What’s Happening at Betelgeuse?

Astronomers are puzzled by the dimming of one of the brightest stars in the sky, Betelgeuse in Orion.

Hertzsprung-Russell (H-R) Diagram plots luminosity vs temperature.

Amateur astronomers quickly learn to identify Betelgeuse, the right shoulder in the constellation Orion. After getting over the initial chuckle that a star could be named beetle-juice (more accurately, betel-jooz, an Arabic name also amusing, meaning ‘armpit of Orion’), the new astronomer learns it is a red giant star 650 light-years away. Astrophysics students learn that red giants are fated to blow up as supernovas, according to stellar evolution theory. They learn the Hertzsprung-Russell diagram that astronomers use to connect the dots between star types, showing how one type evolves into another over billions of years. The diagram was not made for stellar evolution theory, though; it was merely a diagram to conveniently graph luminosity to temperature. Stellar evolution theory followed later.

However, astronomers are not quite sure about the recent observations of the famous red giant. Evan Gough at Universe Today has been following the news about unexpected dimming out there:

Betelgeuse is Continuing to Dim! It’s Down to 1.506 Magnitude (Universe Today, Jan 22).

Betelgeuse keeps getting dimmer and everyone is wondering what exactly that means. The star will go supernova at the end of its life, but that’s not projected to happen for tens of thousands of years or so. So what’s causing the dimming?

Betelgeuse Just Keeps Getting Dimmer, And We Have No Idea Why (Universe Today , Jan 23). Theory tells us that red giants will swell outward, then collapse.

Or could it be something else? We know a lot about stars, but we don’t know everything. We’ve also never been able to observe any other red super-giants the way we can with Betelgeuse.

Scientists enjoy a good surprise, because it usually means more discoveries are coming. It also relieves boredom of thinking everything has already been figured out. Some are wondering if we are about to see a close-by supernova explosion that could rival the moon in brightness. Others are considering more mundane explanations, that maybe interstellar clouds are interfering and causing the dimming. Some stars are known to undergo cycles of brightening and dimming, but usually not red giants like Betelgeuse unless they are near the end of their lives.

Our sun is a speck compared to the red supergiants.

Betelgeuse: star’s weird dimming sparks rumours that its death is imminent (Daniel Brown, astronomer, at The Conversation). It could explode now or any time in the next 100,000 years, Brown says.

But this current substantial dimming is not necessarily a sign of its imminent death. That’s because, at this stage, we do not know enough about how a star’s brightness develops before such an event. That said, this makes Betelgeuse rather interesting for astronomers.

If it did occur, it would become the brightest supernova ever observed. In a matter of days, it would become as bright as the full moon, be visible during day time and be bright enough at night to cast shadows on Earth.

Nobody knows what will happen, if anything. If Betelgeuse were to explode, it would raise questions about why we are around at this time to witness such a rare phenomenon. But it would also be an opportunity to learn more about supernovas, and compare the observations with models.

Betelgeuse, the “armpit of Orion” is a familiar sight in the winter sky. Photo by David Coppedge

Update 2/26/2020: Nature says that Betelgeuse has started to brighten up again. Many stars are cyclical. Perhaps Betelgeuse has several modes of oscillation.

Stellar Evolution Theory Evolves

Astronomers divide supernovas into various types and subtypes (e.g., Type 1a, thought to represent material from a binary system falling onto a white dwarf). Type 1a supernovas have been important “standard candles” for measuring vast distances in space, but occasionally corrections need to be made to fit observations with theory. These articles show that theories are never finished:

Modeling a superluminous supernova (Science Magazine). Keith Smith says, “Superluminous supernovae can be up to 100 times brighter than normal supernovae, but there is no consensus on how such bright transients are produced.” He presents a recent model correction to explain this type.
Mysteriously bright supernova may have smashed up a huge gas cloud (New Scientist). Leah Crane shows how theories need to evolve to fit “weird” situations and “strange variants” of supernova types that astronomers thought they understood.

The sort of supernova that creates enough iron to match this one is called a type Ia, but those are usually 100 times dimmer than SN 2006gy. The best way the researchers found to make a type Ia supernova 100 times brighter is for it to slam into a cloud of material as it explodes, converting the kinetic energy of the blast into light.

The scenario that Jerkstrand and his team found that best matches SN 2006gy starts with a pair of stars orbiting one another in a shared cloud of gas. As the two spiralled towards one another, the gas was blown off, creating a cloud around the stars. When they collided, they blew up and the blast crashed through that cloud in an explosion of light.

“Scenarios” are stories trying to compare theory to observations, but if the observations cannot be made, the scenario is little more than an idle tale. As one astronomer quipped, ‘No observation should be considered valid until it has been confirmed by theory.” (That’s backward, folks.)

Theories of stars can be very sophisticated, with detailed mathematical equations describing their structure and expected behavior. Still, a model is only a simulation of reality – not reality itself. In astrophysics, unlike in biological evolution “scenarios,” scientists can apply known physical laws to the observations. They can try to refine their models, but never reach absolute certainty. “Strange variants” continue to arise. When enough anomalies arise within a paradigm, a scientific revolution may follow.

Nobody knows what will happen to Betelgeuse, but it’s intriguing to observe it and try to understand it. That’s a legitimate human enterprise. Invoking unseen occult forces is not.