How many galaxies are there? Tough question, even for Hubble

How many galaxies are there in the night sky? A lot more than we realised - 20 times more in fact, leading astronomers to predict there are about 2 trillion of them up there.

Someone could be criticised for delivering such a wrong estimate, until you realise that we can only study about 10 per cent of the total number of galaxies that we can currently see.

The best telescopes we have just cannot see far enough yet, so 90 per cent of them are too far away to study until better telescopes come on stream.

Knowing how many galaxies there are in the universe is a fundamental question in astronomy, so with funding from the Royal Astronomical Society, an international team of astronomers assembled to update the latest estimate.

Prof Christopher Conselice, professor of astrophysics at the University of Nottingham, led the group and came up with the 2 trillion estimate after a 15-year effort.

Images of the universe captured by the Hubble Space Telescope were studied, allowing the astronomers to say they could see about 100 billion galaxies. They then started using other telescopes around the world to measure the density of galaxies in one small region of space after another.

Slow work

It was slow work, but it allowed them to find how many galaxies we have missed, with the astronomers likening it to an "intergalactic archaeological dig". Their results appear on Wednesday in the Astrophysical Journal.

It really is like counting the bones of old stars, achieved by estimating galaxy numbers at different epochs in the universe’s history.

When it was young, only a few billion years old, there were 10 times as many galaxies in a given volume of space.

But 13.7 billion years after the Big Bang the head count diminished as galaxies collided and coalesced and merged to form a smaller number of larger galaxies.

“It boggles the mind that over 90 per cent of the galaxies in the cosmos have yet to be studied,” said Prof Conselice.

“Who knows what interesting properties we will find when we study these galaxies with the next generation of telescopes?”