Measuring how far to nearby stars - the parallax technique More distant stars - using absolute brightness Very distant objects - the red shift technique Light years

On a clear dark night, the stars can seem to be very close indeed and they all seem to be simply hanging in the sky. Measuring the distances to the stars has been virtually impossible without modern telescopes and measurement techniques.

We now know that the stars are at incredible distances from us. Not only is there a considerable range of distances to all of the different stars and other objects within our galaxy, but there are also billions of other galaxies. In this topic we shall investigate how to measure the distances to the stars.

Measuring how far to nearby stars - the parallax technique
The distance to a nearby star can be measured using its apparent change in position against the far distant stars over six months. During a six month period, the Earth travels halfway around the Sun. By sighting a nearby star when we are on one side of the Sun and again sighting it against the background of stars six months later, an estimate of how far it is to the star can be made.

More distant stars - using absolute brightness
The farther a star is from us, the smaller the angle through which it will seem to move as the Earth orbits the Sun. Stars that are approximately 1% of the distance across our galaxy are too far for their distances to be measured using the parallax technique.

However, astronomers have devised other methods of distance measurement, which typically rely on analysing the light of stars - or clusters of stars - to find how bright they really are, then inferring their distance by observing how bright they appear to us. This could be likened to finding how bright a torch globe really is, and then shining it towards an observer. Armed with knowledge of the real brightness, the observer could find the distance to the torch by measuring its brightness as seen from his location (the farther away it is, the fainter it would appear to him).

Very distant objects - the red shift technique
When astronomers peer toward distant galaxies, they can see very few - or none at all - of its individual constituent stars. Another technique is used which relies on an important discovery made early in the 20th century. Astronomers found that, on a large scale, the galaxies are moving apart from one another due to the overall expansion of the Universe. In addition, there is a connection between the distance to a galaxy and the speed at which it is moving away from us: the more distant galaxies are receding more quickly.

The speed of recession from us can be found by observing the galaxy's light: by measuring how much the light is shifted toward the red end of the spectrum, it is possible to find its speed (a galaxy with blue-shifted light would be approaching us). The distances to galaxies can therefore be inferred using this technique.

Light years
Stars and other distant objects are so far away that we do not measure their distances in kilometres or even billions of kilometres, but in "light years". A light year is the distance that light can travel through space in one year. Light travels at 300 000 km/sec and over a year this is equivalent to 9.5 trillion (thousand billion) km!

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