Mining for Metals in the Sky
WHY YOU SHOULD CARE
Because Carl Sagan was right — we really are all made of star stuff.
By Melissa Pandika
Mansi Kasliwal is not one to sit still. She sleeps little. Those who know her describe her as “hyperactive” and filled with “tremendous energy.” Now, the Caltech astronomer is putting her vim and vigor to the test by chasing equally dynamic astrophysical transients — ephemeral explosions a million or billion times brighter than the sun.
As principal investigator of the recently launched Global Relay of Observatories Watching Transients Happen (GROWTH), Kasliwal scours the night sky for astrophysical transients using a network of powerful telescopes that encircle the Earth. Astrophysical transients are explosions that occur when celestial objects — like a neutron star or black hole — slam into each other, emanating a brief flash of flight. Here’s how it works: The Palomar Transient Factory (PTF), a system of telescopes at Palomar Observatory outside San Diego, spots a transient; telescopes at the closest observatory to the west home in on the flash, taking photographs and collecting data before the sunrise renders it invisible. Then, if the PTF spots a transient above Los Angeles, for instance, telescopes in Hawaii train their gaze toward it, followed by telescopes in Taiwan, Japan, Israel, Sweden and Germany. At the end of this chain of events, Kasliwal and her team members analyze the light from these explosions, some of which scientists theorize to be giant factories that produce gold, uranium and other heavy elements. One of her main goals: Identify the celestial objects involved and where they collided. Already, she’s discovered a rare class of transients responsible for churning out around half the calcium in the universe. Understanding these esoteric explosions and other transient events can yield clues about the origins of the cosmos and the materials that constitute it … and us.
Kasliwal’s discovery of new types of cosmic explosions has “carved new fields,” says Enrico J. Ramirez-Ruiz, professor and chair of astronomy and astrophysics at the University of California, Santa Cruz — this in a discipline where cosmic-explosion research has garnered Nobel nods. Kasliwal may not have a Nobel in her immediate future, but Ramirez-Ruiz christened her “one of the best young astronomers of her generation.” Her research heralds a shift from studying still images of the sky to forming dynamic understandings of how heavenly bodies change over time. She’s building on a nascent field called celestial cinematography, made possible by powerful telescopes that, in the last decade or so, have been able to shoot a rapid series of photos over larger swaths of sky, essentially creating a film of the universe. Now researchers like Kasliwal can observe events occurring over a span of hours versus, say, billions of years, Ruiz says. Think of it like a slow-motion reel of the night sky.
Was nature not creative enough? Might there be other sorts of explosions?
Skyping from her office at Caltech, Kasliwal evokes the geeky-quirky fashion sense of The Big Bang Theory’s Bernadette Rostentowsky, sporting a long ponytail and geometrically patterned blouse. She describes growing up on her parents’ dairy farm in Indore, India, devouring “beautiful encyclopedias.” She muses: “Solving a physics problem or getting to the heart of an astronomy mystery and looking at the beautiful images and realizing it actually looks like this out there — it made me happy.”
At 15, Kasliwal uprooted from India to attend boarding school in Connecticut. During her junior year of high school, she took college classes before enrolling at Cornell, where she studied applied engineering physics. Watching groundbreaking images stream in from the Spitzer Space Telescope left her enamored by the heavens. After graduation, she turned down a cushy Wall Street job to pursue a Ph.D. in astrophysics at Caltech.
Her quest to understand the inner workings of astrophysical transients began with a riddle presented by her Ph.D. adviser, Shrinivas Kulkarni, while on a drive to Palomar Observatory. For over a century, scientists had known of two types of cosmic flashes: novae, about a million times brighter than the sun, triggered by explosions of dim, burned-out stars called white dwarfs, and supernovae, about a billion times brighter than the sun, which result from the collapse of massive stars or when a white dwarf siphons off excessive amounts of gas from a binary companion star. Kulkarni asked the question: Was nature not creative enough? Might there be other sorts of explosions? Kasliwal took the challenge, designing three experiments to search for distinct explosions. Kulkarni cautioned that she might find nothing, but, he says, “she never asked me about a backup program.” Not even when her first, then second, experiments flopped.
But the third try, a digital survey of the sky using the PTF, unveiled a class of flashes whose brightness falls between that of novae and supernovae — “quite literally, an explosion of new explosions,” Kasliwal says. What were they? Kasliwal faced a research problem: If a transient appeared shortly before sunrise, Kasliwal couldn’t wait until the next night to analyze it — by then, the flash would have long faded. She decided to build a network of telescopes to chase the flashes. She phoned researchers around the world, persuading them to “trust [her] with their biggest and best toys,” she says. Not everyone was convinced.
In 2015 — within less than a year of landing a faculty position and giving birth to a son — Kasliwal won a $4.5 million National Science Foundation grant. And since launching last October, GROWTH has spotted a handful of progenitor stars before they explode as supernovae.
Kasliwal’s also plumbing the cosmos for the mines that give rise to chemical elements, a hunt whose technology to roboticize telescopes and tackle big data questions might have applications in other research fields. Scientists know cosmic explosions synthesize many elements in the universe, but they still don’t know where the explosions occur in the sky. GROWTH telescopes can locate these explosions, and Kasliwal can then identify the elements they contain based on the spectra of color found when the light is split with a prism. The remnants of these and other explosions eventually form everything from planets to the calcium in our bones, the carbon in our DNA, the iron in our blood and the oxygen that fills our lungs. Kasliwal’s hypothesis: The explosions that produce heavy elements result when neutron stars collide with each other or with a black hole.
Kasliwal recently broadened her search for transients to the infrared spectrum, already detecting 14 explosions falling between novae and supernovae, which may represent the birth of black holes or massive star binaries. “We’re right at the tip of an iceberg,” she says. “That’s what my gut tells me.”
- Melissa PandikaContact Melissa Pandika