When it comes to hunting down the explosive deaths of massive stars in the early universe, the James Webb Space Telescope (JWST) is a true cosmic detective. This celestial Sherlock Holmes has found evidence of 80 new early supernovae in a patch of sky as wide as a grain of rice, held at arm’s length.
Not only is this ten times more supernovae than have previously been discovered in such early cosmic history, but the sample also includes the earliest and farthest supernova ever seen. It’s one that exploded when the 13.8 billion year old universe was only 1.8 billion years old.
Data from the JWST Advanced Deep Extragalactic Survey (JADES) program helped a team of scientists find this unprecedented collection of supernovae, which also includes Type Ia explosions that astronomers call “standard candles” and can use to measure cosmic distances.
Before the JWST launched in the summer of 2022, only a handful of supernovae had been found dating back to when the universe was only 3.3 billion years old, equivalent to about 25% of its current age. However, the JADES sample contains many supernovae that exploded even further back in the past. Some even erupted when the universe was less than 2 billion years old.
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“The JWST is a supernova discovery machine,” team member Christa DeCoursey, a third-year student at the Steward Observatory and the University of Arizona in Tucson, said in a statement. “The sheer number of detections plus the large distances to these supernovae are the two most exciting results of our research.”
The JWST’s unparalleled infrared sensitivity means it detects supernovae almost everywhere in the cosmos.
The supernova detective
As light wavelengths travel through the cosmos, the expansion of the fabric of space stretches those wavelengths. This causes the light to move further through the electromagnetic spectrum in terms of classification, from the bluer end to the redder end. This phenomenon is known as ‘redshift’.
The longer light travels through space, the more extreme the degree of redshift it undergoes. Thus, light from bodies about 12 billion light-years away, such as these supernovae, has experienced an extreme wavelength extension, or “cosmological redshift.”
That shifts this supernova light into the infrared region of the electromagnetic spectrum, a region in which the JWST is adept at viewing the universe.
The Hubble Space Telescope had previously allowed astronomers to view supernovae so distant that they existed when the universe was in its “young adult” phase. However, with JADES and the JWST, astronomers can observe supernovae when the cosmos is in its ‘teens’ or even ‘pre-teens’.
In the future, scientists hope to look back to the universe’s “toddler” stage – or even back to its cosmic infancy, ideally encountering the death of the first generation of massive stars.
To obtain this new cavalcade of supernova observations, the JADES team took multiple images of the same patch of sky at yearly intervals. Then they compared the images. Because supernovas are “transients,” meaning they brighten and fade over time, by observing changes in the images the scientists were able to distinguish which points of light were indeed exploding stars and which were likely other phenomena.
“This is really our first example of what the high-redshift universe looks like for transient science,” JADES team member Justin Pierel, a NASA Einstein Fellow at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, said in the statement. ‘We are trying to determine whether distant supernovae are fundamentally different from or very similar to what we see in the nearby universe.’
Not all supernovas observed by the JADES team were “core collapse” supernovas, which were triggered when massive stars ran out of fuel needed for nuclear fusion in their cores and collapsed under their own gravity, creating a black hole or neutron star.
As mentioned, some were type Ia supernovae that were triggered when stellar corpses called “white dwarfs” cannibalistically feed on material stripped from a companion or donor star. This material builds up on the white dwarf’s surface until it causes a runaway thermonuclear explosion that completely destroys the white dwarf.
The light outputs from these events are uniform with the same intrinsic brightness, seemingly regardless of distance. This means they can be used as cosmic rulers to measure distances and can also serve as markers to measure the rate at which the fabric of space is expanding. However, should the intrinsic brightness of Type Ia supernovae change at high redshifts, their usefulness in measuring large cosmic distances would be limited.
The team’s observations of a Type Ia that erupted about 11 billion years ago indicated that its brightness had not varied, despite its light undergoing a cosmological redshift.
The ‘pre-teen’ universe was a very different place than we see today, with much more extreme environments. In addition, because the universe was composed primarily of hydrogen and helium at the time, astronomers expect to see ancient supernovae caused by the death of stars containing far fewer heavy chemical elements or “metals” than the current generation of “metal-rich” stars. like the sun.
So comparing these ancient supernovas to massive stars exploding in the local universe could help scientists better understand how stars are enriched during their formation by metals forged by early stars and spread through the cosmos as they died.
“We’re essentially opening a new window on the ephemeral universe,” said Matthew Siebert, leader of the spectroscopic analysis of the JADES supernovae. “Historically, whenever we’ve done that, we’ve found extremely exciting things — things we didn’t expect.”
The team’s findings were presented Monday (June 10) at a press conference during the 244th meeting of the American Astronomical Society in Madison, Wisconsin.