Space, as the wonderful Hitchhikers Guide to the Galaxy once said, "...is big. Really big."
And this causes us problems when we want to describe distances. The vastness of space quickly confounds the units we use in everyday life. Talk of billions of miles becomes unwieldy before we have even got out of our own solar system. We would run out of noughts, out of page-space, out of interest if we insisted on writing interstellar distances in miles. More importantly, we would simply lose a sense of what it means. We could write interstellar distances in index form, but that becomes unimaginable quickly too.
The units used by astronomers and astrophysicists are therefore designed to make discussion about distances in space easier, and to make the vastness of the universe easier to imagine.
An AU is an Astronomical Unit. It is the distance from the earth to the sun - roughly 93 million miles. AUs are used to describe distances on a fairly local level. For example the very edge of the solar system, the Outer Oort Cloud (a somewhat hypothetical region believed to be where comets originate), is around 100 000 AUs away. As you can see, we haven’t even got into the wider galaxy yet, let alone the universe, and we are already on six-figure numbers. AUs are good for describing the solar system and other planetary systems, but they are impractical for wider use because they quickly suffer the same problem of scale as miles do.
So there is a larger unit, known as a light-year, which most of us will use when talking about intergalactic distances. The light-year is not a measurement of time. It is a measurement of distance: one light-year is the distance light travels in one year. Remember that light-speed makes a pretty good benchmark for speed and distance, because whatever TV science-fiction has to say, the speed of light in a vacuum is the fastest thing ever at 186 000 miles a second. In a year that’s about 6 trillion miles. To give that some perspective, the nearest star, Proxima Centauri, is around 4 light-years away and our galaxy is around 100 000 light-years across. The nearby Andromeda Galaxy is around 2 million light-years away.
You may have heard discussion in newspapers or on TV about the furthest visible galaxies. They are around 13 billion light-years away. This means that their light has taken 13 billion years –almost the age of the universe itself – to reach us. When astronomers look at these distant galaxies, they are looking into the history of the universe itself.
However, there is a third unit, the parsec, which is widely used by scientists, but is less well-known among the general public. The reason for that is that it is more complicated to explain, and is not quite as relevant to most people’s understanding as the light-year. A parsec is 3.26 light-years in distance, so is again good for using at intergalactic levels.
Think of a star in space, a long way away. Think of a right-angled triangle between the earth, this star and the sun. Its base is the distance between the sun at one end and the earth at the other (1 AU). The line from the earth to the distant star is the hypotenuse of the triangle. Therefore the distance from the sun to the star is the third side, opposite the hypotenuse. Now, if you were looking from the earth at the distant star, and then looked at the same star from the sun, it would appear to have moved slightly in space – that’s because you have. If and only if, the distance the star appears to have moved is 1 arcsecond (1/360th of a degree), then the distance from the sun to the star is defined as one parsec.
Phew. It is a bit complicated and seems more contrived than the light-year but that’s because traditionally, the only way to measure the distance of stars was to measure the position of the star exactly six months apart, knowing that the distance the earth would have travelled in that time was 2 AU. Astronomers then used trigonometry to calculate the distance of the star.
Parsecs are commonly used by professionals in specialist publications. For most general purposes, light-years are enough.
Space is still very, very big, though. Even on our little level, when we look toward the sun we look at an object 1 AU away or eight light-minutes away. It is because of this enormous scale that the different measurements are required.