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The SOAR Telescope, located on Cerro Pachón in Chile, has received a major upgrade with the installation of the SOAR Telescope Echelle Spectrograph (STELES). The long-awaited instrument achieved first light in August with observations of the binary star system Eta Carinae, along with 13 other targets. SOAR is part of U.S. National Science Foundation Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF NOIRLab.

STELES was installed on the SOAR Telescope on 30 July 2025 and on 6 August, from its perch on Cerro Pachón in Chile, it pointed toward the constellation Carina to observe one of the most fascinating pairs of stars in our Milky Way—Eta Carinae.

Eta Carinae is a binary star system—two stars orbiting each other—with a long and curious history of brightening and dimming. The system is best known for its "Great Eruption" in 1837, during which it underwent a tremendous explosion and became one of the brightest objects in the night sky, before dimming again. In the centuries since, astronomers have watched Eta Carinae as it mysteriously fluctuates in brightness.

Current estimates hold that Eta Carinae's larger star is about 90 times the mass of the sun, whereas the smaller star is around 30 times the mass of the sun. And while the system is greater than 5 million times more luminous than the sun, it appears faint in our sky due to being heavily obscured by the Homunculus Nebula—a cloud of material ejected from the larger star during the Great Eruption.

This fascinating object was chosen as a first light target for STELES in recognition of Brazilian astronomer Augusto Damineli, who was the first to propose that Eta Carinae was a binary system and who led the acquisition of most of the funding necessary for the construction and installation of STELES at SOAR.

STELES was designed in Brazil by the Laboratório Nacional de Astrofísica (LNA), part of the Ministério da Ciência, Tecnologia e Inovação (MCTI), and the Instituto de Astronomia, Geofísica e Ciências Atmosféricas from Universidade de São Paulo (IAG/USP). The optical design was done by Bernard Delabre from ESO. Components for the instrument's CCD detectors were designed, fabricated, and tested at CTIO.

The instrument arrived at CTIO in May 2016 with a substantial amount of assembly and testing still needed. For the next nine years, the teams worked diligently, overcoming logistical and technical challenges, delays due to the COVID-19 pandemic, and the need for multiple excursions from Brazil to Chile. On the night of first light, the teams felt a true sense of accomplishment as STELES successfully acquired the spectra of 14 stars, galaxies, and planetary nebulae.

"First light marks the achievement of a major milestone, and we celebrate it as a joint achievement of the LNA and the CTIO/SOAR teams," says Felipe Navarete, researcher at LNA and STELES instrument scientist.

STELES works by dividing a beam of incoming light into two arms, one for the short wavelengths of blue light (300–550 nanometers) and one for longer wavelengths of red light (530–890 nanometers). Echelle gratings in each arm act similarly to a prism, further separating each section of light into its spectrum of constituent colors. The spectrum can tell scientists detailed information about an object's chemical composition, motion, rotation, and distance.

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STELES can see a wide range of visible light in a single shot, meaning it can capture most of the photons that reach it. This large light-collecting capability, combined with a sophisticated detector system and the excellent image quality of the SOAR Telescope, allows STELES to quickly take precise measurements of faint distant stars.

With the high-quality data provided by STELES, scientists will be able to study large numbers of metal-poor stars in and outside of our galaxy. Specifically, STELES will search for the theorized first generation of stars, known as Population III, which are the earliest born stars in the universe's history and contain virtually no metals—elements heavier than helium. These oldest stars have never been directly observed.

"STELES will undoubtedly enhance SOAR's spectroscopic capabilities and will be a boon for researchers in the U.S. and Brazil," says NSF Program Director Chris Davis. "STELES offers a unique combination of high spectral resolution and ultraviolet capability, making it a powerful tool for advancing our understanding of star and planet formation, the interstellar medium, and hot stars."

Scientists anticipate that STELES data will provide insight into the chemical evolution of the Milky Way and unveil secrets of the early universe. Following some additional on-sky engineering tests, STELES will begin its pioneering search for the universe's oldest stars in early 2026.