Betelgeuse, the curiously dimming star, may be covered in giant star spots

New research has suggested that large star spots, like sunspots on our sun, are on the surface of Betelgeuse and causing the dimming. The researchers said their result rules out the dust scenario, which suggested that Betelgeuse ejected dust and it was obscuring the star.

The study published Monday in The Astrophysical Journal Letters.

Betelgeuse is estimated to be a few million years old and is about 700 light-years away. And the "supergiant" name is no joke: According to NASA, the star is thought to be somewhere between the diameter of Mars and Jupiter's orbits in size. It's estimated to be between 11 to 12 times the mass of our sun.

Astronomers expected it to begin dimming in December because the star experiences periods of dimming and subsequent brightening every 425 days.

However, Betelgeuse dropped to 40% of its normal luminosity between October 2019 and April 2020, which surprised astronomers.

Thavisha Dharmawardena, a postdoctoral researcher at the Max Planck Institute for Astronomy in Germany, led a team of international astronomers as they studied Betelgeuse amid this unusual dimming episode. The team's data showed that temperature variations in the surface of the star caused the drop in brightness. And the most probable cause of this would be gigantic star spots covering 50% to 70% of Betelgeuse's surface.

"Towards the end of their lives, stars become red giants," Dharmawardena said in a statement. "As their fuel supply runs out, the processes change by which the stars release energy. As a result, they bloat, become unstable and pulsate with periods of hundreds or even thousands of days, which we see as a fluctuation in brightness."

The star is so massive that the gravitational pull on the surface is less than that of a smaller star, so any pulsating by the star can actually eject layers of it easily. When this gas released by the star cools, it essentially forms dust.

Dharmawardena and her collaborators analyzed new and archival data taken from the Atacama Pathfinder Experiment in Chile and the James Clerk Maxwell telescope in Hawaii to search for this dust. Both telescopes can measure radiation in submillimeter waves, which have wavelengths a thousand times greater than that of visible light. This allows them to study interstellar dust, which is otherwise invisible — but can emit a glow in these particular waves.

"What surprised us was that Betelgeuse turned 20% darker even in the submillimetre wave range," said Steve Mairs, study coauthor and researcher at the East Asian Observatory in Hawaii, in a statement.

Star spots on a grand scale

This darkening of Betelgeuse didn't match up with their dust hypothesis. But the data reflected that the star was causing its own change in brightness and a dip in surface temperature.

"Corresponding high-resolution images of Betelgeuse from December 2019 show areas of varying brightness," said Peter Scicluna, study coauthor and researcher at the European Southern Observatory, in a statement. "Together with our result, this is a clear indication of huge star spots covering between 50 and 70% of the visible surface and having a lower temperature than the brighter photosphere [or luminous surface of the star]."

While star spots are common in giant stars, it's not usually on this scale, the researchers said. And they aren't certain about how long these spots can last. But taking into account calculations based on theoretical models, the star spots match up with the drop in Betelgeuse's brightness.

Our own sun experiences sunspots that fluctuate over an 11-year cycle, increasing and decreasing over that time period. The same could be true of more massive stars.

"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," Dharmawardena said.

Betelgeuse will eventually explode, whether it happens in the next few years or 100,000 years from now. Why the uncertainty? Because there are multiple factors we just don't know about Betelgeuse; the star is so bright it makes it harder to observe and study using telescopes.

The star swelled to its current size because it burned through the hydrogen in its core and switched to fusing helium instead. When the helium runs out, and the star has exhausted its supply of carbon and silicon, it will run out of energy.

When that happens, the star's remaining iron collapses and causes a supernova. The star will implode, releasing shock waves and neutrinos, or ghostly particles, and blow apart. Astronomers have estimated it will likely become a condensed neutron star, but it could also turn into a black hole.