THE HIERARCHAL CLUSTERING MODEL

This graphic shows one possible process by which present-day galaxies might form

1 THE BIG BANG

The universe starts to expand, with matter initially spread fairly uniformly through it.

2 CLUMPS OF DARK MATTER

Fluctuations in the distribution of dark matter cause clumps to form, held together by gravity.

3 ACCRETION OF GAS

Through the action of gravity, the dark matter pulls in surrounding gas, which then starts to form stars.

4 SMALL GALAXIES FORM

The gas falls into a flat disc where further stars form, resulting in the first, relatively small galaxies.

5 GALAXY MERGERS

Over time, neighbouring galaxies bump into each other and merge, creating ever larger galaxies.

6 GIANT ELLIPTICAL GALAXY

The most likely end product of multiple mergers is a giant elliptical galaxy.