They represent the end stage of a certain class of stars

Nov 13, 2009 07:27 GMT  ·  By

Astronomers have known for a long time that the end of stars sees them transforming into a number of structures, including white dwarfs, neutron stars, or black holes. The thing that results after a star's demise is entirely based on the mass of said celestial body. If the mass is large, then the structure will collapse into a black hole or a neutron star. If its mass is considerably smaller than that of our Sun, then it will turn into a white dwarf, a form of star made up entirely of carbon and oxygen, with a few traces of other elements. But what happens with stars that have masses in between the two thresholds has remained a mystery until now.

A new astronomical survey has recently discovered two white dwarfs that may give an answer to the debate. Generally, these stars form when a larger body exhausts its hydrogen supply, and sheds the outer layers of its atmosphere. The core compacts into a space roughly the size of the Earth, but that contains about the same amount of mass as our Sun does now. The new structures begin to burn helium, and are incapable of sustaining nuclear fusion, as their larger counterparts do. They also stop generating important amounts of radiation, and emit most of their energy as heat. According to estimates, the newly discovered structures come from stars about seven and ten times the mass of the Sun.

This means that the original formations were precisely at the threshold between becoming white dwarfs or black holes. “These are literally the first two white dwarfs which have this kind of chemical composition. These stars may define the upper boundary of stars that can make white dwarfs,” University of Warwick expert Boris Gansicke explains. He is also the lead author of a new paper detailing the findings, which appears in the November 13 issue of the top journal Science. Located 400 and 220 light-years away from Earth, respectively, the two formations have been creatively named SDSS 0922+2928 and SDSS 1102+2054.

The reason why the experts believe they have a new find on their hands is the fact that, in theory, larger stars that collapse into white dwarfs have still sufficient energy to modify the standard chemical structure of such a formation. “Models predict that, if you go to the top end of the mass range of white dwarfs, they manage to burn most of the carbon layer. These two stars definitely have lower abundances of carbon,” Gansicke tells Space.