What shape are galaxies? It's a simple question, but one that has had scientists in a spin for years.

The University of Sydney's Dr Caroline Foster and her team have taken a significant step towards answering it, proving their shape is in fact linked to their spin. 

If we can see galaxies through our telescopes, why is it so hard to tell what shape they are? Try a simple experiment. Imagine you are watching a coin being flipped. As the coin tumbles over and over, you can easily tell its dimensions.

But if instead you were handed a single image of that coin mid-flip, working out its shape becomes almost impossible.

Scientists have the same problem with galaxies. We can only view them from a single angle, making it almost impossible to tell their true shape. 

This is the sort of question that interests Dr Foster, who has always been fascinated by questions she doesn't know the answers to. Especially questions so difficult no one seems that interested in solving them at all.

"I guess it's the inaccessibility that drew me to this study," Dr Foster said.

"If you do solar system science, people want to go to planets, so they send probes. But there is no way I'll ever reach those galaxies – so how can we try and trick nature to solve the problem, without ever going there? It was like this inaccessibility combined with my innate curiosity: 'What do you mean there is no way I can know that?' "

Dr Foster's team, drawn from several Australian and international universities, pioneered a new approach.

In a study published this week in the Monthly Notices of the Royal Astronomical Society, they combined still images of galaxies with data from the Coonabarabran-based multi-object integral field spectrograph, a device that can be used to measure the movement of stars within a galaxy.

"That wasn't really possible until right now, thanks to the spectrograph," Dr Foster said.

When you track the movement of the stars within it, the spin of a galaxy becomes obvious. In a spinning galaxy, one side moves towards you while the other moves away. The faster the movement, the faster the spin.

The spectrograph's data allowed Dr Foster's team to calculate the speed at which each galaxy being looked at spun – and that revealed an important insight: generally, the faster the spin, the flatter the galaxy.

A high-spin galaxy as it spreads its arms. Photo: Dan Taranu / University of Western Australia

It's a breakthrough, but it's also common sense. Imagine an ice skater twirling on a rink, Dr Foster says. With arms outstretched, she spins slowly. Bring them closer to her chest and her revolutions increase.

Galaxies are the same. By tracking the spin speed, Dr Foster found she was able to accurately predict the shape of the galaxy.

A slowly spinning galaxy. Photo: Dan Taranu / University of Western Australia

Galaxies generally start out as clumps of stars and gas, Dr Foster says. Gravity compresses the stars towards each other, increasing the spin, just like the ballerina.

Many bump into each other, merging their billions of stars and throwing each other out of rotation. These new merged galaxies tend to spin slowly, like a wobbly top.

Others, if left undisturbed, slowly increase their spin. As they do, the spinning edges of the galaxy stretch out into twinkling tendrils, like those that characterise our home, the Milky Way.