Hubble Gets Clear View of Massive Stars

Very massive stars are often the hardest to see, as they're typically embedded in nebulas of gas and dust.

So it is with two of our Milky Way Galaxy's most massive stars. But the Hubble Space Telescope has just offered a better view.

The image shows a pair of colossal stars, WR 25 and Tr16-244, located within the open cluster Trumpler 16. This cluster is embedded within the Carina Nebula, an immense cauldron of gas and dust that lies 7,500 light-years from Earth. The nebula contains several ultra-hot stars, including these two star systems and the widely studied, explosive star Eta Carinae, which has the highest luminosity yet confirmed.

The stars are hot and bright, emitting most of their radiation in the ultraviolet and therefore appearing blue. They are so powerful that they burn through their hydrogen fuel source faster than other types of stars. They live fast, and they will die young. While our sun is middle-aged at 4.6 billion years, the hottest stars live only tens or hundreds of millions of years.

The stars interest astronomers because they are associated with star-forming nebulas, and they influence the structure and evolution of galaxies.

WR 25 is likely to be the most massive and interesting of the two. Its true nature was revealed two years ago when an international group of astronomers led by Roberto Gamen, then at the Universidad de La Serena in Chile, discovered that it is composed of at least two stars. The more massive is a Wolf-Rayet star and may weigh more than 50 times the mass of our sun. It is losing mass rapidly through powerful stellar winds that have expelled the majority of its outermost hydrogen-rich layers, while its more mundane binary companion is probably about half as massive as the Wolf-Rayet star, and orbits around it once every 208 days.

Massive stars are usually formed in compact clusters. Often the individual stars are so physically close to each other that it is very difficult to resolve them in telescopes as separate objects.

These Hubble observations have revealed that the Tr16-244 system is actually a triple star. Two of the stars are so close to each other that they look like a single object, but Hubble's Advanced Camera for Surveys shows them as two. The third star takes tens or hundreds of thousands of years to orbit the other two. The brightness and proximity of the components of such massive double and triple stars makes it particularly challenging to discover the properties of massive stars.

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