Transients are sudden, short-lived cosmic events—from exploding stars to objects torn apart by black holes—that evolve on human timescales. Modern wide-field surveys have driven discoveries from a few per year to tens of thousands, enabling demographic and physical studies. Multiwavelength observations and new space telescopes (Roman, Einstein Probe, Ultraviolet Explorer) will be crucial to identify energy sources and link diverse classes.
Mysterious Cosmic Flashes: How New Surveys Are Revealing Extreme, Short-Lived Events
Ron Miller (illustrations) and Jen Christiansen (graphic)
Long before humans watched the skies, a cluster of stars orbited a galaxy-scale black hole. About 200 million years ago, a collision in that cluster sent a sun-sized star plunging inward. The black hole—roughly a million times the star’s mass—tore the star apart: some gas formed an orbiting stream that flattened into an accretion disk, while the rest was shredded in a single, brilliant flash.
That flash was recorded on Sept. 19, 2019, by the Zwicky Transient Facility (ZTF). Astronomers labeled the event AT2019qiz; images taken three days earlier showed nothing. On Sept. 25, 2019, the Keck I telescope identified AT2019qiz as a tidal disruption event, in which a star is ripped apart by a black hole’s tidal forces. The disrupted star briefly radiated roughly 10 billion times the Sun’s luminosity.
AT2019qiz remained active. An unrelated star on a crossing orbit repeatedly plunged through the newly formed accretion disk, producing smaller but clear flashes. From December 2023 onward the system repeatedly brightened and dimmed—peaking, fading, then rising again nine times—on a roughly 48‑hour cadence. Between 2019 and 2024, telescopes on the ground and in space observed AT2019qiz across x-ray, ultraviolet, optical and infrared bands; the combined multiwavelength data show it began as a tidal disruption event and later produced quasi-periodic eruptions.
What Are Transients?
Transients are astronomical sources that appear suddenly and typically fade on human timescales—seconds to months—challenging the intuition that the cosmos evolves only over billions of years. They range from stellar deaths (supernovae, magnetar flares, fast radio bursts, fast optical blue transients) to violent events near supermassive black holes (tidal disruption events, quasi-periodic eruptions, changing-look quasars and ambiguous nuclear transients).
Why Detections Have Exploded
Detection rates have surged because of wide, fast sky surveys. From 1976–2012 the International Astronomical Union’s Transient Name Server logged roughly five transients per year; that rose to about 100 per year in 2013–2015 and to roughly 20,000 per year since 2019. At press time the server listed about 175,953 transients total.
Facilities such as ZTF scan the northern sky every two nights; the Vera C. Rubin Observatory (which began full operations in 2025) surveys the southern sky every three nights and can flag changes within 60 seconds. Together, these programs produce near–real-time movies of the sky and create an enormous, rapidly growing catalog of transient phenomena.
Types And Open Questions
Supernovae remain the most numerous stellar-death transients and fall into two broad classes: thermonuclear explosions (white-dwarf systems that detonate after accreting mass) and core-collapse supernovae (massive stars whose cores implode, leaving neutron-star remnants). Beyond classical categories are many puzzling variants:
- FBOTs (Fast Optical Blue Transients)—very bright but evolving in days rather than months (e.g., AT2018cow).
- Gamma-Ray Bursts—brief, extremely luminous high-energy flashes often linked to collapsing massive stars or compact-object mergers.
- Fast Radio Bursts (FRBs)—millisecond radio flashes from cosmological distances that can emit in one millisecond as much radio energy as the Sun emits in ~100 years.
- Magnetar-Related Phenomena—neutron stars with magnetic fields ~10^14 gauss that produce powerful flares and may explain several transient classes.
Nuclear transients, which arise in galactic centers, present additional puzzles. Tidal disruption events (TDEs) occur when stars are torn apart by supermassive black holes; quasi-periodic eruptions can follow when another star repeatedly intersects the newly formed accretion disk. Other nuclear transients—changing-look quasars and ambiguous nuclear transients (ANTs)—rapidly change brightness or spectral character in ways that challenge existing accretion models.
How Astronomers Will Learn More
Discriminating among transient types requires multiwavelength coverage: different photon energies trace different physics (for example, ultraviolet from shock breakout; x-rays and radio from ejecta interacting with surrounding gas). New and upcoming observatories will expand that coverage: China’s Einstein Probe (x-ray; collecting since mid‑2025), NASA’s Nancy Grace Roman Space Telescope (infrared; planned mid‑2027), and NASA’s Ultraviolet Explorer (UV; planned ~2030), alongside ground-based networks.
Transients are not only fascinating oddities: they are tools. Supernovae calibrate cosmic distances; TDEs and quasi-periodic eruptions probe dormant and intermediate-mass black holes; FRBs can map the distribution of ordinary matter across cosmic volumes. Above all, transients are natural laboratories for physics under conditions unattainable on Earth.
Bottom line: Rapid, wide-field surveys and coordinated multiwavelength follow-up have transformed transients from isolated curiosities into a rich, data-driven field. Each new detection helps reveal the physics of the most extreme, short-lived events in the universe.
