Published online 25 August 2006 | Nature | doi:10.1038/news060821-13


Key stars have different birthdays

Astronomers tear up textbooks over stellar nurseries.

A range of colours in globular clusters hints that something odd is going on.A range of colours in globular clusters hints that something odd is going on.NASA

In a complex Universe, astronomers thought they had at least one simple system to tell them how stars are born. Turns out they were wrong.

Results from Hubble confirm what some had feared for years: stars in 'globular clusters' are born in several bursts, rather than all at once. This means that globular clusters — small, dense groups of stars found orbiting galaxies — aren't as simple as astronomers used to think.

"It's changing our ideas completely," says Giampaolo Piotto of the University of Padua in Italy. "We have to change our textbooks."

The nature of these stellar nurseries means that previous models of these apparently simple systems are now wrong. And that means trouble for modelling more complex creatures, such as galaxies. "If you have problems reproducing star formation in globular clusters, you will have problems with a galaxy," says Piotto

Colour conundrum

“We have to change our textbooks.”

Giampaolo Piotto
University of Padua, Italy.

Astronomers had long assumed that globular clusters were the simplest stellar systems in existence. They thought that the hundreds of thousands of stars in each cluster were born in one go, condensing from a dust cloud early in the Universe's history, billions of years in the past. These dense balls of stars are now found in orbit around galaxies such as the Milky Way.

But some evidence counted against this simple picture.

Astronomers noticed as much as 30 years ago that helium-burning stars within a globular cluster come in a range of colours. Colour is usually linked to a property such as age or the chemical make-up of a star. But astronomers still thought that globular clusters ought to be uniform in age and composition, so they assumed something else — some unknown parameter — had to be responsible.

This became known as the 'second parameter problem', but it proved impossible to resolve.

Then, in 2004, researchers reported that hydrogen-burning stars in a globular cluster known as Omega Centauri (NGC 5139) seemed to fall into two distinct classes1. One set of stars was somewhat bluer than the other.

This was stronger evidence that something funny was going on. But it wasn't entirely convincing, because Omega Centauri is odd in other ways, too.

Alison Sills of McMaster University in Hamilton, Canada, says her reaction was to "file it under 'Omega Cen equals weird' and be done".

Normal surprise

"Now we can't do that," she says. On 23 August, in Prague, Piotto presented data to the General Assembly of the International Astronomical Union, showing that a normal globular cluster also has two sets of stars.

An analysis of Hubble Space Telescope images taken on 9 August of object NGC 2808, a globular cluster considered utterly normal, shows that its hydrogen-burning stars fall into two groups. "You see two sequences, there's no doubt about that," says Piotto.

"It is a beautiful achievement," says Francesca D'Antona of the Astronomical Observatory of Rome in Monteporzio, Italy.

Double burst

So how are the stars born? D'Antona suggests that the first burst of stars evolve in just a few million years into extended, billowing states that shed matter rich in heavy elements, such as helium, before collapsing into black holes and neutron stars. The debris could then condense into a second burst of stars.


At first, Sills was sceptical. Surely this would result in an odd-looking globular cluster, she thought. So she ran a computer simulation of this scenario, expecting that the stars' distribution would turn out differently from what we observe. She was wrong: the simulation showed that a cluster made in two bursts would look just as today's do.

This "tells us how we could have been fooled", says Sills.

It is unclear what the results will mean for our understanding of the Universe. The age of stars in globular clusters has been used to help constrain the age of the Universe, for example. Luckily, however, the two bursts of stellar formation seem to happen quite close together, within the first few million years of their many-billion-year life span. So the finding shouldn't upset long-term calculations of age too much.

But, astronomers add, they haven't yet had time to work out all the implications.

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University of Padua, Italy.