BBC NEWS | Science/Nature | Astronomers count the stars

From the darkest parts of Earth, the naked human eye can see about 5,000 stars; from a brightly lit city street, only about 100.

The Australian astronomers used some of the world's most powerful instruments to measure the brightness of all the galaxies in one sector of the cosmos - and then calculated how many stars they must have contained.

From that measurement, they proceeded to work out a figure for the whole of the visible Universe, which they believe is much more accurate than previous estimates.

That figure - presented to the International Astronomical Union conference in Sydney - is the kind that really can be called astronomical: 70 sextillion, or seven followed by 22 zeroes.

That is more than the total number of grains of sand in all the Earth's beaches and deserts. But that is only the stars in the visible Universe within range of our telescopes.

Dr Simon Driver, of the Australian National University, says the actual total could be much, much bigger still. He believes that many of the stars out there have planets, and some of those probably have life.

But they are so far away from Earth, he says, that we may never be able to contact anyone living on them.