The ANDROMEDA NEBULA, M31, is a GIANT SPIRAL galaxy. With a mass of 300 billion solar masses (M0) it is the most massive member of the Local Group; it is twice the mass of the Galaxy. Its total absolute magnitude Mv = —21.1 makes it also the brightest member. In the sky, the optical image measures 75 X 245 minutes of arc, which at its distance of 690kpc corresponds to a disc with radius 25kpc, seen at an inclination of 77°. This rather high inclination makes it difficult to decide whether the numerous ionized hydrogen (H+) regions, of which about a thousand have been found, form continuous rings or spiral arms. Other spiral tracers show the same difficulty: the OB associations, of which there are about 200, mostly in the plane of the galaxy between nine and 18 kpc from the centre, and the Cepheids occur in bands which are so foreshortened that they might be spiral arms or closed circles. By analogy with other spiral galaxies it is generally assumed that M 31 is a spiral galaxy with tightly wound arms. Nearly 200 globular clusters accompany M 31; the furthest are at a distance of 30 kpc from the centre. These findings show that M 31 has stellar populations similar to those of our Galaxy. A remark¬able fact is that the properties of the populations of the disc and the lower halo are dependent on the distance to the centre: what we are seeing in these stars is that the amount of elements heavier than helium (mostly carbon, nitrogen and oxygen), increases towards the galactic centre. This indicates that the processing of material through stars, via rapid mass loss and enrichment of the inter¬stellar medium, is larger in the central parts of M 31 than it is in its outskirts. Information of this type is important because it enables astronomers to work out what happened to the galaxy in the past.
The Andromeda Nebula has a nucleus which at optical wave-lengths looks like an ellipse with axes of five and eight parsecs, and a very large surface brightness compared with the rest of the galaxy. Its power output in the infrared is about equal to that of the nucleus of our Galaxy, and since most of this radiation is scattered starlight it follows that the structure of the star clouds in both nuclei are very much alike. But in the radio continuum, the galaxy’s nucleus is more than 20 times fainter than is Sagittarius A.

Our Galaxy ranks second in the Local Group, and the third most massive member is the TRIANGULUM NEBULA, Messier 33 (M33). It is clearly a spiral galaxy with an open-armed pat¬tern, covering 68 x 40 minutes of arc in the sky, corresponding to a disc with a radius of seven kiloparsecs with an inclination of 50°. It contains a Population II component, with perhaps an increase of heavy elements towards the centre similar to that observed in M 31. But the most striking feature of this galaxy is its very strong Population I component. Even in the nucleus, a bright region of five parsecs diameter, young blue stars and accompanying regions of ionized hydrogen are observed. Along the spiral arms many more of these are found, sometimes clumped together as in the giant H+ region in the northeast which measures 270 pc across.

The remaining members of the Local Group are relatively unimportant, for together they comprise only a tenth of the total mass of the group. The galaxies NGC 6822 and 1C 1613 resemble the Magellanic Clouds. They all measure a few kiloparsecs across and have masses between 4(H) million and 6 billion M0. Roughly speaking, they contain the selection of objects to be found in the giant spirals, although they do not show the regular plane-halo arrangement of the three dominant galaxies. They look rather irregular, with perhaps a tendency to have an elongated concentration of stars and gas in their central regions. Both populations are present, and there are indications that the amount of heavy elements they contain is rather less than in our Galaxy.

This difference in chemical composition shows that the Clouds have a history rather different from that of our Galaxy. Probably they collapsed more slowly, and their initial star formation was some five times less efficient than was this process in the Galaxy Therefore the Clouds contain much more gas relative to their total mass. Also, later star formation was less efficient, so that overall the fraction of heavy elements added to the interstellar gas in the Clouds was less than in the Galaxy. This also explains why the SMC is practically free from dust particles, since these form from heavy elements.

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