NASA's Hubble Space Telescope has provided new insights into how stars might have formed many billions of years ago in the early universe. Hubble observations of a pair of star clusters suggest they might be linked through stellar evolution processes.
The pair of clusters are 166,000 light-years away in the Large Magellanic Cloud (LMC) in the southern constellation Doradus. The clusters are unusually close together for being distinct and separate objects, according to Hubble astronomers.
Previously, such detailed stellar population studies were restricted to nearby star-forming regions within the plane of our Milky Way Galaxy. However, Hubble's high-quality images extend these stellar studies one hundred times farther into the universe, out to the distance of a neighboring galaxy. Because the LMC lies outside of our Milky Way Galaxy, it is a natural laboratory for studying the birth and evolution of stars. The stars in the LMC have few heavy elements and, so, their composition is more primordial - like the stars which first formed in the early universe.
A preliminary assessment of the HST observation indicates that these compact clusters contained many more massive stars than expected. "If this were also the case billions of years ago, it would have altered drastically the early history of the universe," says Dr. Nino Panagia of the Space Telescope Science Institute (STScI) in Baltimore, Maryland and the European Space Agency ESA).
Panagia and R. Gilmozzi (also of STScI/ESA) and co-investigators utilized fully HST's unique capabilities - ultraviolet sensitivity, ability to see faint stars, and high resolution - to identify three separate populations in this concentration of nearly 10,000 stars. (Previous observations with ground-based telescopes resolved less than 1,000 stars in this region.) About 60 percent of the stars belong to the dense cluster called NGC 1850, which is estimated to be 50 million years old. A loose distribution of extremely hot massive stars in the same region are only about 4 million years old and represent about 20 percent of the stars in the image. (The remainder are field stars in the LMC.) The significant difference between the two cluster ages suggests these are two separate star groups that lie along the same line of sight. The younger, more open cluster probably lies 200 light-years beyond the older cluster, says Panagia. He emphasizes that if it were in the foreground, then dust in the younger cluster would obscure stars in the older cluster.
Because having two well-defined star populations so separated by such a small gap of space is unusual, this juxtaposition suggests that they might be linked in an evolutionary sense. A possible scenario is that an expanding "bubble" of hot gas, from more than 1,000 supernova explosions in the older cluster, might have triggered the birth of the younger cluster. This would have happened when the bubble expanded across space for 45 million years before plowing into a wall of cool gas and dust. The shock front then caused the gas to contract, precipitating a new generation of star formation. The massive, hot stars are destined to explode in a few million years, and thus create yet a new expanding bubble of gas.
The findings will be published in the November 1, 1994 issue of the Astrophysical Journal Letters.
