A short comment of mine on FB triggered such a response I thought I’d blog on the subject in a little more detail:
It takes the Earth 12 months to orbit the Sun once. The radius of the Earth’s orbit is 1 AU (roughly 150 million km). So at intervals six months apart, the Earth is at opposite ends of a diameter measuring 2 AU. This is such a long baseline that in photographs of the night sky taken six months apart, a very few of the stars seem to have shifted their position slightly, because of the parallax effect. To give you some idea how small the effect is, in the 1830s the first star studied in this way (known as 61 Cygni) was found to have a parallax shift of just 0.3136 seconds of arc. In comparison, the full Moon covers 30 seconds of arc on the sky. So the apparent shift in 61 Cygni as the Earth goes round the Sun is equivalent to about one-hundredth of the apparent diameter of the Moon.
The distances to the stars are so great that astronomers had to invent new units with which to describe them. If you were so far away from Earth that the distance between the Earth and the Sun (the radius of the Earth’s orbit, 1 AU) covered just one second of arc in the sky, then you would be 1 parsec away from Earth (parsec is a contraction of parallax second of arc). A parsec is just over 30 million million kilometres, a number so big that it is hard to comprehend. You can also look at it in terms of the speed of light. Light travels at just under 300,000 km per second, and covers 9.46 million million km in a year, a distance known as a light year. So a parsec is 3.26 light years. Converting the parallax measurement into distance, we find that 61 Cygni is 3.4 parsecs away, or just over 11 light years from us. And this makes it one of the closest stars to our Sun.
The sky on a dark and cloud-free night seems to contain countless numbers of stars and poets have waxed lyrical about the view. But the human eye is not very sensitive to faint light and even under perfect conditions, with no Moon or cloud, and far from city lights, the most you can see at any one time is about 3000. Under more ordinary viewing conditions, you are lucky to see 1000.
We now have a clear idea of the sizes of stars and the distances between them. The Sun has a diameter of 1.39 million km, about 109 times the diameter of the Earth, and this is typical for a star during the main period of its life. But the typical distance from one star to even its nearest neighbours is tens of millions of times its own diameter (except, of course, for systems where two or more stars orbit around one another). There is a useful analogy for expressing these incomprehensibly large numbers in perspective. If the Sun were the size of an aspirin, the nearest star would be another aspirin 140 km away. The distances between stars are absolutely enormous, even compared with the sizes of the stars themselves.
The overall shape of our Galaxy is a flattened disc containing hundreds of billions of stars, all more or less the same as our Sun, spread over a diameter of about 28 thousand parsecs (28 kiloparsecs). The disc is only 300 parsecs thick at its outer regions (roughly 1 per cent as thick as it is wide), but it has a bulge in the middle measuring 7 kiloparsecs across and 1 kiloparsec thick. If we could view our Galaxy from the outside it would look rather like a huge fried egg.
Surrounding the whole disc is a halo of about 150 bright star systems called globular clusters, each one a ball of stars containing hundreds of thousands, or even millions, of individual stars, so close to one another that there may be 1000 stars in a single cubic parsec of space. From the way stars move, astronomers also infer that there is a great deal of dark matter surrounding the whole Galaxy and holding it in a gravitational grip.
The Sun is travelling at a speed of about 250 km per second in its own orbit around the centre of the disc, carrying our Solar System with it; but the Galaxy is so large that even at this speed it takes our Solar System about 225 million years to orbit just once, a journey it has completed about 20 times since it was born some 4.5 billion years ago.
The Sun and its family of planets orbits the Galaxy at a distance of about 9 kiloparsecs from the centre, two-thirds of the way out to the edge of the disc, on the inside edge of a feature known as the Orion Arm. We are not in the centre; there is nothing particularly special about our place in the Milky Way Galaxy.
As well as disc (spiral) galaxies like the Milky Way, there are also much larger, elliptical galaxies, which do not have a disc or spiral shape, but are ellipsoidal (like a rugby ball). These are thought to have been built up by a kind of cosmic cannibalism, from mergers between disc galaxies. There are also smaller, elliptical galaxies (resembling the globular clusters) and small, irregular galaxies which have no distinct shape. The largest ellipticals contain several thousand billion (few x 1000,000,000,000) stars. Disc galaxies, such as the Milky Way, have diameters of a few tens of kiloparsecs and contain a few hundred billion stars. But galaxies are much closer together, in terms of their own size, than the stars are to one another. Again, it’s a matter of perspective. If we adapt the aspirin analogy to galaxies, and represent the Milky Way by a single aspirin, we find that the nearest large disc galaxy to us, the Andromeda Galaxy, is represented by another aspirin just 13 centimetres away. And only 3 metres away we would find a huge collection of about 2000 aspirins, spread over the volume of a basketball, representing a group of galaxies known as the Virgo Cluster. On a scale where a single aspirin represents the Milky Way Galaxy, the entire observable Universe would be only a kilometre across, and would contain hundreds of billions of aspirin. In terms of galaxies, the Universe is a crowded place.