Mysterious obscuration of Betelgeuse explained? | Space



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

This 3-panel artist’s concept illustrates new research, explaining why the bright red super-giant star Betelgeuse suddenly faded for several months in late 2019 and early 2020. In Panel 1, a glowing, hot plasma drop is ejected from the star. In panel 2, the exiting expelled gas expands rapidly outward and cools to form a huge cloud of obscuring dust. In panel 3, the huge cloud of dust partially blocks the light of Betelgeuse. Image via NASA / ESA / E. Wheatley (STScI) / CfA.

For us sky watchers it was a lot of fun – and totally shocking – when the well-known bright star Betelgeuse unexpectedly dimmed in late 2019 and early 2020. Betelgeuse was such a sight. constant in the night sky throughout our lifetimes (and some of us have been around for quite some time), glowing with a red light at the shoulder of the easy-to-see constellation Orion the Huntsman. The gradation of Betelgeuse was even more exciting as it is a well known fact that this star will one day explode. Was Betelgeuse’s sudden gradation a sign that she would explode soon? Speculation raged for weeks as we looked towards a weaker Betelgeuse than ever before. The star did not explode. In fact, by February it was starting to clear up again. Then last week – August 13, 2020 – scientists published a new study suggesting that Betelgeuse’s sudden dimming was most likely caused by the ejection and cooling of dense hot gases. Meanwhile, as I write this, it looks like Betelgeuse is going down again, about a year ahead of schedule.

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

Between October and November 2019, the 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 the southern hemisphere of Betelgeuse, as if something was blocking light in that region of the star. By February 2020, the star had lost more than two-thirds of its brightness, a gradation visible even to the naked eye, creating the buzz that the star could become a supernova. Continuous spectroscopic observations of ultraviolet light with Hubble provided researchers with a timeline to follow, like bread crumbs traveling back in time to locate the source of the mysterious gradation.

Andrea Dupree is associate director of the Harvard & Smithsonian Center for Astrophysics and lead author of the study, which was published Aug. 13 in the magazine 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 outward through the extended atmosphere of Betelgeuse. We believe this gas cooled millions of miles outside the star to form the dust that blocked the southern part of the star pictured in January and February.

The material was two to four times brighter than the star’s normal brightness. And then, about a month later, the southern part of Betelgeuse visibly darkened as the star faded away. We believe it is possible that a dark cloud resulted from the exit that Hubble detected. Only Hubble gives us this proof which led to the gradation.

Read more: Will the Betelgeuse star ever explode?

This spectral plot is based on observations from the Hubble Space Telescope from March 2019 to February 2020. Hubble recorded a surprising explosion in the atmosphere of the nearby red supergiant star Betelgeuse. Magnesium II emission measurements were used to trace motion in the star’s pulsating atmosphere. The Hubble Space Telescope’s imaging spectrograph captured a dramatic increase in the brightness of the magnesium emission in October 2019, in the south-eastern region of the star, as indicated by the white circle. (Betelgeuse is close enough and large enough for Hubble to resolve the star’s enormous disc.) This traumatic event was different from what is normally observed during the star’s 420-day pulse period. At the same time in October, the star suddenly began to darken. This fading continued until February 2020, when Hubble’s ultraviolet spectral data returned to normal. The explosion is believed to have ejected a cloud of hot plasma that cooled to form dust that blocked a significant portion of the star’s light for a few months. Hubble’s long baseline of star monitoring helped put the pieces of the puzzle together. Image via NASA / ESA / A. Dupree (CfA) / E. Wheatley (STScI) / CfA.

The scientists’ models had suggested that – in this situation – the plasma would have to be ejected from the star’s rotating poles. Hubble’s observations have shown not, however. Dupree said:

Hubble’s observations suggest that the material can be chased from any part of the stellar surface.

Dupree added that recent activity on Betelgeuse was not normal for the star. Dupree noted that Betelgeuse is losing mass at a rate 30 million times greater than the sun, but that recent activity has resulted in a loss of about twice the normal amount of material from the southern hemisphere of the ‘star alone. She says:

All stars lose material to the interstellar medium, and we don’t know how this material is lost. Is it a gentle wind blowing all the time? Or does it come in spurts? Perhaps with an event such as we discovered on Betelgeuse? We know that other hotter and brighter stars lose material and it quickly turns to dust, which makes the star appear much paler.

But, for over a century and a half, this has not happened in Betelgeuse. It is truly unique.

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

The star Betelgeuse and its constellation Orion are behind the sun as seen from Earth in early summer in the northern hemisphere. They always come back to our morning skies around the end of July and the beginning of August. While Betelgeuse was hidden behind the sun for terrestrial observers, scientists turned to NASA’s solar survey observatory – STEREO – to monitor the star’s brightness. These observations revealed another surprise, the scientists said: a more unexpected gradation.

Between late June and early August 2020, STEREO observed Betelgeuse for five separate days, measuring the relative luminosity of the star compared to other stars. Dupree said:

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

Betelgeuse is a variable star, although its rising and falling brightness is not noticeable to casual observers. It typically goes through light cycles lasting around 420 days.

Since the previous low occurred in February 2020, this new gradation is more than a year ahead, the scientists commented.

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

People still want to know if Betelgeuse is going to explode. It is an old star and a supergiant star, so the answer to this question is surely yes. When Betelgeuse dimmed so visibly in late 2019 and early 2020, some scientists agreed that this could be a sign that the star was on the verge of becoming a supernova. These scientists commented in their statement:

Betelgeuse is a bright star in our galaxy, near the end of its life, likely to become a supernova. When the star became very faint in February 2020, it was the weakest ever since measurements began over 150 years ago. The darkness was evident to everyone looking at the constellation Orion; it was very odd, Betelgeuse was almost missing.

At 725 light years away, the light – and the gradation – 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 leading up to its explosion, and there were worrying predictions that Betelgeuse was poised to become a supernova.

There’s a chance it won’t explode in our lifetime, but who knows?

Bottom line: An explanation of Betelgeuse’s mysterious gradation in late 2019 and early 2020.

Source: Spatial resolution ultraviolet spectroscopy of the Betelgeuse great gradation

Via Harvard & Smithsonian Center for Astrophysics

Deborah Byrd



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