In the year 1054, a new star appeared in the constellation Taurus. The faint speck of light brightened rapidly, soon outshining other imposing stars in the northern sky. In a matter of days, the star’s brightness peaked. It stayed visible for weeks, even during the day, before it started to dim and slowly fade into nothingness.
The baffling star that embellished the sky in 1054 was in fact a supernova, just one of many transient sources appearing in the sky — that is, objects related to events that occur on short timescales, often changing visibly from night to night. Taking many forms and colors, some transients originate in the Milky Way, while others are objects exploding in galaxies far away.
The interest in transients has never been greater. Many surveys of the sky are discovering new sources at unprecedented rates. In 2019, astronomers reported about 20,000 newly discovered transient objects at visible wavelengths, about 100 times greater than a decade prior.
This firehose of data has the potential to transform astronomy and provide insight into subjects ranging from dark energy and dark matter to the evolution of our solar system. But it also presents unique challenges — how to make sense of the data, and how to follow up on it.
When a trickle becomes a flood
Surveys find transients by imaging the same parts of the sky with a certain cadence. A sequence of images reveals new sources and their change in brightness over time. Such information is not always enough to classify a transient. For that, one would need to obtain a spectrum of the transient, and perhaps even observe it at infrared, X-rays, or radio wavelengths.
However, the era when astronomers could follow up on every object that came along has already gone away — there are now simply too many being found. “We’re already for many years in a regime when you have to make choices [about] what you classify spectroscopically and what you don’t, and that depends on science,” says Daniel Perley, a researcher at Liverpool John Moores University in the U.K. It is telling that the community obtained the spectra for only about 10 percent of transients discovered in 2019.
Perley’s goal is to take stock of the bright transient population detected by the Zwicky Transient Facility (ZTF), which has been one of the most productive transient surveys since it began operating in 2018, spotting supernovae and fast-moving asteroids alike. By limiting the study to the brightest sources, it is feasible to take useful spectra of every object in the survey and learn, for example, how many supernovae of a certain spectral type explode in the universe.
Scientists interested in particular types of transients, like those exhibiting an especially red or blue color, have to take a different approach. For them, the initial limited amount of information determines whether a transient merits a long follow-up observation campaign. Their decision to use additional resources on a transient is based on experience, yet it always carries a bit of risk — you don’t really know a transient until you take that additional observation.