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Doom’s Day: Betelgeuse May Soon Go Supernova

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Astronomers have determined the cause of the dramatic dimming observed last year and earlier this year of one of the brightest stars in the night sky, a colossus called Betelgeuse that appears to be on its way toward a violent death.

Based on Hubble Space Telescope observations, scientists said they believe Betelgeuse ejected a huge hot, dense cloud of material into space that cooled to form dust, shielding the star’s light and making it appear dimmer from the perspective of viewers on Earth.

Betelgeuse is classified as a red supergiant, the largest type of star. It is more than 10 times the mass of our sun. If it resided at the center of our solar system, its surface would extend to the planet Jupiter.

Scientists suspect Betelgeuse – pronounced “beetle juice” – is nearing the end of its life cycle when it will use up its nuclear fuel and explode, relatively soon in cosmic terms, in an event known as a supernova.

“Frankly, we don’t know for sure how soon Betelgeuse will go supernova,” astrophysicist Andrea Dupree, director of the Solar Stellar Planetary Sciences Division at the Harvard-Smithsonian Center for Astrophysics and leader of the research published this week in the Astrophysical Journal, said on Friday.

Whenever Betelgeuse does blow up, our planet Earth is far enough away so the explosion probably won’t impact Earth. Astrophysicists say we’d have to be within 50 light-years of a supernova for it to harm us. Betelgeuse is nearly 10 times this distance.

An explosion of a nearby star might leave Earth and its surface and ocean life relatively intact. But any relatively nearby explosion would still shower us with gamma rays and other high-energy radiation. … Also, the radiation from a nearby supernova could change our climate.

A supernova is a star explosion – destructive on a scale almost beyond human imagining. If our sun exploded as a supernova, the resulting shock wave probably wouldn’t destroy the whole Earth, but the side of Earth facing the sun would boil away. Scientists estimate that the planet as a whole would increase in temperature to roughly 15 times hotter than our normal sun’s surface. What’s more, Earth wouldn’t stay put in orbit. The sudden decrease in the sun’s mass might free the planet to wander off into space. Clearly, the sun’s distance – 8 light-minutes away – isn’t safe. Fortunately, our sun isn’t the sort of star destined to explode as a supernova. But other stars, beyond our solar system, will. What is the closest safe distance? Scientific literature cites 50 to 100 light-years as the closest safe distance between Earth and a supernova.

No supernova has been known to erupt at this close distance in the known history of humankind. The most recent supernova visible to the eye was Supernova 1987A, in the year 1987. It was approximately 168,000 light-years away.

Before that, the last supernova visible to the eye was was documented by Johannes Kepler in 1604. At about 20,000 light-years, it shone more brightly than any star in the night sky. It was even visible in daylight! But it didn’t cause earthly effects, as far as we know.

What about Betelgeuse? Another star often mentioned in the supernova story is Betelgeuse, one of the brightest stars in our sky, part of the famous constellation Orion. Betelgeuse is a supergiant star. It is intrinsically very brilliant.

What about Betelgeuse? Another star often mentioned in the supernova story is Betelgeuse, one of the brightest stars in our sky, part of the famous constellation Orion. Betelgeuse is a supergiant star. It is intrinsically very brilliant.

Such brilliance comes at a price, however. Betelgeuse is one of the most famous stars in the sky because it’s due to explode someday. Betelgeuse’s enormous energy requires that the fuel be expended quickly (relatively, that is), and in fact Betelgeuse is now near the end of its lifetime. Someday soon (astronomically speaking), it will run out of fuel, collapse under its own weight, and then rebound in a spectacular Type II supernova explosion. When this happens, Betelgeuse will brighten enormously for a few weeks or months, perhaps as bright as the full moon and visible in broad daylight.

When will it happen? Probably not in our lifetimes, but no one really knows. It could be tomorrow or a million years in the future. When it does happen, any beings on Earth will witness a spectacular event in the night sky, but earthly life won’t be harmed. That’s because Betelgeuse is 430 light-years away. 

How often do supernovae erupt in our galaxy? No one knows. Scientists have speculated that the high-energy radiation from supernovae has already caused mutations in earthly species, maybe even human beings. 

One estimate suggests there might be one dangerous supernova event in Earth’s vicinity every 15 million years. Another says that, on average, a supernova explosion occurs within 10 parsecs (33 light-years) of the Earth every 240 million years. So you see we really don’t know. But you can contrast those numbers to the few million years humans are thought to have existed on the planet – and four-and-a-half billion years for the age of Earth itself. 

The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars’ absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures.

And, if you do that, you’ll see that a supernova is certain to occur near Earth – but probably not in the foreseeable future of humanity.

Bottom line: Scientific literature cites 50 to 100 light-years as the closest safe distance between Earth and a supernova.

“It is likely not in our lifetimes. But, we do not know how a star behaves the week before, the night before it explodes,” Dupree added.

In a supernova, huge stars like Betelgeuse expel large amounts of heavy elements, including carbon, oxygen, calcium and iron, into space that become building blocks of new generations of stars.

Betelgeuse is located relatively near our solar system, about 725 light-years away. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

Its dimming began last October and by mid-February had lost more than two-thirds of its brilliance. It returned to its usual brilliance by April but may be dimming again, which researchers are working to confirm.

Recent observations of Betelgeuse have revealed that the star’s unexpected and significant dimming periods in late 2019 and early 2020 were most likely caused by the ejection and cooling of dense hot gases, and that the star may be going through another dimming period more than a year early.

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 naked eye, creating buzz that the star might be going supernova. Continued ultraviolet light spectrosopic observations with Hubble provided a timeline for researchers to follow, like breadcrumbs leading back through time to pinpoint the source of the mysterious dimming.

“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,” said Andrea Dupree, associate director of the Center for Astrophysics | Harvard & Smithsonian, and lead author on the study. “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.”

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. Credit: NASA, ESA, A. Dupree (CfA), and E. Wheatley (STScI)

And Betelgeuse held another surprise for scientists when Hubble observations revealed that the detected plasma was not ejected from the star’s rotational poles as predicted by stellar models. “Hubble observations suggest that material can be driven off from any part of the stellar surface,” said Dupree, adding 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 southern hemisphere alone. “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.”

Complementary observations using the STELLAr Activity Observatory—STELLA—measured changes in the velocity of the star surface as it rose and fell during the pulsation cycle, creating a rippling effect that may have propelled the outflowing plasma through the star’s atmosphere. “We saw all the absorption lines in the spectrum blue shifted and knew that the star was expanding,” said Klaus G. Strassmeier, director of cosmic magnetic fields at Leibniz-Institut fur Astrophysik Potsdam (AIP) and co-author on the study.

“When the dimming started, the blue shift became smaller and smaller and actually reverted to a redshift when the star was faintest. So we knew the dimming must have been related in one or another way to the expansion and contraction of the star’s photosphere, but it alone could not have caused such great dimming.”

An image from the Heliospheric Imager aboard 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. Credits: NASA/STEREO/HI

As the star moved into the daytime sky and out of the view of Hubble and STELLA, researchers turned to NASA’s Solar TErrestrial RElations Observatory—STEREO—to monitor the supergiant’s brightness. But summertime observations revealed a startling surprise: 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. “Our observations of Betelgeuse with STEREO confirm that the star is dimming again.

Betelgeuse typically goes through brightness cycles lasting around 420 days, and since the previous minimum happened in February 2020, this new dimming is over a year early,” said Dupree, who plans to observe Betelgeuse with STEREO again next year, during the star’s maximum, to monitor for unexpected outbursts.

This figure shows measurements of Betelgeuse’s brightness from different observatories from late 2018 to present. The blue and green points represent data from ground-based observatories. The gaps in these measurements happen when Betelgeuse appears in Earth’s day sky, making it impossible to take precise brightness measurements.

During this observation gap in 2020, NASA’s STEREO spacecraft — with measurements shown in red — stepped in to observe Betelgeuse from its unique vantage point, revealing unexpected dimming by the star. The 2018 data point from STEREO was found in the mission’s archival data and was used to calibrate STEREO’s measurements against other telescopes. Credit: Dupree, et al.

Intense interest surrounding Betelgeuse ignited late last year as the star continued to grow dimmer and dimmer, a behavior some scientists said signaled that the old star was about to go supernova.

As a variable star that expands and contracts, dimming and brightening over each cycle, “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,” said Dupree. “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. “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?”

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