Astronomers have witnessed one of the universe’s largest stars transform into a rare stellar body, and the dramatic metamorphosis could be the precursor to a powerful supernova explosion that sees this star born a black hole.
The condemned star it concerns WOH G64 (also known as IRAS 04553–6825), located in a satellite galaxy in The Milky Way known as Large Magellanic Cloud (LMC), about 163,000 light-years away. The star is about 1540 times the size of sunwith nearly 30 times the mass of our star and a staggering 282,000 times its luminosity. Discovered in the 1970s, the WOH G64 has always seemed to be one red supergiant star surrounded by a ring, or torus, of dense dust.
In 2014, however, the appearance of this supergiant began to change. A team of astronomers, led by Gonzalo Muñoz-Sanchez at the National Observatory of Athens, noticed that the star’s color changed along with a corresponding increase in surface temperature. Muñoz-Sanchez and colleagues determined that this must represent the transformation of a red supergiant into a rare yellow hypergiant, which could also mean that astronomers are witnessing a star “dying” in real time.
– The fate of stars with initial masses between 23 and 30 solar masses after developing into red supergiants is still uncertain. In this case, WOH G64 was the most extreme red supergiant known, with an estimated mass of about 28 solar masses, Muñoz-Sanchez told Space.com. “It is still unclear whether such stars explode as supernovaecollapse directly into black holesor evolve from the red supergiant stage to a yellow hypergiant stage before ending their lives. “WOH G64 may 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 over the course of a year – and in a steady, silent manner.
“This is particularly surprising because rapid changes in stars are typically associated with violent or abrupt processes,” continued Muñoz-Sanchez.
However, that was not all the team discovered about this huge star. The researchers also found that WOH G64 is not alone.
Live fast, die young…but not alone
At just 5 million years old, WOH G64 is a cosmic youngster compared to other stars such as our middle-aged, 4.6 billion-year-old Sun, so it may seem a little cosmically unfair that it is facing the end of its life. This is the case because massive stars like this “live fast and die young”, burning through the fuel supply necessary for nuclear fusion faster than modest-sized stars.
Although this short lifespan is true for all massive stars, the final stages of the lives of these stellar titans are not quite so certain. For example, not all red supergiants lose 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,” said Muñoz-Sanchez. “Therefore, only a small number of confirmed yellow hypergiants are currently known, amounting to only a few tens of objects.”
For this yellow hypergiant transformation to occur, a massive star needs a stellar wind strong enough to remove an outer envelope of former 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 transition phase that ultimately leads to the death of the star.
The team also found that the massive star is actually part of a binary system, existing with a companion star. This complicates the potential cause of the transformation if the main star greedily pulls matter from the companion.
“Binary interactions can 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 towards the yellow temperatures.”
The scientist went on to explain that in a binary-driven scenario, which sees the evolution of the star caused by interactions with its companion, binary system would have been embedded in a common envelope, a cocoon of gas surrounding both stars that made it is displayed like a red supergiant. The partial ejection of this envelope would then reveal the two stars.
“Alternatively, although 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,” added Muñoz-Sanchez. “Both possibilities are extremely rare, and witnessing either occur on human timescales is almost unprecedented.”
Thus, the team does not yet know whether the evolution is a consequence of interactions between WOH G64 and its binary companion, or whether 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,” explained Muñoz-Sanchez. “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 demise, and interpreting the supernovae they produce, phenomena that are still not fully understood.”
And understanding the binary nature of WOH G64 is not only the key to understanding life; these details are also integral to its death.
The continued exchange of mass between the stars can cause them to collide and merge the two components. However, if interactions between the stars are small or non-existent, the main star will evolve towards core collapse, eventually resulting in either a supernova explosion or outright 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 time scale ranging from a hundred to a few thousand years,” Muñoz-Sanchez said. “Such an event would be extraordinary, it is still very unlikely to happen in our lifetime.
Although, of course, we are not even sure that this star will explode as a supernova.”
An article about these results was published Monday (Feb. 23) in the journal Nature.
