For more than 130 days astronomers had been monitoring a dying red supergiant located more than 120 million light-years from Earth. They hoped to catch the doomed star in the process of collapsing into a supernova — the biggest type of explosion in the universe — and the faint of most such massive stars at the end of their lifecycle. That’s exactly what happened, much to everyone’s delight as science is now much richer after astronomers learned new things that occur in the moments leading to the great kaboom.
Using two Hawaiian telescopes – the University of Hawaiʻi Institute for Astronomy Pan-STARRS on Haleakalā, Maui and W. M. Keck Observatory on Mauna Kea, Hawaii Island – astronomers first detected the doomed star, known as SN 2020tlf, in 2020 after they were tipped off by a huge amount of light radiating from the cosmic object. Just a few months later, the star lit up the sky when it went supernova. This was the first time that scientists observed this phenomenon in real-time.
“It’s like watching a ticking time bomb. We’ve never confirmed such violent activity in a dying red supergiant star where we see it produce such a luminous emission, then collapse and combust, until now,” says senior author Raffaella Margutti, an associate professor of astronomy at UC Berkeley.
The distant SN 2020tlf belongs to the red supergiant class, which represents the largest stars in the universe whose volume can exceed a thousand times the radius of the sun. Unlike main-sequence stars like the sun, which mainly convert hydrogen into helium via nuclear fusion, red giants can fuse heavier elements like carbon due to their cores becoming hotter and more pressurized as they run out of fuel. Red giants have a low surface temperature hovering around 4,100 degrees Kelvin — that’s very cool for a star and makes them shine with a red color, hence the name.
At the very end of its lifetime, a red supergiant will fuse increasingly heavier elements until the star contains a core of iron. At this point, fusion stops and the star collapses under its own gravity. At this moment, the stars generate a Type II supernova explosion that can outshine entire galaxies.
Every year, astronomers detect dozens of supernovae across the sky but only in the aftermath. Until now, they’ve never seen the entire process play out in real-time, which is why SN 2020tlf is such a spectacular find.
Writing in The Astrophysical Journal, the scientists reported a never-before-seen phenomenon. In the days leading up to the supernova, SN 2020tlf erupted in bright flashes of light generated by giant spires of hot gas. They also observed a dense cloud of gas surrounding the star at the time of its explosion, likely the same kind of material violently ejected in the prior months.
These findings suggest that red supergiants likely undergo significant changes to their internal structure before they collapse. More evidence of this behavior is needed before scientists draw any further conclusions, which is why surveys like the Young Supernova Experiment (YSE), which led to the discovery of SN 2020tlf, will be ramped up.
“I am most excited by all of the new ‘unknowns’ that have been unlocked by this discovery,” says Wynn Jacobson-Galán, an NSF Graduate Research Fellow at UC Berkeley and lead author of the study. “Detecting more events like SN 2020tlf will dramatically impact how we define the final months of stellar evolution, uniting observers and theorists in the quest to solve the mystery on how massive stars spend the final moments of their lives.”
In late 2019 and early 2020 Betelgeuse, a red supergiant in the constellation of Orion, made headlines when it underwent a period of extreme dimming. This dip in brightness for the star, which is usually around the tenth brightest in the night sky over Earth, was so extreme it could even be seen with the naked eye.
Some scientists even speculated that the orange-hued supergiant may be about to go supernova, an event which would have been visible in daylight over Earth for months thanks to its power and relative proximity–700 light-years from Earth. Yet, that supernova didn’t happen and Betelgeuse returned to its normal brightness.
This left the ‘great dimming’ of Betelgeuse–something never seen in 150 years of studying the star–an open mystery for astronomers to investigate.
Now, a team of astronomers led by Miguel Montargès, Observatoire de Paris, France, and KU Leuven, Belgium, and including Emily Cannon, KU Leuven, have found the cause of this dimming, thus finally solving this cosmic mystery. The researchers have discovered that the darkening of Betelgeuse was caused by a cloud of dust partially concealing the red supergiant.
“Our observations show that the Southern part of the star was hidden and that the whole disk of the star was fainter. The modelling is compatible with both a cool spot of the photosphere and a dusty clump in front of the star,” Montargès tells ZME Science. “Since both signatures have been detected by other observers, we conclude that the Great Dimming was caused by a cool patch of material that, due to its lower temperature, caused dust to form in gas cloud ejected by the star months to years before.”
The ‘great dimming’ of this massive star lasted a few months presented a unique opportunity for researchers to study the dimming of stars in real-time.
“The dimming of Betelgeuse was interesting to professional and amateur astronomers because not only was the appearance of the star changing in real time we could also see this change with the naked eye. Being able to resolve the surface of a star during an event like this is unprecedented.”
Emily Cannon, KU Leuven
The team’s research is published in the latest edition of the journal Nature.
A Unique Opportunity to Capture a Dimming Star
Montargès and his team first trained the Very Large Telescope (VLT)–an ESO operated telescope based in the Atacama Desert, Chile–on Betelgeuse when it began to dim in late 2019. The astronomers took advantage of the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the VLT as well as data from the telescope’s GRAVITY instrument. This allowed them to create stunning images tracking the great dimming event allowing them to distinguish it from regular dips in brightness demonstrated by the supergiant stars.
Betelgeuse has been seen to decrease in brightness before as a result of its convection cycle, which causes material to rise and fall throughout the star’s layers based on its temperature. This convection cycle results in a semi-regular dimming cycle that lasts around 400 days.
When the ‘great dimming’ was first observed in October 2019 astronomers had assumed this was due to its natural dimming cycle. That assumption was dismissed by December that same year when the star became the darkest that it had been in a century. The star had returned to its normal brightness by April 2020.
“No other red supergiant star has been seen dimming that way, particularly to the naked eye. Even Betelgeuse that has been closely monitored for 150 years has not shown such behaviour.”
Miguel Montargès, Observatoire de Paris, France
Not only does this finding solve the mystery of this star’s dimming, but it also provides evidence of the cooling of a star causing the creation of stardust which goes on to obscure the star.
Even though Betelgeuse is much younger than the Sun–10 million years old compared to our star’s age of 4.6 billion years–it is much closer to the supernova explosion that will signal the end of its lifecycle. Astronomers had first assumed that dimming was a sign that the red supergiant was exhibiting its death throes ahead of schedule.
Thanks to the work of Montargès and his team, we now know this isn’t the case. The dimming is the result of a veil of stardust obscuring the star’s southern region.
“We have observed dust around red supergiant stars in the past,” Cannon explains. “However, this is the first time we have witnessed the formation of dust in real-time in the line of sight of a red supergiant star,”
This stardust will go on to form the building blocks of the next generation of stars and planets, and the observations made by Montargès, Cannon and the team represent the first time we have seen an ancient supergiant star ‘burping’ this precious material into the cosmos.
The Giant that Burped Stardust
The surface of Betelgeuse–which with its diameter of around 100 times that of the Sun would consume the orbits of the inner planets including Earth were it to sit in our solar system–is subject to regular changes as bubbles of gas move around it, change in size, and swell beneath it. Montargès, Cannon and their colleagues believe that sometime before the great dimming began the red supergiant ‘burped’ out a large bubble of gas.
This bubble moved away from the star leaving a cool patch on its surface. It was within this cool patch that material was able to solidify, creating a cloud of solid stardust. The team’s observations show for the first time that stardust can rapidly form on the surface of a star.
“We have directly witnessed the formation of so-called stardust,” says Montargès. “The dust expelled from cool evolved stars, such as the ejection we’ve just witnessed, could go on to become the building blocks of terrestrial planets and life.”
With regards to the future, the researchers point to the Extremely Large Telescope (ELT), currently under construction in the Atacama Desert as the ideal instrument to conduct further observations of Betelgeuse. “With the ability to reach unparalleled spatial resolutions, the ELT will enable us to directly image Betelgeuse in remarkable detail,” says Cannon. “It will also significantly expand the sample of red supergiants for which we can resolve the surface through direct imaging, further helping us to unravel the mysteries behind the winds of these massive stars.”
For Montargès solving this mystery and observing a phenomenon for the first time, solidifies a lifetime of fascination with Betelgeuse and points towards a deeper understanding of the stardust that is the building blocks of stars, planets, and us. “We have seen the production of star dust, materials we are ourselves made of. We have even seen a star temporarily change its behavior on a human time scale.”
Physicist Kip Thorne and astronomer Anna Zytkow proposed a new theoretical class of stars back in 1975, but it was only very recently that such an example of hybrid star was identified in the universe. The Thorne-Zytkow Objects (TZOs) are a combination between red supergiant and neutron stars, superficially looking like normal red supergiants, for example Betelgeuse in the Orion constellation. The main difference consists in the chemical signatures resulting from particular activity in their stellar interiors.
The formation of the TZOs is believed to happen when the two massive types of stars interact – the red supergiant and the neutron star shaped during the explosion of a supernova – in a close binary system. The exact interaction is still undetermined, but the most popular theory holds that during the evolutionary interaction of the two bodies the neutron star is swallowed by its substantially more massive interaction partner, the red supergiant. It is also believed that the neutron star spirals into the core of the red supergiant.
As it is scientifically agreed upon the fact that the red supergiants classically derive their energy from nuclear fusion in their cores, the TZOs are powered by the odd activity of the absorbed neutron stars in their cores. The discovery provides evidence of a new model of stellar interiors, which wasn’t detected by astronomers until the new finding. The declaration of project leader Emily Levesque of the University of Colorado Colder (who was awarded the American Astronomical Society’s Annie Jump Cannon Award this year) is that:
‘Studying these objects is exciting because it represents a completely new model of how stellar interiors can work. In these interiors we also have a new way of producing heavy elements in our universe. You’ve heard that everything is mae of ‘stellar stuff’ – inside these stars we might now havea new way to make some of it.’
The discovery was made on Las Campanas, in Chile, with a 6.5-meter Magellan Clay telescope. Astronomers investigated the spectrum of light that was emitted from the apparent red supergiants, which allowed them to determine what elements made up the stars. The first time when the spectrum of a particular star, HV 2112 in the Small Magellanic Cloud, was displayed, even the observers were surprised by its unusual features.
At a closer look, Levesque and her colleagues found that the spectrum contained excess rubidium, lithium, and molybdenum. Previous scientific research showed that each of these elements could be created through normal stellar processes. But finding so much of them together at the temperatures that are typical to the red supergiants represents a feature unique for TZOs.
‘Since Kip Thorne and I proposed our models of stars with neutron cores, people were not able to disprove our work. If theory is sound, experimental confirmation shows up sooner or later. So it was a matter of identification of a promising group of stars, getting telescope time and proceeding with the object’, declared astronomer Anna Zytkow.
The team does take prevention measures and points out the chemical characteristics not matching the theoretical models proposed by the two researchers. Phillip Massey, co-author of the study, underlines that:
‘We could, of course, be wrong. There are some minor inconsitencies between some of the details of what we found and what theory predicts. But the theoretical predictions are quite old, and there have been a lot of improvements in the theory since then. Hopefully our discovery will spur additional work on the theoretical side now’.
While there is some level of uncertainty, the detection of a TZO provides the first direct, observable evidence for a completely new model of stellar interiors, which implies never-before-seen nucleosynthesis processes.