R136, NGC 3603, AND THEIR SUPERMASSIVE STARS. One of the sacred numbers in stellar astronomy, the upper limit to stellar masses, has been shattered by a team of astronomers observing highly evolved stars within a huge star cluster called R136 within the Large Magellanic Cloud in the southern constellation Dorado and within the distant, dust-obscured nebula-cluster complex NGC 3603 in Carina. Long thought to be in the neighborhood of 120 to 150 solar masses, a handful of stars has now been estimated to have birth masses that range up to 170 times that of the Sun in NGC 3603 as high as 320 within R136, which powers the huge Tarantula Nebula (and which is so large that if placed at the local Orion Nebula, would fill the constellation). At the very top is R136a1, which, with a luminosity of 8.7 million times that of the Sun, not only tops fabled Eta Carinae, but makes it the most luminous star known that is not in a state of explosion. Currently and fiercely losing mass, these stars are whittling themselves down, and appear as "Wolf-Rayet" stars that have lost much of their outer envelopes and that now appear nitrogen- and helium-rich (Gamma-2 Velorum hosting a carbon-rich version). The workers have been able to exclude a double system by the lack of X- rays from colliding winds. The masses of these stars are not direct measures, but are derived from comparison with computer models that appear to be highly accurate. Their effects on their surroundings is staggering. Though we do not know how such massive stars form, their fates seem to be to explode, the events creating more heavy chemical elements out of which "earths" are made. (The work was done at the European Southern Observatory's Very Large Telescope, coupled with Hubble data. See their press release.)

From the paper "The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit," by P. A. Crowther, O. Schnurr, R. Hirschi, N. Yusof, R. J. Parker, S. P. Goodwin, and H. A. Kassim, which will appear in the Monthly Notices of the Royal Astronomical Society.
Written by Jim Kaler 7/23/10. Return to STARS.