The critical mass for stars
Massive stars eventually collapse under their own weight and explode as a supernova. Smaller stars like our sun simply fizzle out into so-called white dwarfs. Scientists are keen to learn exactly at what mass the transition occurs. Not least because supernova explosions produce most of the universe's heavy elements - the building blocks of dust, planets and people. A team of astronomers from Cambridge University think they have found a white dwarf born from a massive star. Scientists assume that more mass than previously assumed is required for stars to eventually explode as supernovas. "It's pretty exciting," said Mike Bolte, an astronomer at the University of California, Santa Cruz. So far, astronomers have assumed that the "mass limit point" is between five and ten times the mass of our sun. Now the area in question could be further narrowed down.
The researchers, led by Rebecca Elson, found their white dwarf while using the Hubble Space Telescope to study star formation in star cluster NGC 1818 in the Large Magellanic Cloud (one of our neighboring galaxies). They already knew that the cluster's stars formed about 40 million years ago. Only a massive star would burn out within that time span. Moreover, since the dwarf was not only blue and consequently very hot, the burnout must have been recent, it was also very bright.
Because the age of the white dwarf is known, scientists were able to estimate that it is the remains of a star of about 7.6 solar masses, explains team member Steinn Sigurdsson. And as a result, according to the researcher, the lower limit of a "very critical limit" is shifted considerably upwards. The results will be published in a future issue of the Astrophysical Journal Letters.
The new observation increases our knowledge of supernovae, the "fertilizer" of the universe. "The point at which supernovae don't happen and stars end up as white dwarfs determines the chemical composition of the entire universe," says Sigurdsson. Determining the boundary more precisely could also shed light on the rate at which massive stars formed earlier in cosmic history, he adds. "If you really depend on massive stars [to account for all the heavy elements in space] and there aren't many of them now, there must have been more in the past."
