How far away are the stars?

Distances to the stars

Because the stars are so far away, our everyday units like metres and kilometres are not much use. In the solar system the usual measure of distance is the astronomical unit, or AU, which is (almost!) the average distance between the Earth and Sun. One astronomical unit is about hundred and fifty million kilometres.

The AU is again too small for describing distances between the Earth and stars so we then use light years (ly) and parsecs (pc) to measure these distances.

One light year is the distance light travels during the one year. Since it travels at 300,000 kilometres each second, this distance works out at 9,460,500,000,000 kms or 63240 Astronomical Units.

One parsec is the distance at which a far away observer would see the Earth's orbit cover an angle of one arcsec, and is equivalent to 3.16 light years.

So, how far away are the stars?

The closest star (oops, but you know what I mean!) is Proxima Centauri and it is 4.3 light years away from us. That means that light needs to travel for more than four years from Proxima Centauri to reach us. This star is very faint and is not visible to the naked eye, but here are some stars you might recognize by their names.

Imagine someone (or thing) living near these stars had sent you a message (maybe by flashing a bright light) on the day you were born.

Those stars are relatively close to us and distances to the other stars and galaxies for convenience are measured in thousands (kilo) or even millions (mega) of light years or parsecs.

I have already mentioned our own galaxy, the Milky Way, and here is a picture of how we think our galaxy would look from far away. Actually this is a picture of a nearby (only 2,500,00 light years away!) galaxy called M31, but our galaxy is thought to be very similar to it.

Our Sun is approximately 15000 parsecs away from the centre of the Milky Way, and the Milky Way itself consists of at least 10 billion stars, many of them very similar to the Sun.