A dwarf galaxy has been observed in the process of forming stars at a phenomenal rate, particularly considering its size. The supernebula in the galaxy appears to be a throwback to a time when giant star clusters were made, offering us a rare insight into our own galaxy's past.
Stars form inside nebulae, and large nebulae can produce hundreds or thousands of stars at once. However, the latest discovery by Professor Jean Turner of UCLA is unlike anything we have seen close up, generating a cluster weighing a million times the mass of the sun.
“I’ve been searching for the gas cloud that is forming the supernebula and its star cluster for years,” Turner says. “Now we have detected it.”
Cloud D is a supernebula containing a cluster a billion times as bright as the sun. It lies in the galaxy NGC 5253. Only 10.9 million light-years away, exceptionally close by galactic standards, such luminosity might be expected to be highly visible. However, the nebula's dust hides the visible light the stars produce, and Turner had to use the submillimeter radio detectors to find it.
“We are seeing the dust that the stars have created,” Turner says. “Normally when we look at a star cluster, the stars long ago dispersed all their gas and dust, but in this cluster, we see the dust.”
Besides its size, what makes Cloud D exceptional is the efficiency with which it is making stars. More than half the mass of Cloud D has already become stars, or is on its way to doing so. Gas clouds in the Milky Way typically have a tenth of that efficiency, Turner notes in Nature.
However, it was not always thus. Globular clusters, the authors argue, are evidence that ten billion years ago the nebulae around the Milky Way were much more efficient at forming stars. In examining Cloud D, we are getting a chance to see how this process took place. Moreover, we are getting to view it almost from the start. The cluster forming from Cloud D is three million years into what is predicted to be a billion year lifespan.
Less efficient clouds end up losing most of their gas to the galactic disk or bulge and Turner says some astronomers doubted such efficiency remains possible in the modern universe. “We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy,” Turner and her co-authors write.
Although NGC 5253 is known for having had two supernovae in the last 120 years, unusual for a small galaxy, Turner says, “We found no evidence of a supernova yet” within Cloud D. This was surprising because the huge O type stars in which Cloud D specializes burn out and explode very quickly.
Eventually, the numerous supernovae will disperse Cloud D into interstellar space, including most of the heavier elements produced by the giant stars during their brief lives.