A little over a year ago, the bright star Betelgeuse faded in a way previously never seen. While captivating observers with its 50% drop in light, it also frustrated scientists. Located more than 500 light-years away, this giant star can be observed in just enough detail that we knew something weird was going on, but we couldn’t tell what. Early theories pointed to there either being a dusty outburst, giant starspots had formed, or maybe it had just dropped in temperature. It seemed that dust was the most likely culprit, with the color of Betelguese not changing as you’d expect with a temperature drop and with the changes over time looking more dust-like than starspot-like.

But looking more like something, and actually being that something, those aren’t the same.

A new paper accepted to The Astrophysical Journal with lead author Graham Harper uses five years of observations to understand this star in a context that looks at before, during, and after the dimming event. They also start from scratch — calculating their own values for the star’s temperature, size, and location in a consistent way.

With this new analysis, they found Betelgeuse’s behavior can be reproduced in models if the star’s outer photosphere is treated as multiple components of different temperatures. This is consistent with prior observations by the Very Large Telescope in Chile and basically means the outer layers of the star are like nesting balls that aren’t entirely solid, and each shell has different holes that allow the shells beneath to be seen – or not – at different times.

According to the paper: These suggest that the cooling of a large fraction of the visible star has a dynamic origin related to the photospheric motions, perhaps arising from pulsation or large-scale convective motions.

That’s a lot of ideas packed into one sentence. Let’s break it down. 

The first thing to note is they point out that we saw dimming of the visible part of the star and that doesn’t mean the entire star dimmed. They also match that dimming to temperature. This is like a hot fire burning brighter than a cool fire. But if the different layers of the star’s photosphere have different temperatures, and sometimes we see more of some layers than others, it’s possible that the motions of those layers caused us to see one of the cool layers covering a large part of the star we could see. Since only part of the star was cooler, we still saw light from the hot parts, leading to confusion about how the star’s temperature may have changed. 

Depending on how the star was observed, different teams would come to different conclusions, and the dust theory would seem to fit if you didn’t see the place to place temperature changes. This new way of looking at Betelgeuse also explains the timing of this great dimming.

Betelguese is a variable star, and it is pulsing with multiple beats that aren’t always in sync. One of those pulsations is a 430-day beat, and the other is 5.8 years. Like windshield wipers that aren’t in sync, these pulsations do sometimes line up, and last January was one of those times, with the both pulses working together to dim the star. While this wasn’t expected to make Betelguese as faint as we saw — and we certainly hadn’t seen this before — these combined motions could have led to a large fraction of the visible star being cooler.

And the images seem to bear this out. We can see that a large fraction of Betelguese appears fainter.

Is this the final answer? No idea. But this paper does one very important thing: it calls out the need to gather all the data from all the telescopes and analyze it together. There is a tendency for researchers to each take their own data, process it, publish their results, and at a certain level say “mine mine mine”. But this is a bit like blind men each trying to understand an elephant based on what they feel in front of them. Each person has a true and valid observation, but the whole can only be understood by combining all the observations.

Here is hoping that this paper’s authors can pull together all the Betelguese researchers and their data, and work together to understand this giant red star of confusion.

More Information

Kavli IPMU press release

“Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA,” Meridith Joyce et al., 2020 October 13, The Astrophysical Journal (preprint on arxiv.org)