Travelling into deep space

Even in our own Galaxy, the stars are surprisingly distant compared with the planets and the Sun. Enormous though the Sun seems in comparison with the Earth, it is really not a large star at all. Once we have passed PIu to, leaving behind the planets of the Solar System, we find ourselves out in deep space. Our distance is now about six thousand million (6 x 109) kilometres; travelling at the speed of light we are over 5IJ2 hours out from Earth. Around us space is empty, yet strange though it may seem, we may not yet have left the Solar System.

From time to time comets are observed which have such elongated orbits round the Sun that they will not return again for hundreds or even thousands of years. Some astronomers think that there is a vast spherical cloud of cometary material enveloping the whole Solar System. This lies, they believe, between IIJ2 and 15 million million kilometres away - so far that travelling with the speed of light it would still take about 58 days to reach its inner boundary and IIJ2 years to get to its outer rim.

Those who take this view believe that sometimes a passing star causes a gravitational upheaval in the cloud, and so triggers off the despatch of a comet from the cloud towards the Sun; then, and only then, can we on Earth observe the comet. At all events, once we have travelled the 15 million million kilometres we have left the Solar System and are out in the depths of interstellar space. We now move in a seemingly empty space.

Looking back at the Sun we find it is not the bright hot globe we are used to seeing in the sky, but has dwindled until it has become just a very bright star. As we move still further into space the Sun loses more of its brilliance, yet we still meet with no other bodies. We seem to be utterly alone. Not until we have left the Solar System and the cloud of comets far behind; not until we have travelled another 26 million million kilometres do we come close to another star. So even if we left the Earth at the speed of light, after travelling for a whole year we should only find ourselves in the cloud of cometary material. After two years we should be in empty space beyond the last boundaries of the Solar System; after another year we should still be travelling alone. The beginning of our fourth year would find us still in isolation.

Not until we have been going for another four months should we arrive at a star, the triple star ex Centauri. We have now travelled what seems a great distance. If we laid out a scale model in a living room, where the diagonal from one far corner to the other is, say, five metres, the result is surprising. If, ex Centauri is in one far corner and the Sun in the other, then the outer boundary of the comet cloud would be 1.8 metres from the Sun; the inner boundary only 18 cm. Yet the distances being represented are so vast that Pluto would be less then % mm from the Sun, which would only be the size of a single atom. We must come to terms with such vast distances because so far we are only at the very edge of interstellar space.

However, experience shows that it does not take long to become accustomed to them. The a Centauri star system lies 1.3 parsecs or 4.3 light-years away (one parsec equals 3.26 light years) yet it has the nearest stars to us in deep space. Other stars are even further off and, in fact, something like four or so light-years separates one star from another, at least in the volume of space occupied by the Sun and the nearby stars. The a Centauri star system can only be seen by observers living in the Earth's southern hemisphere. It consists of one bright star of a type rather like our Sun, another which is a little redder and is about 31f2 times dimmer, and the third, sometimes called 'Proxima' Centauri - a red star more than 6,300 times dimmer still. The three stars orbit each other. Interestingly enough, the brightest of the three was the first star to have its distance measured; this was done by Thomas Henderson between 1832 and 1833 when he was observing at the Cape of Good Hope in South Africa. But Henderson was a very cautious man, and he did not make his results public at once.

He waited until they could be confirmed by a colleague, but by the time this had happened Friedrich Bessel had already announced his measurement of the distance of the star 61 Cygni, which lies 3.4 parsecs away. Multiple stars like a Centauri and binary systems, where only two stars are in orbit around each other, are quite common. Of the 40 nearest stars to the Sun, two are multiples, and nine are binaries; these include 61 Cygni and the brightest star in the sky - Sirius - which is 23 times brighter than our Sun, but whose companion is 184,000 times dimmer. Stars vary in size, and if we travelled out in the direction of Scorpius for 520 years, we should come to Antares or a Scorpii, a very bright but very red star.

This is a red supergiant star, and though each square centimetre of its surface is much dimmer than an equivalent area of the surface of the Sun, its total brightness is over 8,000 times greater. This is because of its very large size (it has much more surface to shine), for the diameter of Antares is 560 times bigger than the Sun (i.e. 763 million kilometres).

If Antares lay at the centre of our Solar System it would swallow up the Earth and Mars, though it would not reach out as far as the orbit of Jupiter. Antares is one of a number of colossal red supergiant stars. Betelgeuse (a Orionis) is another - you can see it in the 'left' shoulder of Orion. It is a variable star and continually changes in size from between 600 and 900 times the Sun's diameter. So we find that stars come in all kinds of sizes with some of them occupying an immense volume. Yet all of them are grouped together in one vast island in space. We see some of the boundaries of this island when we look up at the Milky Way, and for this reason it has been given the name The Galaxy (Greek galaxias, milky).

Our own Sun lies some way from the middle; probably at a distance of about 10,000 parsecs (33,000 light-years) from the true centre. Certainly, the Galaxy is vast; its thickness towards the centre, where it bulges, is at least 3,000 parsecs (10,000 light-years) and its diameter some 30,000 parsecs (100,000 light-years). We are now getting to very large distances indeed, yet our Galaxy is not unique. It is only one of millions upon millions of other galaxies which lie further out in yet deeper space.