Our Galaxy is some 5 x 1017 kilometres in radius, and our journey is still far from complete. Around us in space, far beyond the edge of our Galaxy, lie other spiral forms reminiscent of the Milky Way but still further away. The metre is not a convenient unit for astronomy, nor is the kilometre, for the numbers quickly become unmanageable. The distance from the Earth to the Sun, the astronomical unit (about 1.5x 108 kilometres) is quickly dwarfed by the- distances to stars. We must define new units in terms of the stars themselves. To this purpose we have the parsec, equal to 3 x 1013 kilometres. Suffice it to say that the nearest stars are a few parsecs distant, while our Galaxy sprawls over 30000pc in dia¬meter, containing some 100 000 000000 stars.

Can we keep on going further ? Such distances each time demand far bigger and far brighter objects to mark them out. Must we now say that all spiral objects have the same diameter and boldly gauge their distances on the uniformity of nature ? Analogy has led us astray before. But even the demands of these distances are not yet too great, because in these distant forests there is still a recognizable flicker of light.

Seen in a handful of clusters in our Galaxy, and spread in greater numbers throughout the Milky Way, is an important class of stars, the Cepheid variables. The signal of their presence is their regular variation in brightness, and they have been found in the distant galaxies. These stars are so bright and their intrinsic properties are so well behaved, compared to other stars, that they dominate the survey of distances to local galaxies. Cepheids change their brightness and colour with precise repetition. We can in fact learn of a Oepheid’s true luminosity by merely measur¬ing its period of variation! Distances follow from a comparison of this true luminosity with the apparent luminosity.

Many secondary stellar distance indicators can be invoked to check the Cepheid distance scale in local galaxies but each one carries its own list of uncertainties which far exceed the uncertain ties involved in using cepheids alone . All other regular variables such as the RR Lyrae stars are more uncertain in their intrinsic properties and, being much fainter than Cepheids, they can be seen only over shorter distances. Exploding stars are so rare and unpredictable that their properties have not been well determined: novae are too faint even at the peak of their explosion to be used much further than Cepheids, while supernovae, which sometimes rival entire galaxies in their brightness, are very rare. Finally, bright stable supergiants have a wide spread of intrinsic magni¬tudes and colours. The best that can be done with these secondary-distance indicators is to show that no tremendously great errors have been made so far.

Our LOCAL GROUP OF GALAXIES contains the Milky Way and its two irregular companions, the Magellanic Clouds, two other spirals in the constellations of Andromeda and Triangulum designated as M31 and M33 respectively, and finally a few dozen fainter systems of spherical, elliptical or irregular shape. Beyond the Local Group only one shaggy spiral galaxy NGC2403 has been studied for Cepheids. This galaxy is very similar to our nearer neighbour M33. While NGC2403 is our first step out of the Local Group, it is also the last step beyond which Cepheids, even at their maximum brightness, fall below the limits of detection of our most powerful telescopes. No sooner have we reached this realm of the nebulae than we find ourselves again searching for new indicators of cosmic distance. Rapidly, too, our units of measure are becoming burdensome; the parsec has already been replaced by the kiloparsec a thousand times larger but to cope with the galaxies we must use the megaparsec, a unit step one million parsecs long.

Our search for new distance indicators, the standard candles and standard metres of the Universe, must be conducted before other indicators have left off. We have the Local Group of galaxies, but it is NGC2403 that holds an important key to further steps. Not far away in the sky from NGC2403 is a grouping of several more galaxies, including the spiral M81. If these now form a physical clustering in space with NGC2403, the way is open for us to include this larger sample in our study. But first, how far is NGC2403?

The cumulative efforts of many years of photography of NGC 2403, using the world’s largest telescopes, revealed the stellar population of this galaxy. Viewing individual stars we can estimate its distance. As in the Local Group of galaxies, the best hope for an accurate distance lies with the Cepheids. Dozens of photographs have been taken to follow the Cepheids through their cycles, but the only result was barely to detect their presence as they reached their peak brightness every few weeks. Some seventeen Cepheids have been discovered in NGC 2403 but their properties leave the answer of the distance ambiguous. These Cepheids are not ob¬served to be the same as local Cepheids; many of them are much too red. Does our assumption of the uniformity of nature fail with the Cepheids ?

If we abandon Cepheids as standard candles to our nearby galaxies, then for some time to come we will have, to relegate the extra-galactic distance1 scale to a realm of highly uncertain guesses, plagued by the vagaries of small samples. One hope still exists. The colours of the Cepheids in NG02403 may have been measured slightly wrong or interstellar reddening may be changing their light before it reaches us.

Meanwhile the possible effects of interstellar reddening are straightforward to evaluate. As for all stars, reddening makes us over-estimate the distance because we incorrectly associate its dimming effect with distance. Other luminous stars might also be affected by this dust, so they cannot be easily used to compare the results of the Cepheids. But a geometrical test is always welcome. Here, the discovery of objects with constant size would be of great benefit.

Great luminous regions of gas surround certain hot stars which have recently formed from the interstellar medium and which are responsible for energizing the surrounding gas. In galaxies where star formation is proceeding at about the same pace, the diameters of the largest excited gas clouds appear to be of about the same size. These regions are a few per cent of the diameters of galaxies themselves and they are both recognizable and measurable over cosmic distances. If we now assume that the largest gas clouds in each of the galaxies of the NGC2403-M81 group are identical in diameter to the clouds in other galaxies of similar type in the Local Group, then we can calculate a distance. The distance we find in this way is identical to the distance found to the Cepheids after correcting for reddening. This agreement increases our confidence, although the distances to the nearest groups of galaxies are still systematically uncertain by as much as 30 per cent.

The larger Virgo cluster of galaxies which can be seen stretching over ten degrees of our night-time sky is the next major step out. Here the last recognizable objects left, besides the gas clouds and the galaxies themselves, are the globular star clusters. In fact for galaxies where stars are no longer forming or where no gas exists, globular dusters are the last familiar objects for survey purposes. Smaller than the galaxies around which they orbit but larger than the loose galactic star clusters, the globular clusters can be seen by the combined light of their millions of members. But only if these systems are the same in age, constitution, and number as globular clusters around our Galaxy can we correctly judge the distances by them.

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