CHARA Array Celebrates 20 Years of Transformative Stellar Research

The Center for High Angular Resolution Astronomy (CHARA), operated by Georgia State University (GSU), has made significant contributions to the field of astronomy over its two decades of operation. Located at Mt. Wilson, California, this six-telescope optical interferometer has published over 275 research papers since its debut in 2005, cementing its role as a leading facility in stellar research.

The CHARA Array was established to enhance our understanding of stars, and it has succeeded remarkably. According to Dr. Hal McAlister, Regents' Professor Emeritus of Astronomy at GSU, "These first results from the CHARA Array provide the first interferometric measurement of gravity darkening in a rapidly rotating star and represent the first detection of gravity darkening in a star that is not a member of an eclipsing binary system." This pioneering work has laid the groundwork for further advancements in the field.

From its inception, the CHARA Array faced skepticism regarding its viability. Theo ten Brummelaar, the array's former director, noted the challenges in securing funding and building such a sophisticated instrument at a time when GSU was not yet recognized as a leading research university. Nonetheless, the support from the university was instrumental in overcoming early hurdles. "We were a very small team of people with little history of designing and building large instruments like this," said ten Brummelaar. "Nevertheless, we had a great deal of support, both financial and moral, from the university, and now CHARA and GSU are leaders in the field of ground-based optical interferometry and the astrophysics it enables."

CHARA's research has been pivotal in measuring stellar sizes and understanding their evolution. Combined with data from missions like Gaia and Hipparcos, CHARA's findings have allowed scientists to delve deeper into stellar evolution models. The facility has also resolved circumstellar disks around Be stars, revealing critical information about the material ejected from these stars.

In the current era of exoplanet discovery, understanding the characteristics of the stars these planets orbit is essential. Douglas Gies, the current director of CHARA, underscored this importance, stating, "Without understanding stars, we'll never understand planets." The facility has also excelled in studying rapidly rotating stars, which push the boundaries of stellar physics. These stars tend to exhibit gravity darkening, a phenomenon where the equator appears cooler than the poles due to their oblate shapes.

Another remarkable contribution from CHARA has been in the study of Nova explosions. The facility has imaged the expanding fireballs of Novae, revealing complex structures that evolve as the gas expands and cools. Research published by Schaefer et al. in 2014 in *Nature* highlighted CHARA's ability to accurately measure the expansion rates of these explosions, showing that a bipolar structure forms as early as the second day.

In recognition of its contributions, the National Science Foundation awarded CHARA $3.5 million in 2024 to enhance access for researchers. Gies remarked, "The National Science Foundation award is the key to open the array to the best ideas about new avenues for research. There will be remarkable new results coming soon about stars, planets, and distant active galaxies."

Recent upgrades to CHARA have included new instruments and the addition of a seventh mobile telescope, which will significantly increase the array's baseline from 330 meters to 550 meters. This enhancement is expected to improve the array's ability to image larger stars and further advance research capabilities.

As the CHARA Array enters its third decade, it continues to lead in optical and infrared interferometry, providing unprecedented observational capabilities. Nigel Sharp, a program director in the National Science Foundation’s Division of Astronomical Sciences, stated, "CHARA runs the best optical and infrared interferometer in the world and delivers the highest resolution observations possible at these wavelengths. It is exciting to see that such observations can be delivered routinely and that CHARA’s sought-after capabilities are now available to non-experts in the research community." The future of CHARA looks promising, with the potential for new discoveries that could reshape our understanding of the universe.

In conclusion, the CHARA Array represents a remarkable achievement in astronomical research, facilitating groundbreaking studies that continue to unravel the mysteries of stellar phenomena. As it moves into its third decade, the facility's ongoing advancements promise continued contributions to our understanding of the cosmos.