“The Earth spinning around on its axis once gives us the length of a day, and a complete orbit of the Earth around the Sun gives us a year,” said Professor Gerhardt Meurer, from the University of Western Australia node of the International Centre for Radio Astronomy Research.

“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.

He said that by using simple maths, you can show 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.”

Professor Meurer and his colleagues from the University of Western Australia, the Spitzer Science Center at Caltech and China’s National Astronomical Observatories also found evidence of older stars existing out to the edge of galaxies.

“Based on existing models, we expected to find a thin population of young stars at the very edge of the galactic disks we studied,” Professor Meurer 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 added.

“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.”

Source: Sci News