The spinning speed of spiral galaxies, such as the Milky Way, determines if they bulge or are flat discs of stars, Perth astronomers say.
Research led by the International Centre for Radio Astronomy Research (ICRAR) reveals that fast-rotating spiral galaxies are flat discs of stars, while similar-size galaxies that rotate slowly are fatter and even spherical.
ICRAR research associate professor Danail Obreschkow, from the University of WA, said much of the past century of research had been dedicated to understanding the diversity of galaxies in the universe.
"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," he said.
The study looked at 16 galaxies between 10 million and 50 million light years from Earth, using data from a survey called THINGS.
"The THINGS survey shows you the cold gas in the galaxies - not only where it is but how it moves," Dr Obreschkow said.
He said the shape of a spiral galaxy was determined by its spin and mass, and if it was left on its own for billions of years, both quantities would stay the same.
The Milky Way is a relatively flat disc with only a small bulge, which can be seen at night.
"However, when you look right at the centre near the Sagittarius constellation, you can actually see a thickening of the Milky Way, which is the bulge," Dr Obreschkow said.
The study, co-authored by Swinburne University professor Karl Glazebrook, is published on Friday in the Astrophysical Journal.
