The stars that form our Galaxy, the Milky Way, are still remote, but not untouched by our travels. Parallax has no theoretical limit in its application ; the distances to all visible objects in the Universe could, in principle, be determined by this method. If our eyes could detect shifts in angles ninny times smaller than they can, it would be possible to experience the dimensions of our Galaxy immediately. To see how unrealistic this is is to receive another taste of the vastness of the Universe.

We can achieve the equivalent of stereo vision by taking successive photographs of a scene as we move relative to it. Our Earth is constantly moving with respect to the starry skies. We can photograph after moving a few centimetres, alter moving 12000 km. the distance the Earth will rotate us in half a day (diurnal parallax) or after 150 000 000km, halfway across the orbit around the Sun (annual parallax). By measuring angles a thousand times smaller than can be detected by the human eye, by ‘blinking’ after our ‘eyes’ have separated a million million (1012) times over, then and only then can we perceive the distance to the stars, but even then only the nearest ones.

On Earth, when the stereo effects of our vision fail us, we not uncommonly estimate a distance from the apparent size of distant objects such as large buildings or hills. For example, we can estimate the distance of a forest by comparing the apparent size of its individual trees to the known sizes of trees around us. Of course we must be able to distinguish at a distance small saplings from their older, taller parents. Indeed, we must take care not to compare nearby birches with distant oaks. In precisely the same way we may judge the distances to stars, individually or in groups. And like the sizes and types of trees we find that stars come in a variety of types with different intrinsic brightness and colours.

Like studying a forest, the study of a cluster of stars provides us with a group of objects all at a common distance. In a cluster we can discover the relative properties of many stars and later require that we know the true distance to only one of the stars in order to define the absolute properties of them all. It turns out that for the ordinary stars the intrinsic brightness of the star is determined by an easily-measurable quantity, the colour. So we can measure a colour, then calculate an intrinsic brightness and infer the distance from how bright a star appears. Here only distance is assumed to account for any difference between the intrinsic and apparent brightnesses.
Of great importance in the distance determination was the discovery of interstellar dust. Space is not devoid of matter between the stars. The environment in which a star is embedded and the dust in space through which its light must pass in reaching us can seriously affect the apparent luminosity of a star. This dimming, however, is quite independent of distance and special techniques had to be developed to detect it and remove its effects before distances were calculated. Fortunately interstellar absorption, as it is called, always makes stars appear redder in a very precise way. Furthermore, as the star is made fainter and redder by absorption, the details of its spectrum are still not affected, and thus it has been possible to separate the effects of reddening and distance independently.

Clusters of stars contain a wealth of information and their importance cannot be overstated. They have provided valuable keys in the understanding of stellar evolution, the structure of our Galaxy, and in the establishment of that amazing concept, the distance scale for objects beyond our Galaxy.

Within our Galaxy, supergiant stars of high luminosity can be used to peer out through the dust to great distances. Plotting the positions of these stars around us, we discover that we are living in a disc galaxy where stars are forming only in selected regions. When seen from afar, these regions form a two-armed spiral pat¬tern which radiates out from the nucleus of our Galaxy and passes through the solar neighbourhood after several turns. Our optical picture soon runs out because of the obscuring effects of the dust, but the gas in these regions where stars are forming can be detected by other means.

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