Zwicky Transient Facility Leads to Classification of 10,000 Supernovae

In the 1930s, Caltech astronomer Fritz Zwicky would brave the chill atop Palomar Mountain near San Diego to peer through a small survey telescope in search of eruptions in the night sky. In the 1940s, he and his collaborator, astronomer Walter Baade, continued the quest using a larger survey telescope, the 48-inch Samuel Oschin telescope at Palomar Observatory, still in operation today.

The pair would ultimately go on to discover many stars whose lives ended in fiery explosions, and even coined a term for them: supernovae. In fact, Zwicky held the record for the most supernova discoveries—more than 120—until 2009.

Fast forward to now, and Zwicky's namesake, the Zwicky Transient Facility (ZTF)—a National Science Foundation-funded sky survey that began operations in 2017 using the 48-inch telescope—has detected about a hundred thousand supernovae. These detections, in turn, have led to the spectroscopic classification and confirmation of more than 10,000 supernovae, making ZTF the largest supernova survey to date.

"There are trillions of stars in the universe, and about every second, one of them explodes. Reaching 10,000 classifications is amazing, but what we truly should celebrate is the incredible progress we have made in our ability to browse the universe for transients, or objects that change in the sky, and the science our rich data will enable," says Christoffer Fremling, a staff astronomer at Caltech. Fremling leads the Bright Transient Survey (BTS), ZTF project that discovers and classifies new supernovae.

Nearly 16,000 supernovae have been discovered and confirmed since 2012, when astronomical databases started to officially keep track of all the discoveries. Of these objects, more than 10,000 have been detected by ZTF.

ZTF detects hundreds of thousands of events each night—everything from moving asteroids to merging stars, black holes munching on stars, and more. Verified ZTF detections are quickly sent to the current astronomical database, called the Transient Name Server, which makes the observations available to the wider astronomical community. As the supernovae candidates roll in, telescopes around the world have the option to follow up and obtain a spectrum.

These spectra—detailed measurements of the different wavelengths of light streaming in from a supernova blast—enable astronomers to characterize a supernova's distance from Earth, as well as its chemistry, energetics, and the type that occurred. For example, Type I supernovae are thought to occur when matter flows from a companion onto a white dwarf star, which triggers a thermonuclear explosion. A Type II supernova occurs when a massive star collapses under its own gravity.

Many of the supernovae detected by ZTF were subsequently classified by the ZTF project itself using a spectrograph at the 200-inch Hale Telescope at Palomar Observatory as well as a smaller automated spectrograph called SEDM (Spectral Energy Distribution Machine), which is housed in a small dome next to the 48-inch telescope at Palomar. However, spectra for about 30 percent of the 10,000 classified supernovae were captured by other telescopes, used by the wider astronomical community.

"Back when we started this project, we didn't know how many astronomers would follow up on our detections," says Fremling. "To see that so many have is a testament to why we built ZTF: to survey the whole sky for changing objects and share those data as rapidly as possible with astronomers around the world. That's the purpose of the Transient Name Server. Everything is public in hopes that the community will come together and make the most of it. This way we don't have, say, 10 telescopes across the world doing the same thing and wasting time."

Mansi Kasliwal (PhD '11), who is the principal investigator of ZTF and a professor of astronomy at Caltech, adds that the "name of the game" for studying supernovae is to obtain spectra. "Discovery is only the first step," she says. "Spectra are the most valuable currency in this field. You don't know the physics and chemistry of the object without that."

Among ZTF's impressive catch are some of the rarest supernovae found yet, such as "SN Zwicky," which was so warped by the gravity of an object in front of it that it was split across multiple images.

"I was observing that night and was absolutely stunned when I saw the lensed image of SN Zwicky," says Fremling in a Caltech news story. "We catch and classify thousands of transients with the Bright Transient Survey, and that gives us a unique ability to find very rare phenomena such as SN Zwicky." 

Altogether, ZTF's trove of data will help answer questions about stellar evolution, how dark energy drives the expansion of the universe, and how exactly stars die.

If Fritz Zwicky—who helped lead the development of the 48-inch telescope for exactly the purpose of finding rare, erupting stars—were alive today, he would likely be very proud.

Read the full story, including more about how ZTF is automating the supernovae discovery and classification process, at the ZTF website.

ZTF is funded by the National Science Foundation and an international collaboration of partners. Additional support comes from the Heising–Simons Foundation and from Caltech. ZTF data are processed and archived by Caltech's IPAC astronomy center.