Exploding stars come in different types, and these different types of supernovae show astronomers different things about the cosmos. There’s a scientific appetite to find more of them and boost our knowledge about these exotic events. The Nancy Grace Roman Space Telescope should be able to feed that appetite.
The Roman is due to launch in about two years, and will make its way to its station at the Sun-Earth L2 orbit. After commissioning, it’ll begin operations. One of its three primary surveys is the High-Latitude Time-Domain Survey. In that survey, the powerful space telescope will image the same section of sky beyond the Milky Way every five days for two years. The team behind the Roman will stitch these scenes together into one comprehensive movie, a sort of cosmic cinema.
These movies will reveal the presence of Type 1a supernovae. These occur in binary star systems where one star is a white dwarf. White dwarfs have immense gravitational force because they’re extremely dense objects. They draw material away from their companion stars, which could be anything from another white dwarf to a giant star. That material builds up on the white dwarf’s surface, and when it reaches a critical mass, it triggers a runaway reaction and a supernova explosion.
Type 1a are different from what we can call standard supernovae. Those are core-collapse supernovae, where a massive star collapses into a neutron star or a black hole, or is completely destroyed and leaves behind only a diffuse nebula.
Since Type 1a supernovae explode at a fixed mass, their peak luminosity is known. For that reason, they serve as standard candles, tools astronomers use to accurately gauge the distance to their home galaxies. These accurate distances allow cosmologists to trace the expansion of the Universe.
The Roman’s High-Latitude Time-Domain Survey is a critical part of its mission and is aimed at finding Type 1a supernovae and other transients. According to new research and simulations, it should find about 27,000 of them, a shocking number that’s about ten times greater than the current number of known Type 1a SN. This comprehensive data set should help cosmologists in their quest to map the expansion of the Universe, a critical part of understanding dark energy.
“Evidence is mounting that dark energy has changed over time, and Roman will help us understand that change by exploring cosmic history in ways other telescopes can’t.” – Dr. Ben Rose – Dept. of Physics and Astronomy, Baylor University
The 27,000 number comes from new research published in The Astrophysical Journal titled “The Hourglass Simulation: A Catalog for the Roman High-latitude Time-domain Core Community Survey.” The lead author is Dr. Ben Rose, an assistant Professor of Physics in the Department of Physics and Astronomy at Baylor University.
The Roman will find these explosions by observing light from distant galaxies and looking back in time. The Roman will push that time boundary and allow astronomers to see Type 1a SN further back than ever. Most of the T1a SN observed so far exploded in the last 8 billion years. The Roman’s High-Latitude Time-Domain Survey (HLTDS) will uncover thousands that exploded longer than 10 billion years ago, and dozens that exploded even earlier than that. These standard candles will fill a missing gap and are critical evidence of the Universe’s expansion in its early age.
This graphic outlines the Nancy Grace Roman Space Telescope’s High-Latitude Time Domain Survey. The survey’s main component will cover over 18 square degrees — a region of sky as large as 90 full moons — and will detect supernovae that occurred up to about 8 billion years ago. Smaller areas within the survey can look even further back in time, potentially back to when the universe was around a billion years old. The survey will be split between the northern and southern hemispheres, located in regions of the sky that will be continuously visible to Roman. The bulk of the survey will consist of 30-hour observations every five days for two years in the middle of Roman’s five-year primary mission. Image Credit: NASA’s Goddard Space Flight Center
“Filling these data gaps could also fill in gaps in our understanding of dark energy,” lead author Rose said in a press release. “Evidence is mounting that dark energy has changed over time, and Roman will help us understand that change by exploring cosmic history in ways other telescopes can’t.”
This figure compares the Roman’s expected haul of Type 1a SN with the Dark Energy Survey’s cosmological sample of the same. “DES has over 1500 SNe in its cosmological sample with very few at z > 1. However, we expect Roman to have nearly 19,000 SN Ia, with the majority above z > 1,” the authors write. Image Credit: Rose et al. 2025. TApJ
Every supernova is essentially a flash in the cosmos, and dissecting the light from the flash reveals what type of event released it. Core collapse SN and T1a SN aren’t easy to distinguish at such great distances, but the light changes over time, and can be split apart with spectroscopy to learn more about it. The Roman carries two instruments, and one of them, the Wide-Field Instrument (WFI), allows the telescope to do large-scale spectroscopic surveys.
“By seeing the way an object’s light changes over time and splitting it into spectra — individual colors with patterns that reveal information about the object that emitted the light—we can distinguish between all the different types of flashes Roman will see,” said Rebekah Hounsell, study co-author and assistant research scientist at the University of Maryland-Baltimore County working at NASA’s Goddard Space Flight Center.
The Hourglass Simulation “uses the most up-to-date spectral energy distribution models and rate measurements for 10 extragalactic time-domain sources,” the authors explain in their research. “We simulate these models through the design reference Roman Space Telescope survey.”
“In total, Hourglass has over 64,000 transient objects, 11,000,000 photometric observations, and 500,000 spectra,” the authors write. Hourglass showed that the Roman can expect to find “approximately 21,000 Type Ia supernovae, 40,000 core-collapse supernovae, around 70 superluminous supernovae, ∼35 tidal disruption events, three kilonovae, and possibly pair-instability supernovae.”
This impressive data set will drive the study and understanding not only of dark energy, but of many other transient events too. As of 2024, for example, astronomers knew of only about 260 superluminous supernovae (SLSNe). These explosions can be 10p times as luminous as other SN. Only massive stars greater than 40 solar masses are expected to explode as SLSNe, yet astrophysicists aren’t certain what causes them. Finding an additional 70 could provide answers to some outstanding questions.
This artist’s illustration shows the explosion of SN 2006gy, a superluminous supernova about 238 million light-years away. Image Credit: By Credit: NASA/CXC/M.Weiss – http://chandra.harvard.edu/photo/2007/sn2006gy/more.html#sn2006gy_xray, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2080784
The Hourglass Simulation is designed to prepare the science community for the Roman’s deluge of data. With its tens of thousands of transients, millions of photometric observations, and hundreds of thousands of spectra, Hourglass will serve as a training tool. “Additionally, Hourglass is a useful data set to train machine learning classification algorithms.”
“With the dataset we’ve created, scientists can train machine-learning algorithms to distinguish between different types of objects and sift through Roman’s downpour of data to find them,” Hounsell added in the press release. “While searching for type Ia supernovae, Roman is going to collect a lot of cosmic ‘bycatch’—other phenomena that aren’t useful to some scientists, but will be invaluable to others.”
Among those other phenomena are Tidal Disruption Events (TDE), which occur when a black hole consumes a star. Astronomers know of about 100 of them, and they can reveal the presence of black holes that are otherwise dormant and undetectable. If the Roman can find an additional 35, that will undoubtedly help them answer some of their questions. Not only are their outstanding questions about black holes’ masses and spina, but there are also questions about how stars behave in the dense regions near galactic centers.
Kilonovae are another type of cosmic explosion and occur when two neutron stars or a neutron star and a black hole collide. Though they’re fainter than SN, Kilonovae release gravitational waves and also produce substantial amounts of heavy elements like gold, platinum, and uranium. There’s only one confirmed kilonova explosion, and there are many outstanding questions about them. Astrophysicists want to understand the composition of these elements in their ejecta, and how often they occur and if there are multiple types. If the Roman can find three more, that’s a massive increase in the dataset scientists have to work with.
This artist’s illustration shows two neutron stars merging, releasing gravitational waves and exploding as a kilonova. There’s only one confirmed kilonova, so if the Roman can find three more, that’s a massive jump in data. Image Credit: By University of Warwick/Mark Garlick, CC BY 4.0
Pair-instability supernovae are another exotic type of stellar explosion that scientists want to know more about. Only extremely massive stars between about 130 to 250 solar masses can explode as pair-instability supernovae (PISNe), and they don’t leave neutron stars or black holes behind. The progenitor stars is completely destroyed, and only an expanding nebula of gas and dust, including heavy elements synthesized in the explosion, is left behind. Astrophysicists want to know the exact stellar mass of their progenitors and what role metallicity plays.
As it stands now, astrophysicists have only a small handful of candidate PISNe, and if the Roman can find ten of them like the simulation suggests, researchers will have a lot more data to work with.
“I think Roman will make the first confirmed detection of a pair-instability supernova,” Rose said. “They’re incredibly far away and very rare, so you need a telescope that can survey a lot of the sky at a deep exposure level in near-infrared light, and that’s Roman.”
As NASA’s next flagship astrophysics mission, the Nancy Grace Roman Space Telescope will make an enormous contribution to our understanding of different types of cosmic explosions. By stitching together its observations into movies that show how different cosmic explosions take place, it will advance our scientific knowledge considerably.
“Whether you want to explore dark energy, dying stars, galactic powerhouses, or probably even entirely new things we’ve never seen before, this survey will be a gold mine,” said Rose.
Each time a new telescope mission is launched, it’s after years or even decades of preliminary work, including figuring out what questions need to be asked and what instruments are needed to find the answers. Simulations like the Hourglass simulation are becoming more common, as the astronomy community anticipates and prepares for new data from upcoming missions.
But each mission also produces surprises, and though they’re unpredictable, scientists often mention how excited they are to find surprising new things.
“Roman’s going to find a whole bunch of weird and wonderful things out in space, including some we haven’t even thought of yet,” Hounsell said. “We’re definitely expecting the unexpected.”
An illustration the Nancy Grace Roman Space Telescope, set to launch in 2027, if it can survive budget cuts. Image Credit: NASA/GSFC/SVS
Sadly, the current US administration has taken aim at NASA’s budget and announced that the Roman’s funding will be cut. Since the current administration has gained a reputation for confusing announcements that are sometimes later rescinded, the mission’s future is unclear.
If it is approved and launched, its precious dataset will be a feast for astrophysicists around the world and will help drive a deeper understanding of Nature and some of its most extreme objects and events.