Nearby Massive Star Cluster Yields Insights Into Early Universe

NASA's Hubble Space Telescope has taken a "family portrait" of young, ultra-bright stars nested in their embryonic cloud of glowing gases. The celestial maternity ward, called N81, is located 200,000 light- years away in the Small Magellanic Cloud (SMC), a small irregular satellite galaxy of our Milky Way. These are probably the youngest massive stars ever seen in the SMC.

The nebula offers a unique opportunity for a close-up glimpse at the "firestorm" accompanying the birth of extremely massive stars, each blazing with the brilliance of 300,000 of our suns. Such galactic fireworks were much more common billions of years ago in the early universe, when most star formation took place.

"This is giving us new insights into the physical mechanisms governing star formation in far away galaxies that existed long ago," says Mohammad Heydari-Malayeri, who headed the international team of astronomers who made the discovery using Hubble's Wide Field and Planetary Camera 2.

Because the stars of the SMC are deficient in heavier elements, they too evolve much like the universe's earliest stars, which were made almost exclusively of primordial elements hydrogen and helium that were cooked up in the big bang. In fact, the SMC is a unique laboratory for studying star formation in the early universe since it is the closest and best seen galaxy containing so-called “metal-poor” first and second generation type stars.

Hubble's exquisite resolution allows astronomers to pinpoint 50 separate stars tightly packed in the nebula's core within a 10 light-year diameter - slightly more than twice the distance between earth and the nearest star to our sun. The closest pair of stars is only 1/3 of a light-year apart.

These observations show that massive stars may form in groups. "As a result, it is more likely some of these stars are members of double and multiple star systems," says Heydari-Malayeri. "The multiple systems will affect stellar evolution considerably by ejecting a great deal of matter into space.”

This furious rate of mass loss from these stars is evident in the Hubble picture, which reveals dramatic shapes sculpted in the nebula's wall of glowing gases by violent stellar winds and shock waves. "This implies a very turbulent environment typical of young star formation regions." Heydari-Malayeri adds.

He believes one of the members of the cluster may be an extremely rare and short-lived class of super-hot star (50,000 degrees Kelvin) called a Wolf-Rayet. This star represents a violent, transitional phase in the final years of a massive star's existence - before it ultimately explodes as a supernova.

"If confirmed by future Hubble observations, this finding will have a far reaching impact on stellar evolutionary models," says Heydari-Malayeri. "That's because the Wolf-Rayet candidate is fainter than other such stars in that galaxy, in contrast with the predictions of these models.”

Before the Hubble observations, N81 was simply dubbed "The Blob" because its features were indistinguishable in ground-based telescopes.

The Hubble observations of N81 were conducted by the European astronomers Mohammad Heydari-Malayeri (Paris Observatory, France) and co-investigators Michael Rosa (Space Telescope-European Coordinating Facility, European Southern Observatory, Germany), Hans Zinnecker (Astrophysics Institute, Potsdam, Germany), Lise Deharveng (Marseille Observatory, France), and Vassilis Charmandaris (Paris Observatory).

Their work will be shortly submitted for publication in the European journal Astronomy and Astrophysics.