Astronomer discovered something unusual happening in a distant galaxy in recent images from the James Webb Space Telescope – something that wasn’t there when Hubble last looked at the same galaxy.
“We suspect it’s a supernova,” astronomer Mike Engesser from the Space Telescope Science Institute (STScI) tells Reverse. Finding short-lived cosmic events like supernovae isn’t what Webb was designed to do, but the newly operational space telescope appears to be full of surprises. And this could open the door to searching for the death throes of the universe’s first generations of massive stars.
What is new – Engesser and his colleagues say the bright object is likely the first supernova detected by the Webb telescope. It’s extremely bright compared to the rest of the galaxy, for one thing. And Webb observed the galaxy, called SDSS.J141930.11+5251593, twice, five days apart; the object dimmed, just a little, during those five days – classic supernova behavior.
“We’ll need more time series data to decide, but the data we have is consistent with a supernova, so it’s a very good candidate,” says Engesser.
The galaxy, whose unwieldy astronomical name we won’t repeat, is between 3 and 4 billion light-years away, so astronomers see the slowly fading light from an explosion that occurred 3 to 4 billion years ago. The actual death of a star takes a fraction of a second, but the resulting fireball takes days to grow and brighten, then gradually disappear over the next few months. It’s a moment in astronomical time, so it’s pure luck that Webb spotted this supernova shortly after its luminosity peaked.
“Primarily, it’s exciting because we’ve shown that we’re able to find and discover new transients with Webb, which was something JWST wasn’t designed to do,” says Engesser. “But it’s one of the things that we’re showing that we’re able to do in a kind of ad hoc way.”
Although versatile, the Webb is not the type of telescope astronomers typically use to hunt for short-lived objects such as supernovae. Most of the time, this work is done by ground-based telescopes here on Earth, which image wide swathes of the sky each night.
Webb, on the other hand, looks much, much deeper into smaller patches of the sky. The telescope’s first deep-field image, released on July 12, covers an area of the sky that you could cover with a grain of sand held at arm’s length.
“So the actual probability that you’ll find a transient in the field you’re looking at is pretty small—or at least we thought it would be small,” says Engesser. “But as you’ve probably heard, every JWST field is a deep field at this point, so there are galaxies everywhere, and now we’re thinking, oh, we might have a very good chance of detecting supernovae all the time.”
Here’s the background – The ground-based telescopes that usually look for supernovae take pictures of the same broad areas of the sky every few nights. Their data processing software compares each new image with previous images of the same area, looking for anything that might have changed.
To do the same with space telescopes, Engesser and his colleague compared new data from Webb’s NIRCam instrument with Hubble images of the same area. They used software to look for any differences that might reveal what astronomers call “transients,” objects that appear, disappear, brighten or dim on a time scale we can actually see in real time.
That’s how the team found the supernova. It’s also how they found a supernova that flared to life in 2013. Astronomers knew about it, called 2013 EJ, but the once-bright fireball has long since faded too much for most telescopes to see.
“There are a lot of questions about this particular object and what kind of star it was, and the environment — like how dusty is it? It seems like it’s very dusty,” says Engesser.
Because supernovae tend to fade within a few months, astronomers usually don’t get to see their much later stages, which can provide more clues about the type of star that exploded, along with the physics of that stellar explosion. Webb’s deep view of the universe could make the wake of a supernova like 2013 EJ easier to track even years later.
“So observing this now with Webb is actually really cool as a way to probe the superslow environment around big, bright supernovae like this,” says Engesser. “We get new insights that we haven’t been able to get before.”
Engesser calls both detections – the newly discovered supernova and the fading that scientists had lost sight of – a proof of concept. He and his colleagues are part of the Transient Science @ Space Telescope research team, led by astrophysicist Ori Fox at STScI. Fox’s team wants to use Webb to look for supernovae in the oldest, most distant galaxies in the universe, the kind that even Hubble has trouble seeing.
“If we can discover new things, then it allows us to do these very quick measurements of possibility propositions, where we discover something and then we can say, ‘Okay, cancel everything else.’ We want to observe this right now; this is time-sensitive,” explains Engesser. “When we have a good repertoire [of supernovae] built up, we can say, “Okay, it’s actually very worthwhile for us to have these time-sensitive observations of supernovae at super-high redshift.”
Why it matters – Ancient, distant supernovae can help astronomers better understand the very fabric of the universe and how it stretches and expands over time. Most physicists now agree that the universe is expanding, and that the expansion is increasing; that’s thanks to a 1998 study that used a specific type of supernova to help measure distances between objects in the cosmos. These supernovae, called Type Ia supernovae, happen when a white dwarf star in a binary star system steals (or collects) so much material from its companion star that the greedy white dwarf collapses under its own mass.
Because Type Ia supernovae always have the same brightness, astronomers can measure how bright a supernova appears to calculate how far away it, and its host galaxy, is. Astronomers call objects of known brightness “standard light”.
“By looking at the redshift of these supernovae, you can measure how fast they’re moving away from you, as well as how far away they are,” explains Engesser. “So one of the things we want to be able to do with Webb is detect high-redshift supernovae to further constrain the cosmological nature of the universe and how it changes over time.”
The oldest supernovae in the universe also contain clues about the short lives and violent deaths of the universe’s first generation of very massive stars. These ancient giant stars may have been very different from the giant stars of the nearby, more recent universe more familiar to us.
“We think that stars in the first million years would have been mainly, almost exclusively, hydrogen and helium, unlike the kinds of stars we have now. They would have been massive – 200 to 300 times the mass of our Sun, and they would definitely lived a kind of ‘live fast, die young’ lifestyle,” says Engesser. “Seeing these kinds of explosions is something we haven’t really done yet.”
What’s next – Engesser and his team have not yet decided whether to take another look at the supernova they discovered.
“Supernovae in the grand scheme of things are actually quite common,” he says. “This supernova is interesting in that Webb was the first to find it, which is amazing. By itself, it might not be that interesting. We’re still looking at whether or not we want to do any follow-up observations of this thing. .”
If the astronomers decide to study their newly discovered supernova further, it could answer a few specific questions. Watching how the light from the supernova dims over time can reveal the type of supernova — such as Type Ia — Engesser and his colleagues found. Measuring the light spectra from the supernova can reveal which chemical elements made up the parent star, and thus which type of star exploded so violently.
But in the meantime, Engesser and his colleagues are focused on finding more transitory bright spots in Webb’s profound view of the universe.
This story was updated on 28 July 2022 to include the last five paragraphs.