All Galaxies Rotate Every Billion Years Like Clockwork

For thousands of years, humans have measured time by the movement of the cosmos. A spin of the Earth is a day, an orbit of the moon is about a month and the Earth's journey around the sun is a year.

We might have to add a new cosmological measure to our calendars, because astronomers have discovered that all galaxies, no matter their size, rotate once every billion years.

3_14_Pinwheel Galaxy
This image captured by the Hubble Space Telescope shows the gigantic Pinwheel galaxy. ESA/NASA

"It's not Swiss watch precision," explained Gerhardt Meurer of the International Centre for Radio Astronomy Research (ICRAR) in a statement. But, big or small, he said, if you sat on the edge of a galaxy's disk as it span, it would take about a billion years to go all the way round. The universe's longest ferris wheel, perhaps.

Meurer and his team used simple mathematics to show that, on average, galaxies of the same size have the same interior density. Their research was published in the Monthly Notices of the Royal Notices of the Astronomical Society.

This regularity, he said, helped the team understand how galaxies tick. It means that a dense galaxy won't rotate faster than one with a smaller size but a lower density.

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"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 said.

The team had also been expected to find just gas and a sprinkling of young stars at the outer edge of galaxies. They were surprised to find a significant number of older stars among the youngsters.

This, Meurer said, will help researchers fine-tune future efforts. "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 explained.

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Powerful telescope systems like the upcoming Square Kilometre Array (SKA) will give scientists masses of data to comb through. Understanding where the boundary of a galaxy lies will help reduce the processing power needed to interpret this data.

Meurer added: "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."