Like people, galaxies are not always as young as they seem, according to new reserach by a group of seven Italian astronomers.
The group, which is made up entirely of women, has demonstrated that a galaxy which appeared very young to experts until now is actually as old as our own Milky Way.
The galaxy, whose name is IZw18m, was thought to be just a few million years old. But by spotting star types known as Red Giants and Cepheid variables, the Italians showed it was actually born two billion years ago.
"This discovery shows that galaxies which haven't apparently evolved have actually been active for billions of years," said Monica Tosi, an astronomer at the National Astrophysics Institute (INAF) in Bologna.
"Of all the galaxies we know about so far, there is not one that began forming stars just a few million years ago".
The research project, just published in the prestigious Astrophysical Journal Letters, was led by Alessandra Aloisi, an Italian researcher working at the Hubble telescope's control centre in the US city of Baltimore.
Along with six other female astronomers at the INAF centres in Naples and Bologna, she carefully studied data gathered by the Hubble space telescope between October 2006 and January 2006.
The seven stargazers identified the tell-tale presence of Red Giants and Cepheids in IZw18m, something which had not been noticed before.
Red Giants are huge stars emitting red light while Cepheids are yellow and their light pulsates over periods ranging from three to 50 days. Both demonstrate that a galaxy is not young.
Cepheids are considered excellent reference points for astronomers because the time period of their light pulses can be used to measure their distances.
So the group was also able to show that the young-looking galaxy, as well as being much older than thought, was also much further away than previously believed.
Thanks to the stars, the Italian astronomers were able to calculate that IZw18m was 55 million light years away, not 25 million as previously thought.
"Even though this galaxy is not so young, it's still a unique object in the universe," Aloisi said.
In fact, IZw18m appears to have an unusually small amount of what astronomers call 'heavy chemical matter' in it. This matter tends to interfere with astronomical studies of distant galaxies.
"Studying Cepheids in an environment with so little heavy matter will allow us to gain more information about the physics of this type of star and thus to estimate distances with increasing accuracy," said Tosi.
