R136a1: Monster star too big to be true?

R136a1 could once have been 320 times as massive as the sun. That's twice as massive as scientists thought a star could be. Perhaps R136a1 is several stars close together, some say.

This image provided by the European Southern Observatory shows a new near-infrared image of the R136 cluster. At birth, the three brightest stars each weighed more than 150 times the mass of the sun. The most massive star, known as R136a1, is located at the center of the image.

Astronomers say they have uncovered the most monstrous star yet seen – far more massive than some had thought possible – deep in the Tarantula Nebula, a brightly glowing region of hydrogen gas in a small companion galaxy to the Milky Way.

The team of astronomers estimates that at its birth the behemoth, called R136a1, could have been 320 times more massive than the sun. Up to this point, it looked as though the upper mass limit for stars in today's universe was around 150 solar masses, based on observations of other researchers.

If the object turns out to be an individual star – rather than a several stars bunched closely together – such monsters on the Milky Way's doorstep could help researchers to tackle fundamental questions about about how such bulked up but short-lived stars form and what happens to them when they run out of gas.

They also may shed light on the universe's first stars – objects whose masses are estimated to have ranged from 300 to 1,000 times the mass of the sun. The earliest stars are thought to have formed 100 million years after the Big Bang – the enormous release of energy that researchers say gave rise to the universe some 13.6 billion years ago.

The object is one of a cluster of stellar giants roughly 1 million to 2 million years old. Their nebula-nursery sits in the Large Magellanic Cloud, a small companion galaxy to the Milky Way. Visible in the southern hemisphere, the mini galaxy is some 165,000 light-years from Earth.

The results represent "a real leap" toward getting a better grasp on how massive stars in the Tarantula Nebula can become, says Scott Kenyon, a specialist in stellar and solar system evolution at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Mysteries of the Tarantula Nebula

In the early 1980s, he explains, astronomers thought that the entire star cluster, of which the new object is a part, was one star. If so, it tipped the cosmic scale at 2,500 solar masses. But improved observations showed it to be a bright cluster of massive young stars, rather than one object.

It's ironic that R136a1, the most massive object the team found in the R136a group, "is a record-holder again – hopefully more permanent than in the 1980s," says Paul Crowther, a Sheffield University astrophysicist who led the team reporting the results in a recent issue of the Monthly Notices of the Royal astronomical Society in Britain.

The object's discovery emerged from a study the team was conducting of two regions of intense star formation, R136a and NGC 3603. NCG 3606 lies inside the Milky Way roughly 22,000 light-years from Earth. There, the team detected stars whose initial masses are estimated at 150 solar masses and more.

Those findings helped them understand what they thought they might be seeing in R136a.

Using data from the Hubble Space Telescope, as well as observations take at the European Southern Observatory's Very Large Telescope in Paranal, Chile, the team noted that several of R136a's stars were burning ferociously, posting temperatures some seven time hotter than the sun's. The team put those up against computer models of stellar evolution and concluded that the stars started burning with masses significantly higher than the purported 150-solar-mass limit.

Indeed, radiation from the four largest stars the team says it has identified in cluster R136a account for more than half of the radiation emitted by the cluster's full complement of 100,000 stars.

Doubts about R136a1

Yet the history of observations of R136a call for caution in interpreting the new results, notes the Harvard-Smithsonian's Dr. Kenyon.

The team has used a range of meticulous observations and marshaled a variety of arguments to support the notion that the objects they see are individual stars. But the observing technology the team used can't resolve closely spaced individual stars 165,000 light-years away, he says. It's still possible that an object the team interprets as a single star could instead be close binary stars with orientations that make them look as though they are a single object.

Others have suggested that the team has detected very tightly packed clusters with stars too close to resolve individually.

Whatever the result, the presence of stars with mass topping 100 solar masses poses some problems for theories of stellar evolution, Kenyon adds.

If they form from a single cloud of gas, it's hard to explain how they could gravitationally accumulate so much mass in the relatively short 100,000-year period in which stars are thought to form. If they form from a collision between several closely spaced, lower-mass stars, the physics behind such mergers remains murky, leaving it as an unsatisfying solution, he adds.

With R136a1 having tipped the scales at 320 solar masses and shining with 10 million times the sun's brightness, "owing to the rarity of these monsters, I think it is unlikely that this new record will be be broken any time soon," Dr. Crowther says.


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