The Flame Nebula is 15 light-years across and 1,400 light-years away in the constellation Orion. This composite image combines X-ray (purple) and infrared (red, green, blue) data. ( NASA/CXC/PSU/K.Getman, E.Feigelson, M.Kuhn & the MYStIX team/JPL-Caltech)
The Flame Nebula is 15 light-years across and 1,400 light-years away in the constellation Orion. This composite image combines X-ray (purple) and infrared (red, green, blue) data. ( NASA/CXC/PSU/K.Getman, E.Feigelson, M.Kuhn & the MYStIX team/JPL-Caltech)

The Oldest Stellar Sisters Live in Starry Suburbs

Finding stars is like finding cockroaches — where there’s one star, there are usually more.

Roughly half the stars in the Milky Way come in pairs, or binary systems, where two stars orbit one another in an endless cosmic dance (well, sometimes the dance ends in cataclysmic, fiery star death). Other stars live in clusters, like the one shown above in the Flame Nebula, 1,400 light-years away. These clusters are kind of like enormous family groups. Here, each stellar sister is born from the same clouds of collapsing gas and dust as her siblings.

Scientists used to think the oldest, first-born stars lived in the cluster’s center, where stellar ingredients are more dense and plentiful and it’s easier to make stars. But two new studies, posted to the arXiv [1,2], suggest this isn’t necessarily true. When astronomers studied sun-like stars in the Flame Nebula, and in another star cluster in the Orion Nebula, they found the oldest stars on the families’ fringes — in regions where it should take longer to light a star.

“Our findings are counterintuitive,” said Konstantin Getman of Penn State University, in a statement. “It means we need to think harder and come up with more ideas of how stars like our sun are formed.”

As with humans, determining stellar ages isn’t as simple as asking a star how old it is. Astronomers deduced stellar ages by measuring how bright a star is in X-rays and infrared light. Teams used NASA’s Chandra X-Ray Observatory to measure X-ray brightness, and then used that measurement to deduce a mass. Then, teams pointed several infrared telescopes at the same stars and took the second measurement. Comparing those brightness measurements with various stellar formation models yielded the stars’ ages.

In the Flame Nebula, the stars in the center were relative infants — just 200,000 years old. Their older sisters in the outskirts were already pushing 1.5 million years old. In the Orion Nebula, ages varied between 1.2 and 2 million years.

Astronomers aren’t yet sure how to explain their observations. One idea suggests that stars are born in the center and migrate outward, as though they’re growing up and leaving home. Another proposes that young stars are still being born in the cluster’s center, while star formation in the suburbs has ceased. And the third hypothesis suggests that young stars emerging from filaments of gas and dust fall inward, ending up in the middle.

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The Flame Nebula in the optical, where dust and gas hide young stars. The Horsehead Nebula is in the lower right. (DSS)