astronomy

Supermassive stars found in nearby galaxy

U. SHEFFIELD (UK)—Astronomers have identified two young star clusters that weigh up to 300 times the mass of the Sun—a figure that doubles the previously accepted limit of solar mass.

NGC 3603 is located 22,000 light-years away from the Sun, and is a cosmic factory within which stars form quickly from the nebula’s ring of gas and dust.

RMC 136a (more commonly known as R136a), another cluster of young, hot stars, is found within the Tarantula nebula, itself within the Large Magellanic Cloud, a neighboring galaxy to the Milky Way.

A team of international astronomers led by Paul Crowther, professor of astrophysics at the University of Sheffield, used the European Southern Observatory’s (ESO) Very Large Telescope and data from the NASA/ESA (European Space Agency) Hubble Space Telescope to study the two clusters of stars.

Details of the findings are reported in the Monthly Notices of the Royal Astronomical Society.

Stars with surface temperatures of over 40,000 C—more than seven times hotter than the Sun—were found, measuring tens of times larger and millions of times brighter.

Many of these stars were born with an even greater mass—losing weight through very powerful winds. Models suggest that several were born in excess of 150 solar masses.

The most massive star found, R136a1 within the R136a cluster, has a current mass of 265 solar masses, and it is thought its birthweight was as much as 320 times that of the Sun. It is also the most luminous star ever found, close to ten million times that of the Sun.

Within R136, only four stars out of an estimated 100,000 stars in the cluster weighed more than 150 solar masses at birth, yet they account for nearly half of the solar wind and radiation in the entire cluster.

In NGC 3603, two stars in a binary system were measured, to validate the models used. The stars A1, B, and C in the cluster were all estimated to have weighed above or close to 150 solar masses at birth.

Such massive stars have the potential of creating exceptionally bright “pair instability supernovae” at the end of the lives, blowing themselves completing apart without any remnants, and dispersing up to ten solar masses of iron into their surroundings.

“Unlike humans, these stars are born heavy and lose weight as they age. Being a little over a million years old, the most extreme star R136a1 is already ‘middle-aged’ and has undergone an intense weight loss program, shedding a fifth of its initial mass over that time, or more than fifty solar masses,” says Crowther.

“Owing to the rarity of these monsters, I think it is unlikely that this new record will be broken any time soon.”

Due to the short lives of these very rare, high mass stars, it remains a challenge for astronomers to identify how they originated.

“Either they were born so big or smaller stars merged together to produce them,” adds Crowther.

If R136a1 replaced the Sun at the centre of our solar system, it would outshine our star by as much as the Sun currently outshines the Moon.

“Its high mass would reduce the length of the Earth’s year to three weeks, and it would bathe the Earth in incredibly intense ultraviolet radiation, rendering life on our planet impossible,” says Raphael Hirschi from Keele University.

Researchers from the University of Sheffield and Keele University, U.K.; Astrophysikalisches Institut Potsdam, Germany; and University of Malaya, Malaysia, contributed to the work. Funding was provided by the U.K. Science and Technology Research Council.

More news from the University of Sheffield: www.shef.ac.uk/mediacentre/

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