Dimming of Betelgeuse Explained by Hubble Telescope — Dare to Know

The dimming of Betelgeuse, a bright red star, excited stargazers last year. Find out how the Hubble Space Telescope helped explain what happened.

As we told readers in a previous story, stargazers were excited about the dimming of Betelgeuse back around Christmas. Betelgeuse is a red super-giant star in the constellation Orion.

Betelgeuse is the reddish star that shapes Orion’s right shoulder in his stick figure in the night sky. It gets its name from the Arabic word for “armpit of Orion.”

Betelgeuse is both huge and very close to us. It’s about 725 light-years away, and if it was our Sun, it would engulf everything in the solar system out past Jupiter.

Betelgeuse Is the Tenth-Brightest Star in the Sky

Astronomers rank Betelgeuse as the tenth-brightest star in the sky. Orion is a very bright, prominent constellation, also containing the very bright star, Rigel. He’s a hunter and he a dog with him called Canis Major.

Canis Major contains the brightest star in the sky, Sirius. Stargazers are very fond of the extensive section of the night sky around Orion. It’s effortless to spot on a long winter’s night, and it’s full of fascinating objects.

Astronomers first noticed the dimming of Betelgeuse in December 2019. A team led by Professor Edward Guinan of Villanova University reported their discovery in The Astronomer’s Telegram.

Rumours that Betelgeuse Was About to Explode

Since astronomy tells us that red giant stars end their lives as supernovas, this caused a big sensation with amateur astronomers. Rumours started spreading that Betelgeuse was about to explode.

This would have given stargazers something spectacular to look at for the next few years. Betelgeuse would have been transformed from its current ruddy colour into a strikingly white star as bright as the full moon.

Sadly, it was not to be. Betelgeuse gradually regained its brightness and returned to normal by around April.

Betelgeuse Will end As Supernova, but Not For 100,000 Years

Most professional astronomers never bought into the supernova hype surrounding the dimming of Betelgeuse. The expert consensus was that, although Betelgeuse will indeed end up as a supernova, the red super-giant should last for another 100,000 years or so.

Betelgeuse has always been a variable star. We’ve known that since the 1830s. John Hershel noted that sometimes Betelgeuse was brighter than Rigel, even though it was dimmer most of the time.

That variability in size and brightness runs on a 420-day cycle. It led scientists to look to less dramatic explanations than that the star was about to blow. Even so, their ideas were more like educated guesses than settled science.

NASA Released a Series of Observations

New results from the Hubble Space Telescope have changed that. Last week, NASA released a series of observations from a study led by Professor Andrea Dupree of the Center for Astrophysics.

Professor Dupree and her colleagues have determined that the dimming of Betelgeuse was caused by something the star ejected into space. It appears to have been an enormous volume of scorching material that formed a dust cloud.

The ejected matter had enough density to block about a quarter of the starlight coming from Betelgeuse from our point of view. Astronomers, both professional and amateur, noticed the dimming but it took the Hubble’s resolution to confirm the cause.

Pattern Provided a History of the Dimming of Betelgeuse

The researchers reviewed several months of ultraviolet observations from the Hubble Telescope. From them, they could see a pattern that provided a history of the dimming of Betelgeuse.

Starting back in September, some hot dense matter had been moving through the atmosphere of Betelgeuse. Then, ground-based observatories began to notice that the bottom half of the star was dimming in early December.

As Professor Dupree explains, “With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere before the dust formed that caused the star to appear to dim.” She continued, “We could see the effect of a dense, hot region in the southeast part of the star moving outward.”

“Hot Dense Region Moving Outward”

The researchers have been studying Betelgeuse since early 2019. They’re conducting a three-year study into what causes the red super-giant’s regular cycle of varying size and brightness.

Looking at the star’s outer atmosphere with the Hubble Telescope offers clues. Hubble can see and study the spectrum of ultraviolet light.

Astronomers need that capability because the outer layers of Betelgeuse are so unimaginably hot that they don’t produce visible light. The star’s surface temperature is over 11,000˚ C.

Gradually Cooled Into Dust After a Few Million Kilometres

The hot dense material that Betelgeuse had ejected kept travelling in a straight line. It gradually cooled into dust after it had gone a few million kilometres.

It just so happens that the resulting cloud of dust was in our line of sight from Earth. The astronomers can tell that Betelgeuse didn’t change its usual cycle of variability, meaning that it didn’t get intrinsically smaller or dimmer.

The explanation for the mysterious dimming of Betelgeuse is simple. Something got in the way. If that seems disappointingly mundane, it may help if readers realize that this new knowledge helps scientists understand some new things.

Only Star Where Scientists Can Study Its Surface

Because Betelgeuse is such a prominent star, it’s the only one other than the Sun where astronomers can study its surface. Using Hubble, they can actually see features on the surface called convection cells. The cells brighten and darken as the star’s hot plasma rises and then falls back as it cools.

We have a vital connection to super-massive stars like Betelgeuse. Carl Sagan famously said, “We’re made of star-stuff.”

It’s only in these enormous stars that the heavier chemical elements can form. In particular, stars like Betelgeuse give off large quantities of carbon, which is the building block of all life.

We’re All Part of the Same Substance

The chemical elements that make up all of our bodies can only come from one place-the stars in the sky. We shouldn’t think of them as distant or separate from us. We’re all part of the same substance.

Professor Dupree and her team are on a bit of a break right now. Betelgeuse is in the daytime sky, so they haven’t been able to observe it for a few weeks.

That will change over the next few days as Betelgeuse returns to the night sky. Next year’s plan is to use NASA’s Solar Terrestrial Relations Observatory (STEREO) to monitor the star’s brightness.

Delve Deeper Into What Happens Inside Stars

As Betelgeuse moves into its expanding/brightening cycle, the team hopes to spot anything else the star unexpectedly disgorges. They want to delver more in-depth into what happens inside stars to prompt these strange emissions.

We always have more to learn if we dare to know.
Learn more:
Hubble Finds That Betelgeuse’s Mysterious Dimming Is Due to a Traumatic Outburst
Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse
Newborn Stars Bringing Forth Solar Systems
Betelgeuse Dimming but Not Ready to Blow
Where Do Heavy Metals Come From? (Not Ozzy Osborne!)

Originally published at https://daretoknow.ca on August 26, 2020.