Measuring distance and the size of our galaxy

Measuring distances in the universe: getting to grips with the scales

Distances in the universe are so big that we use a rather unusual unit, the light year, to measure them.

One light year is the distance travelled by light (in a vacuum) in one year.

Given that light travels at about 300 × 10⁶ metre / second you can calculate that:

one light year is 300 000 000 metre / second  ×  (365  ×  24  ×  60  ×  60 second), which is 9,460,800,000,000,000 metre.

The Milky Way galaxy (ours) is about 100 000 light years across, with the Sun about 33 000 light years out from the centre, so we are long way from the centre.

Pupils are invariably fascinated by the links that can be made from the speed of light to distances across space, and travel times through space. It is really helpful to have some relevant facts and figures at the ready.

For example, Supernova 1987a (a supernova is the explosion at the end of a massive star's lifetime) occurred in a nearby galaxy called the Large Magellanic Cloud. Light from this supernova was observed on Earth in 1987, but the distance to the Large Magellanic Cloud is about 190 000 light years. Thus, we normally say that Supernova 1987a occurred in 1987, but it really happened about 190 000 years earlier. Only in 1987 did the light of the explosion reach the Earth. If we want to know what the Large Magellanic Cloud looks like now, we will have to wait 190 000 years.

In comparison, the Sun is only about 8 light minutes away. That is, it takes 8 minutes for light to travel from the Sun to the Earth. So the light we see from the Sun represents what the Sun looked like 8 minutes ago, and we must wait another 8 minutes to see what it looks like now. It is an interesting fact that if the Sun went out we would not know about it until 8 minutes after the event. Reflected light from the Moon travels to the Earth in about 1 second.

The most distant things that astronomers can see are about 12 000 000 000 light years away. Thus, the light that we presently see from these objects began its journey to us about 12 thousand million years ago.

Since that is close to the estimated age of the universe, this light is a kind of fossil record of the universe not long after its birth! So the observation of very distant objects is in a very real sense equivalent to looking backwards in time. (Closer to home light travels about 30 centimetre in one thousand millionth of a second – a foot per nanosecond, so even on the scale of a room, you only know about the past.)