-dwellers can never see it, because it is drowned by the glare of artificial lights; but in the country it is a superb sight, and during summer evenings it is at its best. It takes the form of a band of light, crossing the sky from one horizon to the other. It is particularly rich in the area of the constellation Cygnus, and with binoculars it is seen to be made up of vast numbers of faint stars.
The first man to realise this was Galileo, the Italian astronomer who turned a telescope skyward in late 1609 and made a whole series of spectacular discoveries, even though his telescope was feeble and defective by modern standards.

Because there are so many stars in the Milky Way band, the general impression is one of overcrowding; it might also be thought that the individual stars are so close together that they almost touch. This is not so. Even in the densest parts of the Milky Way the distances between separate stars still amount to many thousands of millions of miles. Our star-system, or Galaxy, is a flattened system, 150,000 light-years across but nowhere more than 20,000 light-years broad; the Sun, with the Earth and other planets, lies roughly 25,000 light years from the centre or galactic nucleus, so that we are well removed from the middle of the system.

This was one of the errors made by Sir William Herschel, who drew up a very reasonable picture of the Galaxy during an observational career which began in the 1770s and ended only with his death in 1822. Herschel, not irrationally, thought that the Sun must occupy a near-central position.

The center of the Milky Way lies in the dense star clouds in the constellation Sagittarius, the archer, which may be seen low over the southern horizon on British summer evenings. We know that there is a black hole in the centre of our galaxy which is associated with a strong radio source called Sagittarius A. We can’t see it but we know from its effect on nearby nebulae and stars that it exists. It is known to have a mass of 4 million times that of the sun.

Once the shape of the Galaxy is understood, the Milky Way is a mystery no longer. When we look along the main plane of the Galaxy, either toward the centre or else directly away from the centre, we see a great many stars in roughly the same line of sight; this causes the crowded appearance. It is, in fact, nothing more than a line-of-sight effect.

The science of radio astronomy has come to our rescue. Radio waves are not blocked by gas and dust, so that they can reach us even from the galactic centre. Optically, the field of infrared astronomy has given us rewarding views of objects since infrared is not really blocked by the gas and dust.

The nucleus of the Galaxy lies beyond the lovely star-clouds in the constellation of Sagittarius (the Archer).

Radio astronomy has also confirmed that the Galaxy is a spiral system, and that we live near the edge of one of the spiral arms. This revelation was not unexpected, because it had long been known that many of the external galaxies are spiral; such, for instance, is the Great Galaxy in Andromeda, which is more than 2,000,000 light-years away, and is a system appreciably larger than ours. Altogether, the world's largest telescope is capable of photographing about a thousand million galaxies, and modern astronomy can probe out to distances of well over 5,000 million light-years.
Video courtesy (C) European Southern Observatory.

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