Galaxies and Intergalactic space
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With a powerful telescope, we can discern hundreds and thousands of galaxies. In recent years, studies have shown that galaxies are surrounded by intergalactic gas and dust.
It was around 200 years ago that astronomers first began to detect and catalogue galaxies, but without at the time knowing what they were. Only within the last 60 years have astronomers known that we and our Sun were part of one galaxy among millions, and only since then have they discovered that galaxies come in a vast variety of sizes and shapes. Galaxies, large and small, are found in every direction in which we can observe, some gathered together in groups and clusters, others seemingly existing on their own.
To begin with, astronomers had only one band of radiation to bring them such information as they could obtain about galaxies. This was light. Only by photographing these distant objects - for even the nearest are further than 50,000 parsecs (160,000 light-years) away - was it possible to study them. But now there are a host of other wave-lengths in which they can be examined, and of these X-rays and radio waves are the most important. Indeed, radio observations of galaxies have proved of immense significance and there are even 'radio galaxies', the study of which has given us a clue as to the explosive, energetic nature of these objects.
When photographs were first taken of galaxies, most seemed to be separate objects, just as the first photographs of stars made it look as though most stars were separate, whereas we now know that many are really binary or multiple systems. We now recognize that some - probably the majority - of galaxies are connected together in some way our Galaxy extends much further into space than was once believed.
Whether our Galaxy incorporates smaller nearby galaxies like the Magellanic Clouds or not, it is certain that they interact with our Galaxy. Evidence of this interaction is to be seen in what is now called the Magellanic Stream - a stream of neutral hydrogen gas which passes near the south pole of our Galaxy and envelops the Magellanic Clouds. Being neutral (i.e. not ionized), hydrogen does not show up visually, though it can be readily detected by radio-telescopes.
We are not sure how the Magellanic Stream came about, although astronomers at the present time think there are two possibilities. One is that the hydrogen was pulled out of the Clouds by their close approach to our Galaxy some time in the not too distant past.
The other is that the gas stream, which is moving very fast - around 200 km per second in the neighbourhood of the Sun - was never part of the Clouds, or of any other galaxy for that matter. It is suggested instead that the Stream is part of an intergalactic gas cloud which envelops not only our own Galaxy and the Magellanic Clouds, but also the other galaxies which form what astronomers call the Local Group.
Whatever the true explanation of the Magellanic Stream may be, its existence underlines the important fact that gas not only surrounds galaxies but often forms 'lanes' between one galaxy and another. Moreover, new telescopes with superior optics, using more sensitive photographic plates, as well as electronographic cameras and even photon counters, have detected such gas connections in other galaxies. Perhaps some of this is not unexpected. Galaxies are collections - islands, if you like - of dust, gas and stars, and are still the scenes of star formation. What is more surprising, though, are the lanes of intergalactic matter. These show clearly that interactions are taking place between galaxies, especially those in groups or clusters, even though such reactions stretch over tens of thousands and even millions of parsecs.
Detecting and studying such intergalactic material is important because at the present time astronomers are trying to work out how much material - stars, dust and gas - the universe contains. The answer to this crucial question will tell them whether the universe will continue to expand for ever or not. Each new observation of material has, therefore, considerable significance.
Galaxies are always on the move. Observations with the spectroscope show that they all have redshifts and an analysis of this movement is evidence that every galaxy is moving away from every other galaxy. It means, of course, that galaxies were closer together in the past than they are now, and we may wonder whether they ever collided when they were closer together. Is there any chance of galaxies, such as those in a group or cluster, colliding? The answer is that collisions were and are possible, and are not as unlikely as one might imagine.
