More than 10,000 supernovae counted in stellar census

Since 2018 the Zwicky Transient Facility, 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 Bright Transient Survey has been counting and cataloging 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 University of Washington—announced 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 Eric Bellm, a UW research associate professor of astronomy and scientist with the UW’s DiRAC Institute. “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, UW research scientist Melissa Graham and Mario Jurić, UW 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. Fritz Zwicky, 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 Vera C. Rubin Observatory 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. UW 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 Apache Point Observatory in New Mexico. These efforts were especially valuable in catching some of the fainter, fading supernovae that would have been missed at Palomar.

“As UW 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.”

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.”