In class we learned how stars of different masses tend to evolve and die off and end up as either a white dwarf, neutron star or black hole. But we also learned that it is possible for one of these three phases to gain more mass and evolve. For example if a white dwarf surpasses the remnant mass of 1.4 solar masses, it will turn into a neutron star, because the electron degeneracy is not strong enough to support this much weight. If a neutron star surpasses 3 solar masses, it will turn into a black hole, since neutron degeneracy cannot hold anymore. But this occurs often when the star is part of a binary system. A blue straggler star is a blue star that looks "hotter and bluer than they should for their advanced age." according to astronomer Natalie Gosnell. One theory to explain this is that there is one star grows in size as it evolves into a white dwarf while the companion star is already a white dwarf. Hence there is a transfer of mass from the swelled up star to the white dwarf, and while this happens, the initial white dwarf turn hotter and bluer while the other star turns into a white dwarf.
Gosnell studied a star cluster called NGC 188, that has 21 blue stragglers, and she discovered that 7 of the 21 star systems had white dwarfs, and another 7 showed mass transfer between their stars, which is actual observable evidence of how blue staggers might be forming. This shows why stars that are suppose to evolve into red giants are actually turning into blue stars that burn hotter and bluer.
Source:http://mcdonaldobservatory.org/news/releases/20151207
4 points.
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