Betelgeuse, normally one of the brightest stars in the night sky, has dimmed 60% ahead of what some astronomers think is a potentially imminent supernova.
Betelgeuse is a red supergiant, usually the tenth-brightest star in sky, which makes up part of the constellation Orion.
It is also one of the largest stars visible to the naked eye and, if placed at the centre of our solar system, would almost certainly engulf Jupiter.
But for months scientists have been trying to figure out why its luminosity has dropped to about 40% of what is normally expected.
Some astronomers speculated the the 60% swing detected between October last year and this April was related to an imminent supernova – imminent in cosmological terms that is, within the next 100,000 years.
An international team of astronomers, led by Dr Thavisha Dharmawardena from the Max Planck Institute for Astronomy, have demonstrated that the dimming was most probably caused by giant star spots covering up to 70% of Betelgeuse’s surface.
“Towards the end of their lives, stars become red giants. As their fuel supply runs out, the processes change by which the stars release energy,” Dr Dharmawardena explained.
This causes the stars to bloat and become unstable, pulsating to a beat that can last hundreds or even thousands of days, which causes the fluctuations in brightness.
Betelgeuse is so large the gravitational pull on its surface isn’t sufficient to prevent these pulsations from ejecting the outer layers of the star.
The gas that the star exhales then cools and turns into dust – which is how the heavier elements in the universe are produced.
Astronomers had suspected that a collection of this dust was absorbing the light from Betelgeuse – but no such dust was showing up even when the light from the star was studied at different wavelengths.
Dr Dharmawardena and her collaborators measured light from Betelgeuse at a terahertz wavelength – where the wavelengths are a thousand times closer together than in visible light.
At this wavelength the cool dust emitted by stars usually glows.
“What surprised us was that Betelgeuse turned 20% darker even in the sub-millimetre wave range,” reported Steve Mairs from the East Asian Observatory, who collaborated on the study.
This was not consistent with what should have been found if the light from the star was being absorbed by dust.
Instead they reasoned that the star itself must have been dimming – as a result of a reduction in the surface temperature.
Using high-resolution images of the star captured in December 2019 the team found areas of varying brightness – indicating the presence of huge star spots covering up to 70% of its surface.
“Star spots are common in giant stars, but not on this scale,” said the Max Planck Institute.
“Observations in the coming years will tell us whether the sharp decrease in Betelgeuse’s brightness is related to a spot cycle. In any case, Betelgeuse will remain an exciting object for future studies,” Dr Dharmawardena said.