Artist impression of HD 45166

New type of star could solve cosmic magnet mystery

Image credit: ESO/L. Calçada

After its death, the star HD 45166 is expected to collapse under its own gravity and become an extremely compact core with an even more powerful magnetic field.

Astronomers have discovered a new type of astronomical object – massive magnetic helium stars – that could shed light on the origin of magnetars.

The discovery was possible because of observations of the mysterious HD 45166 star, which has been found to have an extraordinarily powerful magnetic field, stronger than that of any other star of its size that has been measured.

HD 45166 is located in a binary system about 3,000 light years away. It is a Wolf-Rayet or helium star because it has blown away its outer layers of hydrogen to reveal the underlying helium.

Scientists have long wondered about the particular qualities of this star. 

“This star was known to be weird for about 100 years,” said Tomer Shenar at the University of Amsterdam. “It didn’t make any sense – it really contradicted theories, so it deserved more scrutiny.”

The team has been able to make a new discovery about the nature of the star, based on a series of new observations made with multiple telescopes around the world, including the Canada-France-Hawaii Telescope and the European Southern Observatory (ESO). 

The researchers found that HD 45166 has an extraordinarily powerful magnetic field, stronger than that of any other star of its size that has been measured. 

As such, they established HD 45166 belongs to a new category of astronomical object: massive magnetic helium stars. 

“Tomer and I refer to HD 45166 as the ‘zombie star’,” said Julia Bodensteiner, a co-author of the study. “This is not only because this star is so unique, but also because I jokingly said that it turns Tomer into a zombie.”

Shenar confirmed that the celestial body “became a bit of an obsession of mine”. 

“I remember having a eureka moment while reading the literature: ‘What if the star is magnetic?’” 

HD 45166 is located in the constellation Monoceros, and is larger than the Sun. Its magnetic field is measured at 43 000 gauss – 100,000 stronger than Earth's – making it the most magnetic massive star found to date. 

Once the observations were in, Shenar asked co-author Gregg Wade, an expert on magnetic fields in stars at the Royal Military College of Canada, to examine the data.

Wade’s response confirmed Shenar’s hunch: “Well my friend, whatever this thing is — it is definitely magnetic.”

These powerful fields mean that when this star collapses in on itself in a few million years, it will most likely become a magnetar, with a magnetic field of around 100 trillion gauss – the most powerful type of magnet in the universe.

About 10 per cent of neutron stars are magnetars, but how exactly they form has been a mystery for decades. If stars like HD 45166 eventually turn into magnetars, that mystery is finally solved.

“It is exciting to uncover a new type of astronomical object,” Shenar said, ”especially when it’s been hiding in plain sight all along.”

At the moment, there are only 31 known magnetars in the universe, though astronomers suspect there are millions of inactive ones waiting to be found.

The findings of the study have been published in the journal Science.

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