Just as Earth is not the only planet in the Solar System, and just as our Sun is not the only star in the Galaxy, so our Galaxy is not the only one in the Universe. But it was only 50 years ago that this im¬portant fart was established, even although SPIRAL NEBULAE -distant galaxies like our own – had been known since the end of the eighteenth century . Throughout the nineteenth cen¬tury, theoretical astronomers worked on the possible extent of the material Universe; however, the establishment of some nebulae as extragalactic systems was wholly a feat of modern observational astronomy. The realization that there are other galaxies far beyond the Milky Way proceeded apace with the technology of optical observation, and it provides a fascinating success story of recent astronomy.

By the beginning of the twentieth century, work with the exist¬ing refracting telescopes had led to the listing of some 8000 nebulae, most of which were known to belong to our immediate neighbour¬hood since they were emission or reflection nebulae associated with bright stars; the Orion Nebula is an example of an emission nebula. But the observers found that some were rather different because of their colour and shape: they were whiter, emitting a smaller fraction of their light in the red Ha line than do the galactic nebulae, and many possessed the distinctive spiral shape

In 1917 the discovery of an exploding star (now known to have been a supernova )in the nebula NGC 6946 prompted a search for stellar outbursts in other nebulae. Within a few years, astronomers found 22 in the Andromeda Nebula. M 31. Comparison of the stars’ peak apparent magnitudes with the peak absolute magnitude of galactic novae (at that time thought to be —4 mag) put M 31 at a distance of 200 kiloparsecs. a good 10 times the estimated diameter of our Galaxy. But this method of distance measurement did not prove the extragalactic nature of the Andromeda Nebula beyond all doubt, for one of the 22 exploding stare was fully 12 magnitudes brighter at its peak than were any of the others: we now know that a supernova caused the most brilliant outburst, whereas the others were due to ordinary novae. Therefore observers sought further evidence to strengthen the extragalactic hypothesis, and this was found when they used large telescopes and new photographic emulsions to resolve M 31 and M 33 (Triangulum Nebula) into individual stars. Comparison of these with galactic stars showed M 33 to be 300 kpc away from the Sun, and therefore extragalactic. But the evidence was denied by some observers who maintained that the images of the individual stars were partially resolved and were therefore clouds, which should not be compared to galactic stars.

The interpretation of many astronomical observations hinges on distance determination, and the final resolution of the problem of the nebulae yielded to this method. The spiral nebulae were once and for all proven to be extragalactic in 1924 when the analysis of Cepheid variables in M 31, M 33 and NGC 6822 showed that their distances far exceed the diameter of our Galaxy. Even so, these three external galaxies are relatively near by: they are in fact members of the Local Group.

In addition to the above methods, there are other ways to deter¬mine the distances of galaxies. One possibility is to use the fact that ionized hydrogen (H+) regions are normally smaller than 200pc diameter: thus by determining the apparent diameters of the H+ regions in a galaxy, its distance can be estimated. For example, if such a diameter were 5 arc sec, then the galaxy would be 8Mpc away. Another method is to measure the apparent brightness of an entire galaxy and to compare it with that of a similar galaxy with known distance. On the assumption that similar-looking galaxies have similar absolute magnitudes, this comparison gives a distance modulus. Both methods are imprecise because of the enormous differences in the size and composition of galaxies.

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