Filippenko Wins Gruber Prize for Supernova Research

UC Berkeley supernova and black hole expert Alex Filippenko will share the 2026 Gruber Cosmology Prize, which was announced today (May 19) by the Gruber Foundation.

The prize, one of the most prestigious awards for research on the origin and fate of the universe, comes with $500,000 to be shared equally with Filippenko's two co-winners, theoretical physicists Ken'ichi Nomoto of the University of Tokyo and Stanford Woosley of UC Santa Cruz.

The scientists were cited for "transforming supernovae from poorly understood stellar explosions into the basis for a quantitative, predictive and empirically validated framework."

An astrophysicist, professor of astronomy and one of Berkeley's most popular teachers, Filippenko played a major role in clarifying the varieties of Type Ia supernovae, allowing these exploding stars to be standardized so that their intrinsic brightness could be used to measure the expansion of the universe. He also was cited for discovering two other variants of Type I supernovae.

"I'm honored, though I can think of numerous other astronomers in this field who are equally deserving," Filippenko said.

At different times, he was a member of two groups of scientists that, in 1998, announced the expansion of the universe was speeding up, leading to the idea that a mysterious dark energy permeates the universe and is fueling the accelerated expansion. The 2011 Nobel Prize in Physics was awarded to the leaders of the two groups - Berkeley physicist Saul Perlmutter and American-Australian astronomer Brian Schmidt - and to Adam Riess, a postdoctoral fellow in Filippenko's group at the time the research was conducted.

According to the Gruber Prize citation, Filippenko, Nomoto and Woosley's "trailblazing work links stellar evolution, explosive nucleosynthesis, the origin of heavy elements and the chemical evolution of the universe and supports the use of supernovae for precision cosmology."

A backwater

In 1985, when Filippenko was a young Miller Postdoctoral Research Fellow at Berkeley, exploding stars were primarily the realm of theorists, with little observational data to vet the theories. Many of the known supernovae had been discovered by amateur astronomers who scanned galaxies night after night in search of new points of light. Filippenko had previously attended part of a supernova conference and said he found it relatively boring. But one night, when he was using the 200-inch telescope at Palomar Observatory to search for evidence of supermassive black holes in nearby galaxies, he discovered his first supernova and got hooked.

"Studies of supernovae were kind of a backwater, but I started working on this subject because I discovered one and it turned out to be weird," he said. "Plus, years earlier, as a budding teenage chemist, I had been fascinated by things that go bang, and exploding stars are among the biggest explosions in the universe."

At the time, astronomers thought supernovae came in just two types: hydrogen-free Type I, which are white dwarf stars that accrete enough mass from a companion star to ignite in a thermonuclear explosion and blow themselves to smithereens; and hydrogen-rich Type II, where the core of a massive star collapses at the end of its life, blowing off its outer layers and leaving behind a neutron star or black hole. The theoretical work of Nomoto and Woosley contributed to this understanding of how stars explode.

Unlike typical Type I supernovae, whose spectra are dominated by signatures of freshly created iron at late times, Filippenko's supernova - SN 1985F - had a spectrum dominated by oxygen, magnesium and calcium lines. These are characteristics of a Type II supernova, though 1985F showed no signs of hydrogen or helium like other Type IIs. This indicated that it had exploded via the core-collapse mechanism yet had lost all of its hydrogen and perhaps all of its helium before exploding.

"My late-time observations definitively showed that the suspicions of astronomers who thought that not all Type I supernovae are of the same physical kind were correct," he said, noting that today it would be classified as a Type Ib or Ic. "SN 1985F was like a wolf in sheep's clothing: this was a Type II supernova in Type I clothing."

Another observation in 1987, when he was a newly appointed assistant professor at Berkeley, confirmed that the type classification astronomers used actually hid similarities between them. Supernova 1987K looked initially like a Type II, based on its spectrum, but then transitioned to look like a Type Ib, showing that the star had lost most (but not all) of its hydrogen envelope prior to exploding. He referred to it as "Type II in youth, Type Ib in old age."

Ultimately, classic white dwarf exploding stars were called Type Ia, while the other varieties - similar to core-collapse Type II supernovae - became known as Type Ib, Ic, etc., as the progenitor star shed more of its outer envelope before exploding. Just last year, Filippenko and others published the first observation of a Type Ie supernova, a star that was in the process of shedding its envelope of silicon and sulfur when it exploded.

"This extended the view astrophysicists have of the full range of core-collapse supernovae," he said. "The whole zoo of core-collapse supernovae is very extensive, and much of that work was done after my first introduction to supernovae in February of 1985 to the early 1990s."

An accelerating universe

In the early 1990s, as one of the top experts on supernovae and how to distinguish the different varieties, Filippenko drew the attention of a team of physicists at Lawrence Berkeley National Laboratory. The team's leader, Perlmutter, hoped to use classic Type Ia supernovae as standard candles to accurately measure the expansion of the universe. If all Type Ia supernovae explode with the same intrinsic brightness or luminosity, it's easy to calculate how far away they and their galaxies are. But any variations in luminosity would throw off these calculations.

Thanks to Filippenko's studies of two Type Ia supernovae with weird spectra and light curves - one too luminous, the other too faint - a colleague, Mark Phillips of the Cerro Tololo Inter-American Observatory, was able to correlate the duration of a supernova's decline in brightness with its peak luminosity and allow even unusual Type Ias to be used as standardizable candles. Filippenko's collaboration with Perlmutter's Supernova Cosmology Project, providing spectra and velocities for many distant supernova candidates using the Keck 10-meter telescopes in Hawaii, allowed the team to use most Type Ias as data points to measure the expanding universe.

Owing to "cultural differences" with the team at the national lab, Filippenko said, he joined a rival group, the High-z Supernova Search Team (z is used to indicate the redshift of light, which tells you a supernova's velocity away from Earth). Miller Research Fellow Adam Riess was put in charge of analyzing that team's supernovae, correcting for differences in peak luminosity, dimming caused by galactic dust and other issues to plot velocity versus distance from Earth.

In 1998, both teams, using different sets of supernovae, reported that for a given redshift, or velocity, supernovae were farther away than expected in a universe whose expansion is constant or slowing down because of gravity. Thus, they concluded that the expansion is now actually speeding up.

Both groups have received numerous accolades for their discovery aside from the 2011 Nobel Prize in Physics, including the 2007 Gruber Cosmology Prize, which was given equally to the teams and their leaders.

Automated supernova search

To speed up the discovery of new supernovae, Filippenko ultimately built his own supernova search telescope at Lick Observatory near San Jose. Called the Katzman Automatic Imaging Telescope (KAIT), between 1998 and 2008 it found more relatively nearby supernovae - nearly 800 - than all the world's other searches combined. The Lick Observatory Supernova Search allowed him to identify subtleties in the observed properties of Type Ia supernovae and further improve their reliability as measures of cosmic distances. Crucially, once he and his team discovered a new supernova, they leveraged their access to other telescopes that are part of the University of California Observatories - primarily at Lick and Keck Observatory - for follow-up studies and spectral analysis.

"Over half of all these nearby supernovae were young in their development, which then led to numerous studies by my team and others, because it's the nearby bright ones that you study for a long time in detail to learn much more about the supernovae so that you can compare them with theoretical models such as those of Woosley and Nomoto," he said. "We found all kinds of interesting ways in which stars behave prior to and during their explosions. For example, in some cases the gases are ejected quite asymmetrically, which theorists need to explain."

Though eclipsed by subsequent all-sky supernova surveys, KAIT is still used to record the light curves of supernovae, optical counterparts of gamma-ray bursts, active galaxies whose central black holes are swallowing gas, and interesting variable stars, Filippenko said. "This was the predecessor to the wide-angle surveys and it was the name of the game back then," he added.

Filippenko has continued to study the expansion of the universe. Most recently, with Riess and others, he published definitive evidence that the current expansion rate is faster than expected based on observations of the early universe, even when taking into account the known acceleration. This "Hubble tension" is now a leading cosmological mystery.

Originally a chemistry major in the College of Creative Studies at UC Santa Barbara, his growing interest in astronomy - and several unintended chemical explosions that could have cost him his eyesight had he not been wearing glasses - prompted his switch to physics with the intention of becoming an astrophysicist. He graduated in 1979 and received his Ph.D. in astronomy from the California Institute of Technology in 1984.

Filippenko is a Distinguished Professor of Astronomy and the Class of 1954 Chair. His many honors include election to the National Academy of Sciences and the American Academy of Arts and Sciences. He was named an American Astronomical Society Fellow and also received the society's Education Prize. As a member of both the High-z Supernova Search Team and the Supernova Cosmology Project, he was a co-recipient of the 2015 Breakthrough Prize in Fundamental Physics. Berkeley students voted him "Best Professor" a record nine times, and in 2006 he was chosen as the CASE/Carnegie National Professor of the Year among doctoral and research institutions.

The three recipients of the Gruber Cosmology Prize will receive it on Nov. 10 at the "Illuminating the Cosmos" conference at the Max Planck Institute for Astronomy and House of Astronomy in Heidelberg, Germany.