Michael Lucy
There are about 100 billion stars in the Milky Way, the galaxy that houses our solar system. That’s a conservative estimate based on the mass of the galaxy. (Other estimates put the number at anywhere up to 400 billion.) And our galaxy is far from being the only one in the sky. For a long time, based on an analysis of the Hubble Deep Field – a very long exposure image of a tiny square of sky, mainly full of distant galaxies – astronomers believed there were perhaps 200 billion of them out there, each with its billions upon billions of stars.
In 2016, they realised that there must also be a lot of galaxies that were too dim and distant to turn up in the Hubble Deep Field, which means that the total number of galaxies was probably at least 10 times higher: that makes it 2 trillion. (If you’re keeping count, the number of stars in the universe would then be in the neighbourhood of two hundred billion trillion – that’s a two followed by 23 zeroes.)
The image above, taken by the Hubble Space Telescope, shows one tiny fraction of those galaxies: the Abell 370 galaxy cluster, some 4 billion lightyears from Earth. This is a group of galaxies loosely bound together by the pull of gravity.
You can see that many of galaxies appear stretched or smeared as a result of gravitational lensing, which occurs when the paths of light from behind the cluster are distorted by its enormous mass. Some even appear in more than one place due to the shape of the ripples in spacetime.
Abell 370 contains a mere several hundred galaxies, which means there are still plenty out there for Hubble to catch.
Related reading: Relics from the galaxy’s past Hubble’s ‘cosmic archaeological dig’