Astronomers are already having a hard time understanding a very young, very small star cluster in a nearby dwarf galaxy. But some newfound characteristics are now making the cluster even more strange. It contains more members of the most luminous stellar class than any other young cluster known. And unlike other star clusters that eventually grow up and learn to shed the clouds of gas and dust from which they were born, this cluster refuses to give them up.
Jean Turner, an astronomer at the University of California at Los Angeles (UCLA), has been trying to understand this cluster for years. She and her colleagues recently confirmed that this radio source in the dwarf galaxy NGC 5253 is in fact a young globular cluster in the making, and includes roughly a million stars less than a million years old. All of these stars are packed into a space just three light-years wide - smaller than the space between the sun and its nearest stellar neighbor - making the cluster incredibly dense.
The team's latest infrared study, reported in Nature earlier this month, reveals that the cluster holds more than four thousand O-type stars, which are hot, massive, and very luminous stars that only live for a few million years.
"This is the first time such a large cluster of O stars, bound with its natal gas, has been observed anywhere in the universe," Turner states. "These O stars should not fit in this small region, yet somehow they do."
The cluster, which is invisible at optical wavelengths, is embedded in a thick cloud of gas and dust. The massive, hot stars in the cluster have tremendous radiation and strong stellar winds that should be blowing away the surrounding star-forming clouds but aren't. Turner's group suspects that the dense cluster's gravity is fighting the dissipation.
"The dense gases are bound by the enormous gravity of the cluster, which makes this cluster different from any other known young cluster," Turner says. "It's truly a unique object. Clusters in our galaxy are not nearly as massive and cannot trap their gas."
Astronomers are also puzzled over how and why the unusual cluster is forming in NGC 5253, while our galaxy hasn't given birth to any new globular clusters in billions of years.
"It's a mystery why this tiny galaxy can form globular clusters at the present time and the Milky Way can't. We hope to be able to solve this mystery," Turner shares. "How a million stars can form in such a small region is also a mystery," she adds.
She and her colleagues imagine that other young clusters throughout the universe may be holding on to their native clouds and are hiding from view as well. They plan to continue studying the odd cluster in NGC 5253 and will look for others like it elsewhere.