Although most stars shine steadily for billions of years, some of them vary, pulsate, flare or explode on timescales of years, weeks, or even a fraction of a second. These rapid changes provide unique insights into the death of stars, the behaviour of matter at extremely high temperatures and densities, the size and age of the universe, and aspects of fundamental physics such as the nuclear equation of state and Einstein’s theory of general relativity.

Discovering and analysing transient events — the subject of time-domain astronomy — is a challenging task that requires sifting through vast databases, identifying rare anomalies, discarding false positives from terrestrial events and other sources, and notifying the astronomy community, ideally within minutes, to enable follow-up studies from other telescopes.

Throughout his career, Kulkarni has made a sustained series of fundamental discoveries in time-domain optical and radio astronomy. As a student, he and his collaborators discovered the first millisecond pulsar, a rapidly rotating neutron star that emitted precisely spaced pulses over 600 times per second. Known millisecond pulsars now number in the hundreds. They are the most precise astronomical clocks in the universe, and are used to test Einstein’s general theory of relativity and to look for gravitational waves from merging supermassive black holes.

Brief, intense bursts of gamma-rays from across the sky were first detected in the 1960s, but their origin remained mysterious for decades. In 1997, Kulkarni and his collaborators made a critical breakthrough by determining the distance to a gamma-ray burst. They showed that the burst originated in the distant universe, far beyond our own Galaxy, and so must have been an extremely energetic event. We now know that most gamma-ray bursts come from similar distances.

Fast radio bursts (FRBs) are intense bursts of radio emission lasting as little as a thousandth of a second. A type of neutron star known as a magnetar, with extremely strong magnetic fields, has long been a candidate for the source of FRBs (the Shaw Prize in Astronomy was awarded in 2021 for work on magnetars and in 2023 for work on FRBs). Kulkarni and his collaborators built — quickly and inexpensively — STARE2, a set of three radio detectors dispersed across the southwestern United States, designed to detect nearby FRBs. In 2020, STARE2 was one of two telescopes that detected an FRB from a magnetar located in our Galaxy, showing for the first time that magnetars can generate FRBs.

Kulkarni’s contributions culminated in the construction of the Palomar Transient Factory (PTF, 2009) and its successor, the Zwicky Transient Facility (ZTF, 2017), two novel astronomical surveys using a seventy-year-old telescope at Palomar Observatory in southern California. ZTF scans the entire Northern sky every two days, analyses the data with automated software, and communicates its discoveries through an alert system that within minutes provides astronomers around the world with notifications of transient events. The flood of data from PTF and ZTF have enabled the discovery of a wide variety of astronomical transients and variable sources. ZTF has discovered thousands of rare events, including extremely bright supernovae, luminous red novae, calcium-rich gap transients, and disruptions of stars by black holes. ZTF has also found a star swallowing one of its planets, one of the nearest and brightest supernovae in history, a new orbital class of asteroids, binary stars with orbital periods as short as seven minutes that are strong sources of low-frequency gravitational radiation, and many other exotic systems and rare events whose properties are just beginning to be understood. PTF and ZTF have trained a generation of young astronomers now leading the field of time-domain astronomy.

This award is also intended to recognise Kulkarni’s discoveries in other areas of stellar astronomy, in particular his role in the discovery of one of the first “brown dwarfs” — stars so small that they cannot burn hydrogen by nuclear fusion. Brown dwarfs bridge the gap between giant planets like Jupiter and hydrogen-burning stars like the Sun, and this discovery revealed the existence of brown dwarfs with atmospheric properties similar to planets and set the stage for decades of work on the atmospheres of sub-stellar objects.

12 November 2024 Hong Kong