Scientists have uncovered a previously unknown generation of stars hidden within globular clusters.
The discovery reported in this week's Astrophysical Journal changes our understanding about how these star-filled objects form.
Globular clusters are densely populated masses of stars, usually found orbiting the centre of galaxies.
They contain hundreds of thousands of stars and until now, those found in our galaxy were thought to contain stars that formed in a single 'burst' event around 10 billion years ago.
However Dr Kenji Bekki, an astrophysicist with the International Centre for Radio Astronomy Research (ICRAR) - a joint venture between Curtin University and The University of Western Australia - has found these clusters contain at least two separate populations of stars.
Multiple periods of star formation
Using detailed spectrographic readings, Bekki found the composition of the stars inside these globular clusters indicated two different stellar generations.
"An initial first generation, followed about 100 million years later by a second generation of stars which formed out of the gas from the first generation," he says.
Bekki says confirming the origin of the second generation stars was the key to understanding what was actually going on.
The study shows these multiple star populations occur in globular clusters, but not in open clusters.
"Open clusters don't have enough mass for multi-generational stellar populations," says Bekki.
He says the number of stellar generations in a globular cluster may not be limited to just two.
"The Omega Centauri globular cluster may have more."
As big as a dwarf galaxy
Using 3D computer simulations of cluster evolution, Bekki realised the globular clusters observed today may have evolved from far more massive 'super clusters' in the distant past.
Over billions of years much of the original stellar population has disappeared through tidal stripping by the host galaxy.
"The computer simulations show the globular clusters in the Milky Way, could have been up to 25 times more massive than the clusters we see today," says Bekki.
"These super clusters may not be classifiable as clusters at all, because they would once have been as massive as dwarf galaxies."