Mysterious dimming of Betelgeuse explained?

Observations with the Hubble Space Telescope revealed a large amount of dense hot gas moving outwards through Betelgeuse’s extended atmosphere. This gas might have cooled and formed a dust cloud that partially blocked the star’s light as seen from Earth, earlier this year.

3 panels showing an artist's concept of a red star emitting a cloud of dust and then being partially blocked by it.

This 3-paneled artist’s concept illustrates new research, explaining why the bright red supergiant star Betelgeuse suddenly became fainter for several months during late 2019 and early 2020. In panel 1, a bright, hot blob of plasma is ejected from the star. In panel 2, outflowing expelled gas rapidly expands outward and cools to form an enormous cloud of obscuring dust. In panel 3, the huge dust cloud partially blocks Betelgeuse’s light. Image via NASA/ ESA/ E. Wheatley (STScI)/ CfA.

For us skywatchers, it was lots of fun – and entirely shocking – when the well-known bright star Betelgeuse unexpectedly dimmed in late 2019 and early 2020. Betelgeuse has been such a steadfast sight in the night sky throughout our lives (and some of us have been around awhile), shining with a red light at the shoulder of the easy-to-see constellation Orion the Hunter. The dimming of Betelgeuse was even more exciting because it’s a well-known fact that this star will someday explode. Was the sudden dimming of Betelgeuse a sign that it would explode soon? Speculation raged for weeks, as we gazed toward a fainter Betelgeuse than we’d ever seen before. The star did not explode. In fact, by February, it was starting to brighten again. Then last week – on August 13, 2020 – scientists released a new study suggesting that the sudden dimming of Betelgeuse was most likely caused by the ejection and cooling of dense hot gases. In the meantime, as I write this, it appears Betelgeuse is dimming once more, about a year earlier than expected.

A statement from the Harvard & Smithsonian Center for Astrophysics (CfA) explained:

Between October and November 2019, Hubble Space Telescope observed dense, heated material moving outward through the star’s extended atmosphere at 200,000 miles per hour. The following month, several ground-based telescopes observed a decrease in brightness in Betelgeuse’s southern hemisphere, as if something was blocking light in this region of the star. By February 2020, the star had lost more than two-thirds of its brilliance, a dimming visible even to the unaided eye, creating buzz that the star might be going supernova. Continued ultraviolet light spectroscopic observations with Hubble provided a timeline for researchers to follow, like breadcrumbs leading back through time to pinpoint the source of the mysterious dimming.

Andrea Dupree is associate director of the Harvard & Smithsonian Center for Astrophysics and lead author of the study, which was published on August 13 in the peer-reviewed The Astrophysical Journal. She said in the scientists’ statement:

With Hubble, we had previously observed hot convection cells on the surface of Betelgeuse and in the fall of 2019 we discovered a large amount of dense hot gas moving outwards through Betelgeuse’s extended atmosphere. We think this gas cooled down millions of miles outside the star to form the dust that blocked the southern part of the star imaged in January and February.

The material was two to four times more luminous than the star’s normal brightness. And then, about a month later the south part of Betelgeuse dimmed conspicuously as the star grew fainter. We think it possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence that led up to the dimming.

Read more: Will the star Betelgeuse explode someday?

This spectral plot is based on Hubble Space Telescope observations from March 2019 to February 2020. Hubble recorded a surprising outburst in the atmosphere of the nearby red supergiant star Betelgeuse. Measurements of emission from magnesium II were used to trace motion in the star’s pulsating atmosphere. Hubble’s Space Telescope Imaging Spectrograph captured a dramatic increase in the brightness of magnesium emission in October 2019, in the southeast region of the star, as outlined by the white circle. (Betelgeuse is close enough and big enough for Hubble to resolve the star’s enormous disk.) This traumatic event was different from what is normally seen in the star’s 420-day pulsation period. At the same time in October, the star abruptly began dimming. This fading continued until February 2020, at which time the Hubble ultraviolet spectral data had returned to normal. The outburst is suspected to have ejected a cloud of hot plasma that cooled to form dust that blocked out a significant portion of the star’s light for a few months. Hubble’s long baseline of monitoring the star helped put the puzzle pieces together. Image via NASA/ ESA/ A. Dupree (CfA)/ E. Wheatley (STScI)/ CfA.

Scientists’ models had suggested that – in this situation – the plasma should be ejected from the star’s rotational poles. The Hubble observations showed it was not, however. Dupree said:

Hubble observations suggest that material can be driven off from any part of the stellar surface.

Dupree added that recent activity on Betelgeuse was not normal for this star. Dupree noted that Betelgeuse is losing mass at a rate 30 million times higher than the sun, but that recent activity resulted in a loss of roughly two times the normal amount of material from the star’s southern hemisphere alone. She said:

All stars are losing material to the interstellar medium, and we don’t know how this material is lost. Is it a smooth wind blowing all the time? Or does it come in fits and starts? Perhaps with an event such as we discovered on Betelgeuse? We know that other hotter, luminous stars lose material and it quickly turns to dust making the star appear much fainter.

But, in over a century and a half, this has not happened to Betelgeuse. It’s very unique.

An image from NASA’s STEREO spacecraft shows the star Betelgeuse, circled. For several weeks in 2020, STEREO was the only observatory making measurements of Betelgeuse because of the spacecraft’s unique position in space. Between late June and early August 2020, STEREO observed Betelgeuse on five separate days, measuring the star’s relative brightness in comparison to other stars. Image via NASA/ STEREO.

The star Betelgeuse, and its constellation Orion, are behind the sun as seen from Earth in early Northern Hemisphere summer. They always return to our early morning sky around late July and early August. While Betelgeuse was hidden behind the sun for earthly observers, scientists turned to NASA’s Solar TErrestrial RElations Observatory – STEREO – to monitor the star’s brightness. Those observations revealed another surprise, the scientists said: more unexpected dimming.

Between late June and early August 2020, STEREO observed Betelgeuse on five separate days, measuring the star’s relative brightness in comparison to other stars. Dupree said:

Our observations of Betelgeuse with STEREO confirm that the star is dimming again.

Betelgeuse is a variable star, though its rising and falling in brightness isn’t noticeable to casual observers. It typically goes through brightness cycles lasting around 420 days.

Since the previous minimum happened in February 2020, this new dimming is over a year early, the scientists commented.

Dupree said she plans to observe Betelgeuse with STEREO again next year, during the star’s maximum, to monitor for unexpected outbursts.

People always want to know if Betelgeuse will explode. It is an old star and a supergiant star, and so the answer to that question is surely yes. When Betelgeuse dimmed so noticeably in late 2019 and early 2020, some scientists agreed it might be a sign that the star was about to go supernova. These scientists commented in their statement:

Betelgeuse is a bright star in our galaxy, near the end of its life, that is likely to become a supernova. When the star became very faint in February 2020, this was the faintest that it had ever been since measurements began over 150 years ago. The dimming was obvious to everyone when looking at the constellation Orion; it was very weird, Betelgeuse was almost missing.

At 725 light-years away, light – and dimming – seen from Betelgeuse today on Earth left the star in the year 1300. Dupree said:

No one knows how a star behaves in the weeks before it explodes, and there were some ominous predictions that Betelgeuse was ready to become a supernova.

Chances are, however, that it will not explode during our lifetime, but who knows?

Bottom line: An explanation for the mysterious dimming of Betelgeuse in late 2019 and early 2020.

Source: Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse

Via Harvard & Smithsonian Center for Astrophysics

Deborah Byrd