PERTH, Australia, Feb. 28 (UPI) -- Australian astronomers say they've discovered why some spiral galaxies are fat and bulging while others are flat disks, and it's all down to how fast they spin.
Research led by the International Center for Radio Astronomy Research in Perth found that fast-rotating spiral galaxies are flat and thin while equally sized galaxies that rotate slowly are fatter, a center release reported Friday.
Why galaxies of similar size can look so different from each other has been a much-debated mystery, the researchers said.
"Much of the last century of research has been dedicated to understanding this diversity of galaxies in the Universe and with this paper we've made a step towards understanding how this came about by showing that the rotation of spiral galaxies is a key driver for their shape," ICRAR researcher Danail Obreschkow, from The University of Western Australia, said.
The study looked at 16 galaxies -- all between 10 million and 50 million light years from Earth.
Shape is determined by both a galaxy's spin and its mass, and if you leave a galaxy on its own for billions of years both quantities will stay the same, Obreschkow said.
The way galaxies are formed looks a bit similar to a carousel made of an elastic disc, he said.
"If the carousel is at rest, the elastic disc is quite small," he said. "But when the whole thing is spinning the elastic disc becomes larger because it's feeling the effects of centrifugal force."