Clayton, of Balclutha Primary School, asks :-

How far does light travel?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

Light keeps going till it bumps into something. When it hits something then two things happen: some of the light is absorbed, some is reflected. When sunlight hits the ground some of the light is absorbed and warms the ground. Some is reflected so we see the ground (fields, hills, whatever.)

When you shine a light into the dark night sky you can see the beam for a certain distance. You see it because some of the light is reflected back from dust, smoke or fog in the air. We call this kind of reflection 'scattering' because the light is reflected in all directions, not in one direction like it would be from a mirror. The rest of the light continues on into space.

As light travels it spreads out, so it becomes fainter. At double the distance light is one-quarter as bright. Three times further way it is one-ninth as bright.

In space light from stars can travel great distances. The Milky Way is the combined light of millions of stars at great distances. With our eyes (in really dark skies!) we can easily see two nearby galaxies -- the Large and Small Clouds of Magellan -- more than 150 000 light years away. (A light year is nearly 10 million million km.) The Andromeda Galaxy can also be seen by eye, but not easily from NZ. It is more than two million light years away. But light in space can also be stopped by clouds of gas and dust. One sees gaps or holes in the Milky Way where giant clouds of dust and gas absorb the light of distant stars.

For really far away galaxies light gets stretched. This is because the universe is expanding; space is getting bigger. As the light travels over millions or billions of years the expansion of the universe stretches the light waves. This makes the light redder. We can measure this 'red shift' and use it to estimate the distances of remote galaxies. So with big telescopes we see light that has travelled over a thousand billion billion km.