Astronomers have witnessed one of our universe's biggest stars transforming into a rare stellar body, and the dramatic metamorphosis may be the prequel to a powerful supernova explosion that sees this star birth a black hole.
The doomed star in question is WOH G64 (also known as IRAS 04553–6825), located in a satellite galaxy of the Milky Way known as the Large Magellanic Cloud (LMC), around 163,000 light-years away. The star is around 1,540 times the size of the sun, with almost 30 times the mass of our star and a staggering 282,000 times its brightness. Discovered in the 1970s, WOH G64 has always appeared to be a red supergiant star surrounded by a ring, or torus, of dense dust.
However, in 2014, the appearance of this supergiant began to change. A team of astronomers, led by Gonzalo Muñoz-Sanchez at the National Observatory of Athens, noticed the star's color changing along with a corresponding increase in its surface temperature. Muñoz-Sanchez and colleagues determined this must represent the transformation of a red supergiant into a rare yellow hypergiant, which could also mean astronomers are witnessing a star "die" in real time.
"The fate of stars with initial masses between 23 and 30 solar masses after evolving into red supergiants is still uncertain. In this case, WOH G64 was the most extreme red supergiant known, with an estimated mass of around 28 solar masses," Muñoz-Sanchez told Space.com. "It remains unclear whether such stars explode as supernovas, collapse directly into black holes, or evolve from the red supergiant phase into a yellow hypergiant stage before ending their lives. "WOH G64 might be the solution to this question."
The team's results represent the first evidence that an extreme stellar object can change its temperature and evolve from red to yellow in the span of a year — and in a smooth, silent manner at that.
"This is especially surprising because rapid changes in stars are typically associated with violent or abrupt processes," Muñoz-Sanchez continued.
That wasn't all the team discovered about this immense star, however. The scientists also found that WOH G64 isn't alone.
Live fast, die young … but not alone
At just 5 million years old, WOH G64 is a cosmic youngster in comparison to other stars such as our middle-aged, 4.6-billion-year-old sun, so it may seem a touch cosmically unjust that it is facing the end of its life. This is the case because massive stars such as this "live fast and die young," burning through their fuel supply needed for nuclear fusion more rapidly than modestly sized stars.
Though this short life span is true for all massive stars, the end stages of the lives of these stellar titans aren't quite so certain. For instance, not all red supergiants shed their outer layers as their cores contract to become yellow hypergiants.
"Yellow hypergiants are extremely rare because they represent a short-lived transitional phase between the red supergiant stage and the eventual supernova explosion," Muñoz-Sanchez said. "Consequently, only a small number of confirmed yellow hypergiants are currently known, amounting to just a few tens of objects."
For this yellow hypergiant transformation to happen, a massive star needs a stellar wind that is strong enough to strip away an outer envelope of previously shed stellar material, a process that drives up its temperature. However, only the brightest red supergiants can drive outflows of material powerful enough to trigger this transitional phase that leads eventually to the death of the star.
The team also found that the huge star actually is part of a binary system, existing with a companion star. This complicates the potential cause of its transformation if the main star is greedily dragging matter from its companion.
"Binary interactions may also play a crucial role in the formation of yellow hypergiants," Muñoz-Sanchez said. "If mass transfer or envelope stripping occurs in a binary system, the envelope of a red supergiant can be partially removed, potentially driving its evolution toward the yellow temperatures."
The researcher continued by explaining that in a binary-driven scenario, which sees the evolution of the star caused by interactions with its companion, the binary system would have been embedded in a common envelope, a cocoon of gas surrounding both stars that made it appear as a red supergiant. The partial ejection of this envelope would then reveal the two stars.
"Alternatively, even though the system is binary, the transition may have been driven by intrinsic stellar processes. In this case, the star may have undergone an extraordinary eruptive episode lasting more than 30 years and is now returning to a yellow, quiescent state," Muñoz-Sanchez added. "Both possibilities are extremely rare, and witnessing either occur on human timescales is nearly unprecedented."
Thus, the team does not yet know whether its evolution is a consequence of interactions between WOH G64 and its binary stellar companion or if the metamorphosis is intrinsic to the star itself.
"Recent observations suggest that some of the other extreme red supergiants may also be in binary systems," Muñoz-Sanchez explained. "Understanding whether the extreme properties of these stars arise from their intrinsic nature or from binary interactions is crucial for studying the populations of evolved massive stars, predicting their deaths, and interpreting the supernovas they produce, phenomena that are still not fully understood."
And understanding the binary nature of WOH G64 isn't just key to understanding its life; these details are integral to its death, too.
The continued exchange of mass between the stars could lead to their collision and the merger of the two components. However, if interactions between the stars are slight or non-existent, the main star would evolve toward core collapse, ultimately resulting in either a supernova explosion or direct collapse into a black hole. "In astronomical terms, WOH G64 appears to be a highly evolved system, and it is possible that it could undergo core collapse 'soon.' In this context, 'soon' corresponds to a timescale ranging from a hundred to a few thousand years," Muñoz-Sanchez said. "Such an event would be extraordinary, it remains highly unlikely that it will occur within our lifetime.
Although, of course, we are not even sure that this star will explode as a supernova."
A paper about these results was published on Monday (Feb. 23) in the journal Nature.
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