One of the brightest stars in the sky evolves and dies before our eyes

Nothing lasts forever, including the stars in our night sky. One of the brighter and more conspicuous stars in our sky is Betelgeuse, the bright red supergiant in Orion’s shoulder.

In late 2019, astronomers around the world were giddy with excitement as we watched this giant star grow fainter than we’ve ever seen it before. Since Betelgeuse is in the final stages of its life, there has been speculation that it could be a death rattle before the end.

But the cause of the “great eclipse” was not entirely clear until now. New preprint research pending peer review, led by Andrea Dupree of the Harvard & Smithsonian Center for Astrophysics, has used the Hubble Space Telescope to help unravel one of the greatest astronomical mysteries of this century: the cause of the sudden strange behavior of Betelgeuse.



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A star on the brink of death

From this latest research, it was discovered that Betelgeuse likely experienced a massive surface mass ejection (SME) in 2019. An SME occurs when a star expels large amounts of plasma and magnetic flux into the surrounding space.

We don’t fully understand what caused these SME’s, but if they have similar precursors to the coronal mass ejections we’ve seen on our own sun, they could be caused by the destabilization of large-scale magnetic structures in the star corona.

Artist’s impression of the aftermath of the SME, where the mass cooled and formed a cloud of dust that briefly dimmed the star. The chart below outlines Betelgeuse’s actual and expected brightness changes during this time.
Credit: NASA, ESA, Elizabeth Wheatley (STScI)

It is suspected that Betelgeuse lost much of its surface material in this remarkable event. In fact, the amount of material ejected is the largest SME event we’ve ever seen on a star, in modern astronomy.

What is truly remarkable is that Betelgeuse was cast out 400 billion times more mass than a typical event on other stars. This is several times the mass of the moon, pushed out at incredible speeds.

The artist’s animation shows a close-up of the irregular surface of Betelgeuse. The drop in the star’s brightness was the result of a “dusty veil” formed from material emerging from the star, partially obscuring its southern portion. ESO/L. Calcada.

Stellar evolution in real time

Betelgeuse looks a lot like an astronomical jack-in-the-box. Astronomers know that sooner or later it will “pop” and implode in a spectacular supernovabut we don’t know when. (We do know that when it does, it might even be visible in the daytime sky!)

Stars are born in many different sizes; some start small and grow big while others are born big. Betelgeuse is a red supergiant and would have started out smaller before expanding its outer shell over tens of millions of years. Once great red supergiants, they don’t have long before they reach a point where their cores produce iron and can no longer sustain it nuclear fusion.

We have seen the death of many thousands of distant stars before, in galaxies far, far away. But the appeal of studying the process in near real time on our galactic threshold is too good to pass up. In our star district, Betelgeuse offers us the best chance of success.

A diagram showing various cosmic objects in orange, white, and yellow on a galaxy background
Stellar evolution of low-mass stars (left cycle) and high-mass stars (right cycle), such as Betelgeuse.
Wikimedia/NASA’s Goddard Space Flight Center, CC BY

We’ve put together the secret lives of stars by studying things like globular clustersdistant supernovas and stellar nebulae. From this we can understand the birth, life and death of a star.

However, there are often gaps. Betelgeuse gives us a glimpse into the ‘before’ the end of a star, the last tens of thousands of years before the great event – ​​the twinkling of an eye in astronomical terms.

With this last result, we are already beginning to better understand how large stars like Betelgeuse lose mass through surface mass ejections as they age. As Dupree explains:

We’ve never seen a massive mass ejection from the surface of a star… It’s a totally new phenomenon that we can observe directly and resolve surface details with Hubble. We watch the evolution of stars in real time.

The night sky is overlapped by a blue sky diagram, with a red circle indicating Betelgeuse
Wide view of the region of the sky where Betelgeuse is located, in Orion’s ‘right shoulder’.
ESO/N. Risinger (skysurvey.org), CC BY

Surprising aftermath

One of the most interesting things we see about Betelgeuse in the wake of its superficial injury is a significant acceleration in its pulsation rate.

For more than 200 years, astronomers have faithfully tracked the brightening and dimming of Betelgeuse using its very constant 400-day cycle.

The massive ejection of material may have disrupted the star’s entire internal structure, potentially sloshing the inner layers and disrupting its typical pulsation rate.

Time will tell if it can recover to pre-emission pulsation as we continue to monitor Betelgeuse’s brightness closely.

While we don’t think Betelgeuse is ready to die yet, we wouldn’t know until about 640 years later. Thanks to the limitations of the speed of light, everything we see in the cosmos is a glimpse back in time – even the stars in our night sky.



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