Galaxies like our own Milky Way formed easily and have also been the largest spiral galaxies in the universe for almost 4 billion years.

This is shown in a new study by associate professor Kambiz Fathi of the Department of Astronomy at Stockholm University. The study is now published in the scientific journal The Astrophysical Journal Letters.

Astronomers believe that galaxies are formed by a complex interplay between the processes that affect not only the stars and gas but also to supermassive black holes and possibly also the mysterious dark matter that surrounds every galaxy and whose nature remains unexplained.

"Galaxy formation history may be telling us something about the places in the universe where life can form," says Kambiz Fathi. Large galaxies like the Milky Way swim in a sea of many smaller galaxies, and in this process assimilate the small galaxies, sort of in the same fashion as "mergers and acquisitions" in the financial world.

"The new results show that spiral galaxies, such as the Milky Way, appears to have worked in the same way, easily formed, and have been the largest spiral galaxies in the universe for at least the last 3.4 billion years," says Kambiz Fathi.

Since the 1970s, astronomers have noticed that the number of stars in these unremarkable, middle regions of spiral galaxies is never much larger than in the Milky Way. This upper limit is known as Freeman's Law, named after the Australian astronomer Ken Freeman, who first described it. Astronomers have previously checked Freeman's Law for a few tens of galaxies. Kambiz Fathi has made a far more comprehensive investigation and found that this applies more widely than previously thought.

To come to this conclusion, Kambiz Fathi measured the images of 30,000 galaxies using the resources from the European Virtual Observatory, which gives astronomers the opportunity to use the power of the internet and large databases to reuse and combine observations from many different telescopes in innovative ways.

Since the speed of light is finite, we see distant galaxies as they were when the universe is younger than it is now. This effect allows astronomers to investigate how the universe and its galaxies have changed just by looking at objects at very large distances.

For each of the 30,000 galaxies, Kambiz Fathi estimated the number of stars in the parts of the galaxies where spiral arms are prominent. Our own sun occupies just such a place in the Milky Way.