When it comes to searching for the explosive deaths of massive stars in the early universe, the James Webb Space Telescope (JWST) is quite the cosmic detective. This celestial Sherlock Holmes has found evidence of 80 new early supernovas in a patch of sky as large as a grain of rice held at arm’s length.
Not only are these 10 times more supernovas than previously discovered in early cosmic history, but the sample also includes the oldest and most distant supernova ever observed. This is 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 set of supernovas, which additionally includes Type Ia explosions that astronomers call “standard candles” and can use to measure the cosmic distances.
Before JWST began operations in summer 2022, only a handful of supernovas had been discovered, dating back to when the universe was only 3.3 billion years old, or about 25% of its age current. The JADES sample, however, contains many supernovas that exploded even further in the past. In fact, some erupted when the universe was less than 2 billion years old.
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“The JWST is a supernova discovery machine,” team member and third-year student Christa DeCoursey at the Steward Observatory and the University of Arizona in Tucson said in a statement. “The large number of detections as well as the large distances to these supernovas are the two most interesting results of our investigation.”
JWST’s unparalleled infrared sensitivity allows it to discover supernovas almost anywhere it looks in the cosmos.
The supernova detective
As wavelengths of light pass through the cosmos, the expansion of the very fabric of space expands those wavelengths. This causes the light to move further down the electromagnetic spectrum in terms of classification, going from the bluer end to the redr end. This phenomenon is known as “redshift”.
The longer light travels through space, the more extreme the degree of redshift it experiences. Thus, light coming from bodies located around 12 billion light years away, such as these supernovas, has experienced an extreme lengthening of its wavelength, or “cosmological redshift”.
This shifts this supernova light toward the infrared region of the electromagnetic spectrum, a region in which the JWST is adept at observing the universe.
The Hubble Space Telescope had previously allowed astronomers to observe supernovas so distant that they existed when the universe was in its “young adult” phase. With JADES and JWST, however, astronomers can observe supernovas when the cosmos is in its “adolescence” or even its “pre-adolescence.”
In the future, scientists hope to return to the “infant” phase of the universe – or even its cosmic beginnings, ideally by stumbling upon the death of the first generation of massive stars.
To obtain this new cavalcade of supernova observations, the JADES team took several images of the same part of the sky at intervals of a year. Then they compared the images. Because supernovas are “transient,” meaning they brighten and fade over time, observing changes in the images allowed scientists to distinguish which bright spots were actually stars exploding and which were probably other phenomena.
“This is really our first sample of what the high-redshift universe looks like for transient science,” said Justin Pierel, a JADES team member and NASA Einstein Scholar at the Space Telescope Science Institute (STScI). of Baltimore, Maryland, in the release. “We are trying to determine whether distant supernovas are fundamentally different or very similar to what we see in the nearby universe.”
Not all of the supernovas observed by the JADES team were “core collapse” supernovas, triggered when massive stars lacked the fuel supply needed for nuclear fusion in their cores and collapsed under their own gravity , giving rise to a black hole or a neutron star.
As mentioned, some were Type Ia supernovas triggered when stellar corpses called “white dwarfs” fed cannibalistically on material taken from a companion or donor star. This material accumulates on the surface of the white dwarf until it triggers an uncontrollable thermonuclear explosion that completely obliterates the white dwarf.
The light fluxes from these events are uniform with the same intrinsic brightness, apparently regardless of distance. This means they can be used as cosmic rulers to measure distance and also serve as markers to gauge the rate at which the fabric of space is expanding. However, if the intrinsic luminosity of Type Ia supernovae changed at high redshifts, their usefulness for 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 did not vary despite the cosmological redshift of its light.
The “tween” world was a very different place than the one we know today, with much more extreme environments. Additionally, because the universe was mostly hydrogen and helium at that time, astronomers expect to see ancient supernovas triggered by the deaths 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 supernovas with massive stars exploding in the local universe could help scientists better understand how stars become enriched as they form with metals forged by early stars and spread throughout the cosmos when they die.
“We are essentially opening a new window into the transient universe,” said Matthew Siebert, head of JADES supernova spectroscopic analysis. “Historically, every time we’ve done this, we’ve discovered extremely exciting things, things we didn’t expect.”
The team’s results were presented at a press conference at the 244th meeting of the American Astronomical Society in Madison, Wisconsin, Monday (June 10).