Only the Magellanic Clouds are observed to contain a few globular clusters, presumably because the Clouds are more NGC6822 and 1C 1613. Open clusters and association are very patchily distributed through all these galaxies, tracing no discernible structure. Some associations are rather large and the LMC contains a vast clumping of H+ regions, called the 30 DORADUS COMPLEX, spanning 3 X 1 kpc in the sky . The largest single region of this complex has a diameter of 230pc, which is only slightly less than the giant cloud of ionized hydrogen in M 33 Imagine what would happen if 30 Doradus were at the distance of the Orion Nebula: it would be visible throughout the day and cast shadows at night! In the LMC, a dozen probable supernova remnants are observed, and both clouds together have about 50 planetary nebulae. As everything in the clouds is at roughly the same distance from the Sun, they are a good collection of objects for internal comparison purposes. This feature, combined with their closeness, makes the Magellanic Clouds very important, not¬withstanding their modest mass: a 50-cm telescope can easily do as much work on the Clouds as can a 5-metre telescope on the Andromeda Nebula. A dramatic example of this importance was the discovery, after the turn of the century, of a correlation between the brightness and the period of LMC Cepheid variables; because these all are about equally far (50 kpc) from the Sun, this directly yielded the celebrated period-luminosity law. A drawback is that the Magellanic Clouds are not readily comparable with the large spiral galaxies. As has been said, they look very different, and it is found that some of their properties are different too. For example the gas content of the SMC is about a third of its total mass, an although the gas content is smaller in the LMC, the latter may well have a quarter of its mass in gaseous form. This is about six much as the Galaxy or M 31. Also, the dust content of the SMC gas is ten times lower than is that of the Galaxy. These differences have to be taken into account when, for example, we try to information on the rate of star formation

From our vantage point, we have a convenient overall vie the Clouds, which immediately shows that young stars of the O and B types, ionized hydrogen clouds, dense neutral hydrogen clouds and late-type supergiants all occur near each other, in small dense clumps. There is about 200 times more gas in them than there is in the form of stars. Most young clumps measure some tens of parsecs across, so the 30 Doradus nebula is an outsized example of such an aggregate. Only weak magnetic fields are observed. There is a possibility that supernova remnants are present in a few associations. Far away from the young clumps old red stars come to the fore, with practically no associated gas. The life history of a star, of which we see here a snapshot, apparently goes as follows. Firstly, tenuous and cold hydrogen clouds with masses of a few thousand solar mass contract by gravitation, slowly at first, but gradually more and more rapidly. Somehow such a cloud breaks up into fragments, and a smaller number of these (half a per cent or so) manage to contract so far that their temperature and density become high enough for nuclear hydrogen fusion to begin. The most massive of these young stars give off so much ultraviolet radiation that the gas around them is ionized and blown away through the newly-formed association. In a few hundred thousand years, these stars may explode as supernovae. Their less massive, and hence more sedate, brethren slowly evolve and move away to mingle with their distant surroundings. Although almost all stages of stellar evolution are presented to us in a picture of the Clouds, we cannot apply any derived results directly to our Galaxy.

The remaining members of the Local Group are rather feature¬less clumps of stars. They are very numerous (14 out of the 21 members) but have a very small mass, whence they are called DWARF ELLIPTICAL GALAXIES. Typical absolute magnitudes range around Mv= —10, and their surface brightness is so low (about 24 mag. per square second of arc), that almost all were discovered in the last 30 years, when photographic plates improved. Even with today’s electronographic detectors, they are practically undetectable beyond 500 kpc. It is unlikely that this situation will improve until devices are used that automatically remove the unwanted radiation due to the brightness of the night sky, which has a surface brightness about equal to that of a dwarf elliptical galaxy.

Still, there is a striking resemblance in the star content of globulars and dwarf elliptical galaxies: both contain exclusively old Population II stars, with the exception of a very small Population I within a few hundred parsecs of the centres of NGC185 and NGC 205. No interstellar gas has been observed in any dwarf elliptical, but dust appears to be present in a few patches in the centre of NGC 105 and in two sizeable dust lanes across NGC 205. The masses of dust involved are at most a few hundred solar masses. Although the dust indicates the presence of at least silicon and carbon, the spectra of stars in these galaxies show that they contain less heavy elements than do the dominant spirals.

It should be clear from the above, that the members of the Local Group have much in common. They appear to be built out of the same materials, which is something worth considering; for it is not at all obvious that a cloud of stars that has formed 700 kpc away from us should contain essentially the same kind of stars as are to be found in the solar neighbourhood. The building blocks differ in detail, though, and notably the different abundances of the heavy elements show that not all members of the Local Group have gone through the same past evolution, while all do seem to have approximately the same age.

Comments are closed.