Astronomers using the Large Binocular Telescope in Arizona, USA, and the LAMOST survey data set have recently discovered two very strange stars, unlike anything previously known. The two stars, 10 000 light years away, are small but extremely dense and hot, with surface temperatures up to 10 times hotter than those of the Sun. But most interesting is that the surface of both stars is a fairly thick layer of carbon and oxygen, the products of the thermonuclear combustion of helium.
What makes these stars, PG1654+322 and PG1528+025, even more remarkable is that, despite the presence of a fusion product envelope, the cores of these stars still consist of pure helium and their 'fusion reactor' continues to run at full power. Usually, stars with such surface chemistry have already finished burning helium in their cores and are on their way to becoming white dwarf stars. And these two findings represent a major challenge to most of the existing theories of stellar evolution.
It is likely that the two stars found will be assigned to an entirely new class, and scientists are now putting forward theories that could explain the occurrence of such a phenomenon. "Our main theory is that such strange objects could possibly have formed through a very rare type of star merger process," the researchers write, "Under a number of conditions, a relatively small carbon-oxygen white dwarf star could have collapsed, after which its matter was absorbed by a companion star, forming its highly unusual outer shell."
In other words, when two white dwarfs merge, the larger and more massive object destroys the smaller one by its gravity. And instead of completely mixing the matter of the two stars, the matter of the collapsed star simply envelops the surface of the remaining one. "This is very similar to the process of putting a glove on a hand," the researchers write, "only in this case the hand is the large white dwarf star and the glove is the matter of the collapsed companion star."
Next, scientists are going to fit some of the cosmological models to what they have been able to glean from observations of the stars PG1654+322 and PG1528+025. In addition, the discovery of these stars has posed scientists a number of other questions, the answers to which they have yet to find.