Cosmic Revelations: Seeing the Universe Anew with Time-Domain Astronomy in Infrared Light

The universe is still full of surprises. Even though astrophysicists have observed the cosmos across all forms of light—from low-energy radio waves to high-energy gamma rays—for decades now, new phenomena continually, as it were, come to light.

Kishalay De, a postdoctoral scholar at the Massachusetts Institute of Technology's (MIT) Kavli Institute for Astrophysics and Space Research (MKI), is a seeker of these novel celestial happenings. Earlier this year, he made headlines for observations of a star engulfing a planet 12,000 light-years away. De works in the field of time-domain astronomy, which broadly studies how astronomical objects change with the passage of time. This time-domain element has of course always been integral to astronomy—consider ancient civilizations' tracking of the motions of what humanity later learned were planets, or the Chinese records of a "guest star," a supernova, in 1054 CE.

In more recent years, though, time-domain astronomy has zeroed in on highly ephemeral events. Many of these so-called transients are stellar brightening or even outright explosions that may last only days or even mere hours upon the sky, making them difficult to spot. Increasing the challenge further, astronomers have increasingly shown that quite a number of transients never produce optical light that reaches Earth. Instead, these short-lived cosmic detonations can only be discovered by widely scanning the skies in infrared light, an innovative approach that De and his colleagues are spearheading.

"Starting in graduate school, I decided to explore the blooming field of time-domain astronomy, where we study the dynamic universe—literally chasing explosions in the near and distant universe as they happen," says De, who is now a postdoctoral scholar at MKI. "Unlike other areas of astrophysics, where things stick around for millions to billions of years, the study of fleeting explosions in the sky is really at the forefront of discovering exciting new phenomena, some of which have never been observed or even predicted!"

De came to MKI originally as a NASA Einstein Fellow in 2021 and is scheduled to continue his research as an MIT Kavli Postdoctoral Fellow in 2024. Over his career so far, he has helped build the first wide-field infrared time-domain astronomical survey. Called Gattini-IR, it began observations at the Palomar Observatory in California in 2018. Through his research with that telescope, De brought to light a range of transient events and otherwise obscured objects that optical surveys could not discern.

"Because our eyes are sensitive to optical light, astronomy naturally began in the optical bands, and over the last few decades, we've made tremendous strides in understanding the optical sky," says De. "But what we've also realized, thanks to investigations in other wavebands, is that the optical bands do not reveal a plethora of phenomena in the universe."

Infrared light is just a bit lower in energy and longer in wavelength than optical light. These characteristics allow infrared light to pass through clouds of gas and dust that block optical light. Accordingly, observing in the infrared light has been an extremely valuable tool for studying the universe since about the 1960s.

To increasingly extend time-domain astronomy into the infrared, De and colleagues are surmounting the obstacle of detecting ample celestial infrared light on the ground.

"Infrared astronomy is difficult, because unlike the optical sky, our atmosphere happens to be very bright at infrared wavelengths, implying that we have to detect faint astronomical sources 'behind' the atmospheric glow," De explains. "In addition, infrared detectors happen to be much more expensive than optical detectors, so it is very difficult to build large cameras that can survey the sky. To alleviate these effects, I have been working with a number of teams around the world that are using state-of-the-art technologies to build innovative telescopes where we modify the optics or the detector technology to build these cameras."

A major focus of De's work, both in the transient space and in astrophysics more generally, is binary stars. A star living all on its lonesome like our Sun, without a stellar partner or two, is unusual. De points out that astronomical research on stars has been historically focused on the life cycles of isolated Sunlike stars, of which we now have a fairly reasonable understanding. Yet much remains poorly grasped about the commonest star configuration.

"The last few decades of work are beginning to reveal that the lives of most stars may in fact be substantially altered because of the presence of a companion, meaning that we need to rewrite our textbooks about how a 'typical' star lives its life and produces all that we see around us," says De.

His interest in stars goes all the way back to childhood when De's father bought him a telescope around age 10. De recalls taking the telescope out at nights in the big city of Kolkata, India, where he grew up.

"I was always fascinated with stargazing as a kid, primarily as a fun hobby to identify patterns," says De. "During college, I realized that I could use physics to understand the workings of the stars that I was always fascinated by, and I decided to go to graduate school to pursue my childhood fascination."

Following through on this interest, De is now looking to leverage time-domain astronomy towards better understanding how binary stars interact and evolve together, and specifically when the stars interact and transfer stellar material to each other. Over time, the unequal exchanges can cause brightening and ultimately trigger sudden explosions.

"One of the most challenging and transformative phases in the lives of binary stars arise when they start interacting with each other," says De. "It is during this phase, that they become highly variable in brightness—the physics of which we can only study with time domain surveys."

Yet much of the progression through this phase and other connected phases is not well-documented because optical surveys cannot pierce through obscuring dust clouds formed alongside the transfers.

"Many of these mysterious phases happen to be enshrouded in the optical bands because as mass is transferred and ejected from these stars, the mass cools and forms dust, thereby completely making the stars invisible in the optical bands," says De.

A far greater knowledge of binary star behavior in this realm is expected in coming years, though, thanks to infrared time-domain-directed efforts.

"With the current and upcoming generation of time-domain surveys, we are beginning to be able to finally create a complete roadmap of the lives of stars in binary systems by observing the most crucial phases of mass transfer where these stars dramatically vary in brightness," says De.

As the results continue coming in, De looks forward to sharing them with the wider world. De also has an interest in science communication and enjoys spreading the joy he feels about astrophysically adventuring into the final frontier.

"Perhaps the best thing I like about being an astronomer is that it is so easy and fascinating to talk to the general public about it," says De. "So many of us have just stared at the night sky in awe of the wonders out there."

That awe, as De himself knows well since childhood, is innate and can serve as the springboard for others to scientifically follow in De's and his colleagues' footsteps.

"Not only is it a humbling experience to talk to people about the workings of the universe," De says, "but I believe it is a great way to get youngsters interested in science and technology."