Astronomers have discovered that all galaxies, regardless of their size, rotate once every billion years.

The discovery is surprising, but doesn't quite rewrite the rules of the physical universe as we know them.

On Earth, we measure time by the rotation of our planet around its axis. A full rotation gives us a day, and a complete orbit around the Sun gives us one year.

The Milky Way, our galaxy, has a diameter of between 100,000 and 180,000 light-years, depending on where it is measured from.

Scientists have discovered that on the extreme edge of the Milky Way - and indeed any other galaxy - the rotation period lasts for a billion Earth years.


Galactic rotation like clockwork. Images: ICRAR

That said, Professor Gerhardt Meurer from the International Centre for Radio Astronomy Research (ICRAR) explained: "It's not Swiss watch precision."

"But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round."

Prof Meurer explained how all galaxies of the same size have the same average interior density.

"Discovering such regularity in galaxies really helps us to better understand the mechanics that make them tick - you won't find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly," he added.

Prof Meurer's team also discovered that older stars were living out their last years on the very edge of the galaxies they examined.

"Based on existing models, we expected to find a thin population of young stars at the very edge of the galactic disks we studied," he said.

"But instead of finding just gas and newly formed stars at the edges of their disks, we also found a significant population of older stars along with the thin smattering of young stars and interstellar gas."

"This is an important result because knowing where a galaxy ends means we astronomers can limit our observations and not waste time, effort and computer processing power on studying data from beyond that point," he said.

Image: Milky Way viewed from Canberra

"So because of this work, we now know that galaxies rotate once every billion years, with a sharp edge that's populated with a mixture of interstellar gas, with both old and young stars."

Prof Meurer said that this information would be vital to the next generation of radio telescopes which will generate enormous amounts of data when examining the sky.

For telescopes like the soon-to-be-built Square Kilometre Array (SKA), being able to detect where the edge of a galaxy lies will make searching through this data much easier.

"When the SKA comes online in the next decade, we'll need as much help as we can get to characterise the billions of galaxies these telescopes will soon make available to us," Prof Meurer said.