When it comes to searching for the explosive demise of massive stars in the early universe, the James Webb Space Telescope (JWST) is quite the cosmic detective. This celestial Sherlock Holmes 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 10 times more supernovae than had previously been discovered this early in cosmic history, but the sample also contains the oldest and most distant supernova ever observed. It’s the 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 supernova clutch, which also includes Type Ia explosions, which astronomers call “standard candles” and can use to measure cosmic distances.
Before JWST begins operations in the summer of 2022, only a few supernovae have been found from when the universe was only 3.3 billion years old, about 25% of its current age. However, the JADES sample contains many supernovae that exploded even further back in time. In fact, some exploded when the universe was less than 2 billion years old.
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“JWST is a supernova discovery machine,” team member Christa DeCoursey, a third-year student at 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 interesting results of our survey.”
JWST’s unparalleled infrared sensitivity means it is discovering supernovae almost everywhere it looks in space.
Supernova Detective
As light wavelengths travel through space, the expansion of the very fabric of the universe lengthens those wavelengths. This causes the light to move further down the electromagnetic spectrum in terms of classification, progressing from the bluer end to the redder end. This phenomenon is known as “redshift”.
The longer the light travels through space, the more extreme the degree of redshift. Thus, light from objects about 12 billion light-years away, such as these supernovae, has experienced extreme wavelength stretching, or “cosmological redshift.”
This pushes this supernova light down into the infrared region of the electromagnetic spectrum, a region in which JWST is adept at observing space.
The Hubble Space Telescope has previously allowed astronomers to view supernovae so distant that they existed when the universe was in its “young adult” phase. However, with JADES and JWST, astronomers can observe supernovae when the universe is in its “teens” or even “pre-teens.”
In the future, scientists hope to peer back into the “toddler” phase of the universe – or even back into its cosmic origins, ideally encountering the death of the first generation of massive stars.
To get this new cavalcade of supernova observations, the JADES team took multiple images of the same spot in the sky at yearly intervals. Then they compared the pictures. Because supernovae are “transient,” meaning they brighten and fade over time, observing the changes in the images allowed scientists to distinguish which points of light were actually exploding stars and which were likely some other phenomenon.
“This is really our first sample 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 a statement. “We’re trying to find out if distant supernovae are fundamentally different from what we see in the nearby universe, or if they’re very similar.”
Not all supernovae seen by the JADES team were “core-collapse” supernovae, which are triggered when massive stars run out of fuel for nuclear fusion in their cores and collapse under their own gravity, giving birth to a black hole or neutron star.
As mentioned, some were Type Ia supernovae, triggered when corpse stars called “white dwarfs” cannibalistically feed on material stripped from a companion or donor star. This material accumulates on the surface of the white dwarf until it triggers a thermonuclear explosion that obliterates the white dwarf.
The light outputs of these events are uniform with the same intrinsic brightness, seemingly regardless of distance. This means that they can be used as cosmic rulers to measure distance and also serve as markers to measure the rate at which the fabric of the universe is expanding. However, if the intrinsic luminosity of Type Ia supernovae changed at high redshifts, their utility in measuring large cosmic distances would be limited.
Observations of the Type Ia team, which exploded about 11 billion years ago, suggest that its brightness has not changed even though the light has passed through cosmological redshift.
The “pre-teen” universe was a very different place than we see today, with much more extreme environments. Additionally, because the universe was mostly hydrogen and helium at the time, astronomers expect to see ancient supernovae triggered by the death of stars that contain far fewer heavy chemical elements, or “metals,” than the current generation of “metal-rich” stars. like the sun
So comparing these ancient supernovae to massive stars exploding in the local universe could help scientists better understand how stars are enriched during their formation with metals created by early stars and spread through space as they die.
“Essentially, we’re opening a new window into the transient universe,” Matthew Siebert, lead spectroscopic analysis of 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 at a news conference at the 244th meeting of the American Astronomical Society in Madison, Wis., on Monday (June 10).