2026 Gruber Cosmology Prize

Observations of the sudden appearance of a new star, or nova, in the night sky date to ancient times. Not until the 1930s, however, did astronomers adopt the term supernova to differentiate between two types of bright stars. A nova brightens intensely before returning to its previous level of luminosity, while a supernova ends its life in a display of fireworks tens of thousands of times brighter than a nova. 

When the three recipients of the 2026 Gruber Prize in Cosmology started studying supernovae in the late 1970s and early 1980s, sightings of supernovae were rare, and that field was still in its infancy. Over the following four decades the independent investigations of Alex Filippenko (via observations) and Ken Nomoto and Stan Woosley (via theory) have transformed not just the science of exploding stars but astronomy itself.

In retrospect, those days were the end of an era, the last time astronomers trekked to an observatory on a mountaintop, put eye to eyepiece, and captured a photo on a glass plate. The arrival of new technologies revolutionized astronomy: CCD imaging that allowed observers to gather and interpret data farther across space and (the speed of light being finite) time; computer processing that allowed theorists to shorten their model-making and model-testing from months to minutes. Suddenly the supernovae available for study, whether by observers or theorists, were no longer rare and random. Now they were frequent and focused—a feast that Filippenko helped harvest and cultivate and that Nomoto and Woosley helped interpret and advance.

Astronomers had known since the early 1940s that supernovae come in at least two varieties, Type I (those that do not contain hydrogen) and Type II (those that do). Filippenko’s observations in the mid-to-late 1980s and early 1990s helped identify three Type I subcategories, including Type Ia (the other two being Ib and Ic, which astrophysicists now know possess different origins). For theoretical models of Type Ia, Nomoto and Woosley independently studied the evolution of the white dwarfs that undergo mass accretion from the companion stars in close binaries and found that the white dwarf explosion models are in good agreement with the observational features of Type Ia. Filippenko, however, found that Type Ia supernovae come in several varieties whose peak luminosities, durations, and chemical compositions differ. This observational work was crucial in establishing that, despite their common origins, Type Ia supernovae vary too much in intrinsic power (luminosity) to serve as “standard candles”—sources of light whose apparent brightness can be used to measure distance. Shortly thereafter, Mark Phillips of the Cerro Tololo Inter-American Observatory confirmed a suspected correlation between their duration and peak luminosity, meaning that Type Ia supernovae could be standardizable candles. This finding was key to the observational programs of the two rival collaborations that independently concluded the expansion of the Universe is accelerating under the influence of (a still-mysterious) “dark energy.” (Filippenko was a member of both teams, though not at the same time, and therefore shared in each of their 2007 Gruber Prizes in Cosmology.)

Filippenko also conceived the Lick Observatory Supernova Search, which from 1998 to 2008 found more relatively nearby supernovae than all other searches worldwide combined. In addition, he continued to work on identifying subtleties in the observed properties of Type Ia supernovae (such as yet additional variations of this subclass) that further improved their reliability as measures of cosmic distances, he explored the use of Type II supernovae for cosmology, and he studied many different additional kinds of supernovae. Most of this research was conducted with the telescopes at Lick (California) and Keck (Hawaii) Observatories.

In the 1980s, Nomoto and Woosley, mostly working independently of each other but in a complementary fashion, provided the theoretical foundations for the life-cycle processes and inherent properties of Type Ia supernovae. Then in the 1990s, again independently, Nomoto and Woosley worked on models of Type II supernova formation that matched observations of gamma-ray bursts, the most energetic phenomena in the Universe. Woosley concluded that GRBs are the result of stars collapsing into a black hole, and he and Nomoto both explored the observational consequences of such a model. Woosley himself made some of those observations of GRBs (which can be observed only from outside Earth’s obscuring atmosphere) as a coinvestigator on the High Energy Transient Explorer space mission from 2000 to 2008. Since the word supernovae no longer seemed adequate to describe such ultra-energetic events, Nomoto adopted the term hypernovae.

Astronomers have known since the 1950s that successive generations of exploding stars seed the Universe with heavier and heavier elements through thermonuclear processes. (Woosley’s postdoc advisor was one of the four authors of the landmark 1957 paper, Nobel laureate Willy Fowler.) Again working independently, and using codes run on the supercomputers of the day,

Woosley and Nomoto calculated models for individual supernovae that addressed not only the complicated physics of the explosion but the specific amount of each element made and ejected.

When summed over all supernova masses and types, the result agreed accurately with what we see in our Solar System and many other stars formed throughout the history of the galaxy. In effect, Woosley and Nomoto had transformed the study of supernovae into a predictive science.

Taken together, Filippenko, Nomoto, and Woosley’s work links stellar evolution, explosive nucleosynthesis, the origin of heavy elements, and the chemical evolution of the Universe, while also supporting the use of supernovae for precision cosmology.