When astronomers peer up at the heavens, they aren’t just looking for objects we already know to exist. They also hunt for evidence of physical phenomena we think should exist but haven’t found yet. When you add up the various factors in play — the vastness of space, the length of time we’ve been searching, and the current quality of our instruments — there are plenty of things we expect to see but haven’t actually seen yet. The rarer these things are in the universe, the harder it is to see them, period. But we may have found one of the hardest objects to find recently — an incredibly rare neutron star produced by the collision of two white dwarfs.
Stars with insufficient mass to become neutron stars (which is almost all of them) are thought to end their lives as white dwarfs. White dwarfs are stellar remnants composed of extremely dense degenerate matter. They have a maximum stable mass of approximately 1.44 M☉, which translates to 1.4 solar masses. This is known as the Chandrasekhar limit. A white dwarf that acquires enough mass to exceed 1.44 M☉ is massive enough that the electron degeneracy pressure at the star’s core is no longer sufficient to resist its own gravitational self-attraction. At this point, the star implodes and becomes a neutron star or black hole, in a classic Type 1a supernova.
Or at least, that’s what’s supposed to happen. What researchers have discovered is a stellar object, J005311, with some incredibly rare properties. It’s a bright star in the infrared nestled inside a gas cloud, giving off no visible light. It’s ~40,000 times brighter than the sun (in infrared) and it produces a massive stellar wind, at 16,000km/s. Typical solar wind velocity off the largest stars is ~2,000km/s, to give you an idea just how fast this star is spinning. The video below shows an artists’ impression of a white dwarf collision.
“First of all, [these results show] that white dwarf mergers happen,” study co-author Götz Gräfener, an astronomer at the University of Bonn told Gizmodo. “And secondly, it shows that some of these mergers don’t explode.”
When a star is in the end stages of its life, it begins fusing materials other than hydrogen. Which materials it can fuse ultimately is determined by its density. The collision of two white dwarfs sharply boosts the density of the final object, allowing for the fusion of heavier elements. This is supposed to create a runaway fusion reaction that blows the star(s) apart, but that’s not what happened here. Instead, the collision of these two white dwarfs provided enough heat to allow for non-explosive carbon ignition. Because the star is burning, it’s generating enough thermal pressure to stave off the collapse and supernova that would have otherwise resulted. This is an incredibly rare occurrence relative to the expected behavior for a pair of colliding white dwarfs.
And the temperature and speed of the winds currently whirling around J005311 suggest that the object is actually near the end of its life. With a current mass expected to be above the Chandrasekhar limit and an anticipated lifespan of just thousands of years, there’s a very good chance that we caught J005311 during the tiny window of time we actually could observe it (as these things are reckoned). When the star explodes, it’ll likely produce a subluminous Type 1c supernova.
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