Dramatic changes observed in one of universe’s biggest stars

By Will Dunham

WASHINGTON, Feb 28 (Reuters) – The universe’s biggest stars live the life of a rock star: they are born brilliant, live fast and die young. If so, the one named WOH G64 could be considered the stellar equivalent of ‌Jimi Hendrix.

WOH G64, which is 28 times the mass of the sun and resides in a satellite galaxy of the Milky Way called the “Large Magellanic Cloud,” is one of the few largest stars known, just like Hendrix was in rock ‘n’ roll. And observations spanning more than three decades show that it behaves like no other star observed before.

Astronomers have only an incomplete understanding of the life history of the largest stars, and the WOH G64 observations provide new information.

In 2014, researchers observed a change in the star’s color, corresponding to an increase in its surface temperature, as it evolved from red to yellow. The star had been classified as an extreme red supergiant but quickly became a yellow hypergiant. This transition happened quickly, in cosmic terms, and without any evidence of an eruption or explosion.

“In general, the evolution of a star takes place on time scales of billions of years. On the human time scale, we only observe more abrupt and violent events, such as flares, the merger of two stars or their explosive death,” said astronomer Gonzalo Muñoz-Sanchez, lead author of the research published this week in the journal Nature Astronomy.

“No current stellar model can fully explain this transformation” in WOH G64, said Muñoz-Sanchez, who worked on the study at the Athens National Observatory.

Compared to the sun, its luminosity is approximately 300,000 times greater and its diameter approximately 1,500 times greater. If it were in the Sun’s place, its surface would extend over a distance between the orbits of Jupiter and Saturn, the fifth and sixth planets in our solar system. Traveling at the speed of light would take six hours to circle the star’s surface.

WOH G64, approximately 10 million years old, appears to be nearing the end of its lifespan. In contrast, the Sun is about 4.5 billion years old and has about 5 billion more years left. WOH G64 is located approximately 160,000 light years from Earth. A light year is the distance traveled by light in a year, or 9.5 trillion kilometers.

“WOH G64 is a massive star and very different from the Sun,” Muñoz-Sanchez said.

Stars between eight and 23 times the mass of the Sun are expected to evolve into red supergiants and eventually explode as supernovas.

For stars with masses between 23 and 30 times the mass of the sun, their fate is less certain. It is still unclear whether they explode as a supernova, collapse directly to form a black hole, or evolve from a red supergiant phase to a yellow hypergiant stage before ending their lives. Black holes are exceptionally dense objects with gravity so strong that even light cannot escape.

“Therefore, WOH G64 could be the solution to this question,” said Muñoz-Sanchez.

The fact that observations indicate that it is gravitationally bound to another star in what is called a binary system adds another layer of complexity to understanding WOH G64. The researchers were unable to determine the size or characteristics of the stellar companion, but said the two could at some point merge.

Researchers have speculated about recent changes in WOH G64. They said he may have experienced a violent episode prior to the observational data that turned him red and was now returning to his usual yellow resting state. They said it was also possible that an interaction between WOH G64 and its companion temporarily mimicked the appearance of a red supergiant.

“As astronomers continue to monitor this remarkable system, WOH G64 is poised to reshape our understanding of how the most massive stars live and die,” Muñoz-Sanchez said.

(Reporting by Will Dunham; editing by Daniel Wallis)