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"It's like finding a Sumo wrestler sitting in a little 'Smart car'!" This is how Don Neill, member of the Galaxy Evolution Explorer team (California Institute of Technology in Pasadena) described finding the most luminous supernovae, derived from the most massive stars, in dwarf galaxies.
As already noted at the end of the 80s by Neil deGrasse Tyson and John Scalo, and now emphasised by images produced by the Palomar Transient Factory survey, a considerable number of very luminous supernovae explode in very low mass galaxies (even 1000 times less massive than the Milky Way). This is surprising because one may have naively thought that the greater mass of available material in massive galaxies would have allowed the formation of greater numbers of massive stars, and therefore more luminous supernova explosions. But this is not the case.
Studying a large sample of dwarf galaxies at ultraviolet wavelengths with the GALEX satellite, Neill's team (that also includes Michael Rich of UCLA) has measured their star formation rates, and though these rates are very low, as expected, they nonetheless do produce very massive stars.
According to the researchers, what distinguishes the dwarf galaxies is their low metallicity (low levels of heavier elements relative to hydrogen). During the life of a massive star it looses huge amounts of mass in stellar winds, and the amount of mass loss is higher for more metal-rich stars.
Given that the stars in the dwarf galaxies are less metal rich, they loose less mass during their evolution, so that when they explode as supernovae they are larger than their counterparts in more massive, more chemically evolved, galaxies.
These huge supernova explosions far outshine the tiny galaxies which host them, as shown in the above images: the top row shows medium-sized galaxies (centered in the frames) hosting normal supernovae (off centre), while the bottom row shows supernovae that have occurred in dwarf galaxies.
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