Since 2018 the , an international astronomical collaboration based at the Palomar Observatory in California, has scanned the entire sky every two to three nights. As part of this mission, the ZTF’s has been counting and cataloguing supernovae — flashes of light in the sky that are the telltale signs of stars dying in spectacular explosions.

On Dec. 4, ZTF researchers — including astronomers at the 91̽�� — announced that that they have identified more than 10,000 of these stellar events, the largest number ever identified by an astronomical survey.

“There are trillions of stars in the universe, and about every second, one of them explodes,” said Christoffer Fremling, an astronomer at Caltech who leads the Bright Transient Survey. “ZTF detects hundreds of these explosions per night and a handful are then confirmed as supernovae. Systematically doing this for seven years has led to the most complete record of confirmed supernovae to date.”

The Bright Transient Survey is currently the primary discovery pipeline for cosmic flashes — also known as astronomical transients — in the world. To determine which transients are supernovae, ZTF shares a stream of nightly transient detections with the wider astronomical community so that other telescopes around the world can conduct follow-up observations of candidate transients. This includes conducting a spectral analysis, in which instruments on observatory telescopes split the light from a transient object into its individual colors to reveal its distance from Earth and other properties.

“Classifying 10,000 supernovae is a tremendous achievement and will enable unprecedented scientific studies of explosive transients,” said ZTF team member , a 91̽��research associate professor of astronomy and scientist with the UW’s . “Reaching this milestone required careful technical work on scheduling and processing the ZTF discovery images, human and machine vetting of the alerts and obtaining timely follow-up spectra.”

For the Bright Transient Survey, a 60-megapixel wide-field camera mounted on Palomar’s Samuel Oschin telescope scanned the entire visible sky every two nights. To detect new astronomical events, astronomers subtracted images of the same portion of the sky from subsequent scans. Next, members of the ZTF team studied the subtracted images and triggered follow-up spectral observations by a second telescope at Palomar or other observatories.

Bellm, 91̽��research scientist and , 91̽��professor of astronomy and director of the DiRAC Institute, all contributed to the Bright Transient Survey. Bellm managed alerts of new transients and scheduled imaging for the survey. Jurić helped set up the ZTF’s automated system to alert team members around the world of new transients.

Developing automated analysis pipelines and alert systems are critical for the field as more powerful imaging technologies and new generations of observatories continue to transform astronomy into a “big data” endeavor. , a 20th century astronomer who first coined the term “supernova,” identified 120 supernovae in 52 years. The Bright Transient Survey by the ZTF — named for Zwicky — found 10,000 in a fraction of that time.

“The Bright Transient Survey program serves as an exemplar for the kinds of science we hope to do with the in the near future,” said Bellm.

Under construction in Chile, the Vera C. Rubin Observatory is the future home of the Legacy Survey of Space and Time, or LSST, a mission that will take deep images of the sky nightly and detect even more cosmic transients than ZTF. 91̽��scientists with the DiRAC Institute have been heavily involved in planning for the launch of the LSST. Collaborations like the ZTF have been a proving ground for developing and testing methods for use in the LSST.

For the Bright Transient Survey, Graham conducted follow-up spectral analyses of transients at in New Mexico. These efforts were especially valuable in catching some of the fainter, fading supernovae that would have been missed at Palomar.

“As 91̽��astronomers, we are so fortunate to have access to the Apache Point Observatory for our research,” said Graham. “One of the most impactful — and fun — parts of obtaining optical spectra is being surprised by rare transients with peculiar characteristics, which often reveal more about supernova physics than hundreds of ordinary objects. Figuring out how to do this work with the even larger number of LSST supernovae is the next big challenge.”

Most of the transients in the Bright Transient Survey are classified as one of two common types of supernovae: Type Ia, when a white dwarf steals so much material from another nearby star that it explodes, or Type II, when massive stars collapse and die under their own gravity. Thanks to the treasure trove of data from the Bright Transient Survey, astronomers are now better equipped to answer questions about how stars grow and die, as well as how dark energy drives the expansion of the universe.

After its expected 2025 commissioning, the Vera Rubin C. Observatory could discover millions more supernovae.

“The machine learning and AI tools we have developed for ZTF will become essential when the Vera Rubin Observatory begins operations,” said ZTF team member Daniel Perley, an astronomer at Liverpool John Moores University. “We have already planned to work closely with Rubin to transfer our machine learning knowledge and technology.”

With an additional $1.6 million of funding from the National Science Foundation, ZTF will continue to scan the night sky for the next two years.

“The period in 2025 and 2026 when ZTF and Vera Rubin can both operate in tandem is fantastic news for time-domain astronomers,” said Mansi Kasliwal, an astronomy professor at Caltech who will lead ZTF in the next two years. “Combining data from both observatories, astronomers can directly address the physics of why supernovae explode and discover fast and young transients that are inaccessible to ZTF or Rubin alone. I am excited about the future.”

For more information, contact Bellm at ecbellm@uw.edu and Graham at mlg3k@uw.edu.

Adapted from a by Caltech.

These latter events can happen in binary star systems, where two stars attempt to share dominion. While the exploding star gives off lots of evidence about its identity, astronomers must engage in detective work to learn about the errant companion that triggered the explosion.

On Jan. 10 at the in Seattle, an international team of astronomers announced that they have identified the type of companion star that made its partner in a binary system, a carbon-oxygen white dwarf star, explode. Through repeated observations of SN 2015cp, a supernova 545 million light years away, the team detected hydrogen-rich debris that the companion star had shed prior to the explosion.

“The presence of debris means that the companion was either a star or similar star that, prior to making its companion go supernova, had shed large amounts of material,” said 91̽�� astronomer , who presented the discovery and is lead author on the accompanying paper accepted for publication in .

The supernova material smacked into this stellar litter at 10 percent the speed of light, causing it to glow with ultraviolet light that was detected by the and other observatories nearly two years after the initial explosion. By looking for evidence of debris impacts months or years after a supernova in a binary star system, the team believes that astronomers could determine whether the companion had been a messy red giant or a relatively neat and tidy star.

The team made this discovery as part of a wider study of a particular type of supernova known as a . These occur when a carbon-oxygen white dwarf star explodes suddenly due to activity of a binary companion. Carbon-oxygen white dwarfs are small, dense and — for stars — quite stable. They form from the collapsed cores of larger stars and, if left undisturbed, can persist for billions of years.

Type Ia supernovae have been used for cosmological studies because their consistent luminosity makes them ideal “cosmic lighthouses,” according to Graham. They’ve been used to and served as indirect evidence for the existence of .

Yet scientists are not certain what kinds of companion stars could trigger a Type Ia event. Plenty of evidence indicates that, for most Type Ia supernovae, the companion was likely another carbon-oxygen white dwarf, which would leave no hydrogen-rich debris in the aftermath. Yet theoretical models have shown that stars like red giants could also trigger a Type Ia supernova, which could leave hydrogen-rich debris that would be hit by the explosion. Out of the thousands of Type Ia supernovae studied to date, only a small fraction were later observed impacting hydrogen-rich material shed by a companion star. Prior observations of at least two Type Ia supernovae detected glowing debris months after the explosion. But scientists weren’t sure if those events were isolated occurrences, or signs that Type Ia supernovae could have many different kinds of companion stars.

“All of the science to date that has been done using Type Ia supernovae, including research on dark energy and the expansion of the universe, rests on the assumption that we know reasonably well what these ‘cosmic lighthouses’ are and how they work,” said Graham. “It is very important to understand how these events are triggered, and whether only a subset of Type Ia events should be used for certain cosmology studies.”

The team used Hubble Space Telescope observations to look for ultraviolet emissions from 70 Type Ia supernovae approximately one to three years following the initial explosion.

“By looking years after the initial event, we were searching for signs of shocked material that contained hydrogen, which would indicate that the companion was something other than another carbon-oxygen white dwarf,” said Graham.

In the case of SN 2015cp, a supernova first detected in 2015, the scientists found what they were searching for. In 2017, 686 days after the supernova exploded, Hubble picked up an ultraviolet glow of debris. This debris was far from the supernova source — at least 100 billion kilometers, or 62 billion miles, away. For reference, Pluto’s orbit takes it a maximum of 7.4 billion kilometers from our sun.

By comparing SN 2015cp to the other Type Ia supernovae in their survey, the researchers estimate that no more than 6 percent of Type Ia supernovae have such a litterbug companion. Repeated, detailed observations of other Type Ia events would help cement these estimates, Graham said.

The Hubble Space Telescope was essential for detecting the ultraviolet signature of the companion star’s debris for SN 2015cp. In the fall of 2017, the researchers arranged for additional observations of SN 2015cp by the in Hawaii, the in New Mexico, the European Southern Observatory’s and NASA’s , among others. These data proved crucial in confirming the presence of hydrogen and are presented in a companion paper lead by Chelsea Harris, a research associate at Michigan State University.

“The discovery and follow-up of SN 2015cp’s emission really demonstrates how it takes many astronomers, and a wide variety of types of telescopes, working together to understand transient cosmic phenomena,” said Graham. “It is also a perfect example of the role of serendipity in astronomical studies: If Hubble had looked at SN 2015cp just a month or two later, we wouldn’t have seen anything.”

Graham is also a senior fellow with the UW’s and a with the , or LSST.

“In the future, as a part of its regularly scheduled observations, the LSST will automatically detect optical emissions similar to SN 2015cp — from hydrogen impacted by material from Type Ia supernovae,” said Graham said. “It’s going to make my job so much easier!”

Co-authors are Harris; Peter Nugent at the University of California, Berkeley and the Lawrence Berkeley National Laboratory; Kate Maguire at Queen’s University Belfast; Mark Sullivan and Mathew Smith at the University of Southampton; Stefano Valenti at the University of California, Davis; Ariel Goobar at Stockholm University; Ori Fox at the Space Telescope Science Institute; Ken Shen, Tom Brink and Alex Filippenko at the University of California, Berkeley; Patrick Kelly at the University of Minnesota; and Curtis McCully at the University of California, Santa Barbara and the Las Cumbres Observatory. The research was funded by the National Science Foundation, NASA, the European Research Council and the U.K.’s Science and Technology Facilities Council.

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For more information, contact Graham at mlg3k@uw.edu.

Grant numbers: AST-1211916, NNX17AG28G, 615929, 758638