“Super stars” uncovered

Artist's impression of the relative sizes of young stars

This artist's impression shows the relative sizes of young stars, from the smallest ones called "red dwarfs", with about 0.1 solar masses, through low mass "yellow dwarfs" such as the Sun and massive "blue dwarf' stars with more than 8 times the mass of the Sun, to the newly-discovered 300 solar mass star R136a1.

  • Stars found with 100+ times more mass than the Sun
  • Stellar record now stands at 320 solar masses
  • Did they form large, or did smaller stars merge?

“Super stars” born with hundreds of times the mass of our Sun, have been spotted in star clusters within our galaxy and in a neighbouring galaxy.

One of the stars, at birth, would have had over 300 times the mass of the Sun.

The UK-led team of astronomers studied two young star clusters—NGC 3603, located 22,000 light-years from Earth within our Milky Way galaxy; and R136a, located 165,000 light-years away in the Large Magellanic Cloud, a satellite galaxy of the Milky Way.

Both clusters are “star factories” where stars have formed from clouds of gas and dust.

The discovery of the giant stars might help solve a long-standing puzzle in astronomy—just how massive can stars become, and how do they get to be so massive?

Note that by “massive” the astronomers are not referring to the stars’ physical size—although the new “super stars” certainly are very large. Rather, they’re referring to how much mass they contain.

The team used archived data from the Hubble Space Telescope, plus new observations obtained with the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile.

The Tarantula Nebula (left) and star cluster R136a (right)

A region of the Large Magellanic Cloud galaxy seen with the Very Large Telescope (left), as well as a new image of the R136 cluster obtained with the MAD adaptive optics instrument on the Very Large Telescope (right). R136a contains the most massive star found so far.

Several of the stars studied have surface temperatures in excess of 40,000 degrees Celsius, which is almost 8 times hotter than the Sun. The stars are also tens of times bigger and millions of times brighter.

Computer models suggest that stars like these must have started off with masses greater than 150 times the mass of the Sun.

Stellar heavyweight champion

The heavyweight champion, in the RMC 136a cluster, is a star called R136a1. It is the most massive star known, with a current mass around 265 times that of the Sun. At birth, it must have been over 320 solar masses.

Young star cluster R136a

R136 is a cluster of young, massive and hot stars located inside one of the Milky Way's neighbouring galaxies, the Large Magellanic Cloud, 165,000 light-years away.

These kinds of hot, huge stars lose a lot of their mass by blowing off “winds” into space.

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

Two of the heavyweight stars in the NGC 3603 cluster are in orbit around each, a double star system. Using simple physics laws, the astronomers could calculate their masses at 120 and 92 times that of the Sun. At birth, they would have been 148 and 106 solar masses respectively.

Woking out how such huge stars formed in the first place is a major challenge for astronomers.

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

Story by Jonathan Nally, Editor, SpaceInfo.com.au

Images courtesy ESO / P. Crowther / C.J. Evans / M. Kornmesser.

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