Betelgeuse is dimmed again

Just when you thought it was safe to go back to ignoring Betelgeuse, the red giant star began to reappear. After its first round of dimming, and then clear, Betelgeuse has now once again started dimming again.

Moreover, this new dimming is consistent with the current cycle of Betelgeuse variation for brightness – so, again, the star moves into the spotlight.

Betelgeuse, 700 light-years away in the constellation of Orion and one of the brightest stars in our sky, is also one of the most interesting. That’s because it’s very old, about 8 to 8.5 million years old, and practically at the time of death, for a star.

It is thought to be between 10 and 25 times the mass of the Sun, and lived most of its life as a hot, blue-white massive star. Now the days of their sequence of fusing hydrogen into the stellar core are done; Betelgeuse ran out of hydrogen some time ago, and it now fuses helium into carbon and oxygen.

Once made of helium, it will fuse heavier and heavier elements, resulting in a buildup of iron in the nucleus that will eventually make the star supernova. But while dramatic dimming is just expected for the Great Kaboom, this time is not quite upon us yet. It will be a few tens of thousands of years, astronomers predict.

The dimming event that took place between September 2019 and February 2020, nicknamed the Great Fainting, was quite dramatic, dimming the brightness of the star by almost 25 percent.

Betelgeuse is a semi-regular variable star, which means that the light fluctuates slightly on regular cycles. The longest of these cycles is about 5.9 years. Another is 425 days. The Great Fainting was very close to the minimum of both of these cycles, but it turned out that they did not have much to do with the event.

Astronomers are now pretty sure it was just a snowfall: Betelgeuse threw out a bunch of material that it partially hid for a while; not uncommon for a star of such a reverent age.

“We see this all the time in red supergiants, and it’s a normal part of their life cycle,” said astronomer Emily Levesque of the University of Washington in March.

“Red supergiants will sometimes smear material from their surfaces, which condenses around the star like dust. As it cools and disappears, the dust grains will absorb some light towards us and block our view.”

That, that’s what solves mystery. But the new dimming will also need to be investigated. Although not as dramatic as the Great Fainting, it is inconsistent with the star’s variability cycles.

The next peak of Betelgeuse brightness will take place… now, no-ish, August and September 2020. Therefore, it should gradually become clear throughout the year.

The brightness of the star is actually a bit tricky to follow, as Betelgeuse’s position in our sky moved behind the Sun from May to early August. But NASA’s Solar and Terrestrial Relations Observatory (STEREO) in a solar orbit that runs behind Earth, which means that it could keep an eye on Betelgeuse for part of the time that it was hidden from Earth’s view.

stereo betelgeuseSTEREO’s view of Betelgeuse. (NASA / STEREO / HI)

And, from May to July, when STEREO observed it, the star was not bright. The opposite.

“Surprisingly, instead of continuing to increase or increase levels in brightness, Betelgeuse decreased from ~ 0.5 mag from mid-May to mid-July,” wrote a team of scientists led by Andrea Dupree of the Harvard Smithsonian Center for Astrophysics in an Astronomer Telegram broadcast.

“By stretching the STEREO observations, the star expands at a rate of 5 mmag / day.”

The good news is that Betelgeuse is visible in our skies again, so more observations can be made. STEREO’s Heliospheric Imager detects the star’s brightness in visible light, but more instruments can detect temperature changes, for example – to confirm or exclude solar pot activity – and whether the star changes in size, as found in the case of the Great Fainting.

According to the 425-day cycle, Betelgeuse was next due to dim in April 2021. But, apart from its known cycles, the star can be quite unpredictable, and has complex variations in its light that we simply do not quite understand.

So, maybe this premature dimming could actually be luminous, and help us understand what’s going on inside what looks like a very messy, unquiet star indeed. And in turn, this could help us to understand the processes that take place at the end of the life of massive stars, in their dying years.

“It will be important,” the researchers wrote, “to track Betelgeuse through 2020/21.”