Betelgeuse is nearly 50% brighter than normal. What is going on?

By | June 6, 2023

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The red supergiant Betelgeuse. His activity can be confusing and the current clearing is not helping. Credit: ALMA (ESO/NAOJ/NRAO)/E. O Gorman/P. Kervella

Whenever something happens with Betelgeuse, speculation about its explosion as a supernova proliferates. It would be nice if it did. We’re far enough away to be unaffected, so it’s fun to imagine the sky lighting up like this for months.

Now the red supergiant star has brightened by almost 50% and this raises the speculation again.

Betelgeuse will explode as a supernova. On this, there is universal agreement. But the question of when is less certain. The star’s behavior is confusing. How can puny humans find out?

Betelgeuse is not only a red supergiant, it is also a pulsating semiregular variable star. This means that there is some periodicity in its brightness changes, although the amplitudes can vary. It has a cycle of about 400 days in which its brightness changes. It also has a shorter 125-day cycle, another 230-day cycle, and a whopping 2,200-day cycle, all determined by your pulse. All of these cycles can make the star difficult to understand clearly.

A couple of years ago, Betelgeuse went dark and people wondered what that meant. It turns out that the star’s brightness hasn’t actually changed. Instead, the star had ejected material from its surface that cooled into a cloud of dust and blocked out the light. The episode is called “The Great Dimming”.

This graph shows what likely caused Betelgeuse to go dark for some time in 2019. Credits: NASA, ESA and E. Wheatley (STScI)

Now that it’s glowing, it’s attracting the attention of scientists again. They want to know what evolutionary stage he’s in and what all this activity means. New research shows it could explode as a supernova sooner than expected.

The new paper is “The evolutionary stage of Betelgeuse inferred from its pulsation periods” and the lead author is Hideyuki Saio of the Astronomical Institute, Graduate School of Science, at Tohoku University in Japan. Monthly Notices of the Royal Astronomy Society has accepted the article for publication and a version of the article is available at arXiv prepress server.

The juicier parts of new research often grab the headlines. There is no point in railing against this. That’s the way humanity goes.

We don’t blame Dr. Eldridge. She’s not wrong. It’s just that the paper says it’s just one possible outcome. He outlines many others.

In their paper, the authors claim that Betelgeuse could be the Milky Way’s next supernova, regardless of which of their findings turn out to be true. “We conclude that Betelgeuse is in the late stage of core carbon burning and a good candidate for the next galactic supernova,” they write.

As a red supergiant, Betelgeuse has left the main sequence. During its long history of 8 to 8.5 million years, it used large amounts of hydrogen by fusing it into helium and releasing the mass lost from that fusion as energy. (Thanks Einstein.) That means it’s no longer fusing hydrogen into helium like the sun does. When stars like Betelgeuse lose mass, their gravity can no longer contain their outward pressure and they expand into a bulkier envelope. Then, despite the loss of mass, they grow in size.

This diagram shows the scale of the red supergiant Betelgeuse and its circumstellar medium relative to that of the solar system. Credits: L. Calada, European Southern Observatory (ESO)

After stars like Betelgeuse leave the main sequence and no longer fuse hydrogen to helium in their cores, things change dramatically. During the subsequent helium fusion phase, carbon accumulates in their cores. Then they begin a period of burning the central carbon which produces other elements. The authors of the new paper say Betelgeuse is in the latter stages of that period.

But how late? How much time is left? There is no exact answer for this yet.

“Despite the relatively small distance from the Earth, and in some sense because of it, it has been difficult to obtain tight constraints on the distance, luminosity, radius, current and masses of the zero age main sequence (ZAMS) and information on the internal rotation state and associated admixture and thus on the evolutionary status of Betelgeuse and when it might erupt,” write the authors of a new Betelgeuse review. ZAMS is especially important for understanding the evolutionary stage of particular stars. It is fundamental, although not solely responsible.

But the study has some solid possibilities.

The work is a combination of observations and models that each fit the observations in different ways. It’s a complicated business, which is why headlines claiming it could explode in dozens of years are a bit misleading. The nuance rarely attracts attention.

This image is based on data from the Herschel mission and shows the circumstellar medium (CSM) surrounding Betelgeuse as it accelerates through space. There is prominent arc shock at 7 arcmin, evidence of its motion. There is also another feature in the 9 arcminute CSM that could be evidence of a past melting or material ejection from Betelgeuse. Betelgeuse is complex and difficult to understand. Credit: Decin et al. 2012

The central period of carbon burning has several phases. The difficulty in determining when Betelgeuse will go supernova stems in part from determining which of these stages it is in. Betelgeuse pulsates, ejects material, rotates, and what’s more, it’s a runaway star accelerating through space. Its distance from us is also a matter of debate. “Although it is only 200 parsecs from Earth, and thus can be spatially resolved with appropriate instrumentation, uncertainties about its distance remain a critical obstacle to deeper understanding,” the Betelgeuse review explains.

What caught everyone’s attention are these two sentences from the research: “According to this figure, the nucleus will collapse in a few decades after carbon depletion. This indicates that Betelgeuse is a very good candidate for the next galactic supernova, which occurs very close to us.

This is the figure they are talking about.

This study figure shows the abundance of different elements in Betelgeuse. Elemental abundances are like a fingerprint or snapshot of what is happening inside the core, at what stage of the stars’ carbon burning, and when it will explode. Melt products from the core are periodically pulled from the core to the surface by convection, giving the researchers a glimpse into the core. But determining when it will explode also depends on knowing the star’s initial mass, how fast it rotates, and a myriad of other factors, all difficult to determine to varying degrees. Credit: Saio et al. 2023

But what has not attracted so much attention is the following part of the document.

‘Indeed, the exact evolutionary stage cannot be determined, because surface conditions hardly change in the late stage near carbon depletion and beyond,’ the researchers write. Astronomers can only see the surface, but it’s what happens deep inside the star that tells the story.

The authors of the article are really saying that according to observations, data and models, Betelgeuse could explode sooner than previously thought. But and this is crucial, they do not know in which phase of the combustion of the carbon of the core the star is. Carbon burning could continue for a long time, according to some of the models that fit the data.

But not everyone agrees that Betelgeuse is even in the middle stage of carbon burning. The authors of the Betelgeuse review say the star is still in the helium phase. “Because the core helium burn is much longer than the subsequent burn phases, Betelgeuse is most likely in the core helium burn. The pulsation period likely constrains the radius and distance and evolutionary state to the core helium burn.” core helium,” they write, while acknowledging that there are “arguments to the contrary.”

Another way researchers have tried to determine the timing of the Betelgeuse supernova explosion is by comparing its periodic pulsations with patterns of itself.

This research figure presents four patterns that correspond to four cycles or periods of Betelgeuses (alpha Ori). If you’re not an astrophysicist, it’s confusing. (I’m not one, and I’m confused.) But it helps illustrate the complexity behind predicting the Betelgeuse explosion and the uncertainty. Credit: Saio et al. 2023

When it does eventually explode, and no one disagrees with its eventual explosion as a supernova, it’s not likely to produce a lethal gamma-ray burst the way some supernovae do. And while it will eject material and produce powerful X-rays and UV radiation, we’re too far away to be affected. Instead, it will be a light show visible to all of humanity that will change the constellation Orion forever. Scientists say it will likely leave behind a neutron star, possibly a pulsar that will be visible for millions of years. The entire event, from start to finish, will be an unprecedented opportunity to study stellar evolution, supernovae and stellar remnants. Scientists will be able to work back from the explosion to all the research done and all the observations and data and figure out where they were right and where they were wrong. Old Betelgeuse will teach them a lot.

The supernova shock wave will arrive in about 100,000 years and will be easily deflected by our sun’s solar magnetosphere. The biggest effect on Earth will be an increase in cosmic rays hitting our upper atmosphere.

Most of us will see this calamitous outburst and become enraptured by the power of nature, we hope, while others will degenerate into weird conspiracy theories or quasi-religious, pseudo-scientific, cult-like reverence.

If, that is, humanity is still present when the blessed event occurs.

More information:
Hideyuki Saio et al, The evolutionary stage of Betelgeuse inferred from its pulsation periods, arXiv (2023). DOI: 10.48550/arxiv.2306.00287

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