One of the largest known stars in the Universe has changed its appearance completely within just a few years, according to an international team of astronomers that includes Polish researchers from the Astronomical Observatory of the University of Warsaw working within the OGLE project.
The star, WOH G64, is located in the neighbouring galaxy Large Magellanic Cloud. It was discovered in the 1980s.
“It soon became clear that it was a massive, extremely cool star of nearly record-breaking size, with a radius exceeding 1,500 times that of the Sun. Such rare objects are known as red supergiants. Massive stars have relatively short lifespans - lasting only several million years - and the red supergiant phase represents the final stage of their evolution. According to stellar evolution theory, such stars should soon end their lives by exploding as supernovae or collapsing into black holes without a visible explosion. However, these final evolutionary stages remain poorly studied observationally,” said Andrzej Udalski, project leader, and Igor Soszyński from the OGLE team.
WOH G64 has been observed regularly for decades, including by the long-term sky survey Optical Gravitational Lensing Experiment (OGLE) run by the University of Warsaw.
During observations conducted between 2001 and 2009, astronomers detected regular periodic variations in the star’s brightness caused by pulsations similar to those of Mira-type stars, but with an unusually long period of 886 days.
The star was also periodically studied spectroscopically. Researchers later discovered that its properties had changed dramatically.
“Toward the middle of the last decade, researchers found that its spectrum had undergone dramatic changes - WOH G64 now looks completely different from how it appeared 20 years ago. Its surface temperature increased by about 1,000 K,” the researchers wrote in a press release sent to PAP.
“Continuous photometric observations during the fourth phase of the OGLE project (since 2010), supplemented by data from other surveys, revealed that the previously observed periodic variability had disappeared. They also showed a drastic change in the star's colour, consistent with the temperature increase. The turning point came around 2011, when the star's brightness suddenly declined. When it returned to its former luminosity in 2013–2014, it had revealed a new identity,” the researchers added.
Scientists were surprised to find that instead of exploding as a supernova, the star had evolved from a cool red supergiant into a symbiotic star — a binary system composed of a yellow hypergiant and a smaller gravitationally bound companion star.
“The hypergiant is still enormous but significantly hotter than before, and nearly half the size of its previous incarnation as a red supergiant. Its companion is a Sun-like star, though several times more massive, hotter, and brighter than our Sun, emitting primarily blue light. As these observations demonstrate, the late stages of stellar evolution can follow different paths,” the astronomers said.
Researchers are considering two possible explanations for the transformation.
The first involves the binary nature of the system.
“As the envelope of the more massive star expanded, the companion eventually became engulfed beneath its surface, causing the system to appear externally as a single, bloated red supergiant. This stage is known as the common-envelope phase in binary evolution. Interaction between the engulfed star and the envelope of its massive companion likely triggered the ejection of the outer layers. By the early 2010s, the original binary system had re-emerged - now visible as a blue star and a yellow hypergiant, the latter representing the exposed inner layers of the former red supergiant,” the researchers explained.
The second scenario assumes that the yellow hypergiant expelled enormous amounts of mass in the past. This ejected material could have obscured the entire binary system for many years, making it appear as a red supergiant. In the early 2000s, the surrounding matter may have dispersed enough for WOH G64 to reappear in its original form as a symbiotic system composed of a blue star and a yellow hypergiant.
The future of the system remains uncertain. Astronomers predict that the massive component will most likely explode as a supernova. Another possibility is that the system will eventually form a black hole, either through the gravitational collapse of the hypergiant at a later stage of its evolution or as a result of a merger with its companion.
Udalski and Soszyński emphasise that the case of WOH G64 demonstrates the importance of large-scale surveys that monitor millions of stars over long periods.
Such projects make it possible not only to discover rare and unusual objects, but also to track changes in their behaviour over decades.
The research results were published in the journal Nature Astronomy.
The OGLE project is co-financed by the Polish Ministry of Science and Higher Education, the Polish National Science Centre, and the Foundation for Polish Science. (PAP)
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