Looking back billions of years to see the first stars in the universe

The cosmos has always held mysteries, but perhaps none as profound as the story of its inception, featuring the universe’s first stars. These ancient luminaries, known as Population III stars, were quite unlike those we see today.

Formed primarily from hydrogen and helium and devoid of heavier elements, they burned hotter, larger, and faster than stars like our Sun, exhausting their fuel quickly and living short lives.

These stars are not just astronomical entities but are key to understanding how the universe evolved from a simple to a complex state, spawning the diverse galaxy systems we observe today.

“The first-generation metal-free stars, referred to as Population III (Pop III) stars, are believed to be the first objects to form out of the pristine gas in the very early Universe,” noted the researchers.

Revolutionary tools

Peering back across billions of years to observe these stars is no small feat. Enter NASA’s Nancy Grace Roman Space Telescope. Slated for launch by May 2027, Roman is set to revolutionize our view of the cosmos.

With a field of view 200 times greater than that of the Hubble Space Telescope and the ability to survey the sky 1,000 times faster, Roman offers unprecedented capabilities.

The telescope could potentially unveil the elusive Pop III stars by observing their dramatic endings rather than their brilliant beginnings.

Shredded by giants: The demise of the first stars

Instead of seeking intact stars, astronomers will search for the signs of Pop III stars that have met their demise through interactions with black holes.

This catastrophic end results in a tidal disruption event (TDE), where a star is torn apart, and its remnants glow intensely enough to be seen from great distances.

“Since we know that black holes likely exist at these early epochs, catching them as they’re devouring these first stars might offer us the best shot to indirectly detect Pop III stars,” explained Priyamvada Natarajan, a co-author of the study from Yale University.

Detecting the echoes of ancient light

TDEs emit light across a spectrum, including X-ray, radio, UV, and optical light. Due to the universe’s expansion, the light from these early first stars shifts into near-infrared wavelengths, which Roman is designed to detect.

Moreover, the timescale of these events stretches significantly due to their large redshift. Unlike a supernova, which brightens and fades over shorter periods, a Pop III TDE brightens over hundreds to thousands of days, with a decline that could span over a decade.

“The evolution timescales of Pop III TDEs are very long, which is one feature that could distinguish a Pop III TDE from other transients including supernovas,” noted Rudrani Kar Chowdhury, a postdoctoral fellow at the University of Hong Kong and first author of the study.

The cosmic hunt for the first stars

While the James Webb Space Telescope also plays a vital role in observing TDEs, its smaller field of view limits its efficiency as a TDE hunter.

Roman, with its expansive survey capability through the High Latitude Wide Area survey covering about 2,000 square degrees of the sky, is better suited to this task.

“Roman can go very deep and yet cover a very big area of the sky. That’s what’s needed to detect a meaningful sample of these TDEs,” said Jane Dai, a professor of astrophysics.

The spectroscopic follow-up

Once a TDE is spotted, follow-up observations are crucial. Here, Webb’s advanced spectroscopic tools come into play. Since Pop III stars contain only hydrogen and helium, their TDE spectra would lack the metal lines typically found in later star generations.

Identifying these unique spectral signatures could confirm the presence of Pop III stars.

Expanding our cosmic knowledge

This ambitious approach not only holds the promise of detecting the universe’s first stars but also opens up broader opportunities to understand galactic and cosmic phenomena closer to home.

As we stand on the brink of these groundbreaking observations, the Roman Space Telescope may soon offer a window into the very dawn of time, bringing us closer to understanding the grand narrative of the cosmos.

The study is published in the journal Astrophysical Journal letters.

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