Twelve point nine billion years ago, a source in the Sunrise Arc galaxy emitted light that journeyed through stretching space until it arrived on Earth. By the time the Hubble Space Telescope picked it up in 2022, the star dubbed Earendel was the farthest solitary star ever observed, a light from the time that the universe was only 7 percent as old as it is today. But latest observations from the James Webb Space Telescope (JWST) indicate that Earendel might not be an isolated star, but a tightly packed cluster of stars.
The difference matters. Detection of one star at this record-breaking distance depends on an extremely favorable alignment of cosmic geometry. Earendel’s visibility depends on gravitational lensing, a process for which Einstein’s general theory of relativity predicted that light coming from a more distant source would be bent and amplified by a massive foreground object. For Earendel, a galaxy cluster between Earth and the Sunrise Arc warps space-time so intensely that Earendel’s size is amplified. Its apparent size is amplified by at least a factor of 4,000. The object nearly exactly sits on a “caustic,” a thin band of maximum magnification, in which tiny sources appear thousands of times brighter.
Early Hubble and initial JWST Near-Infrared Camera (NIRCam) imaging indicated Earendel might be more than twice hotter than the Sun and a million times brighter, maybe accompanied by a cooler companion star. But the new spectroscopic information from JWST’s Near-Infrared Spectrograph (NIRSpec) complicates the image. Massimo Pascale of the University of California, Berkeley, and colleagues compared Earendel’s spectroscopic continuum, which is the smooth brightness variation with wavelengths, and discovered that it was in close agreement with combined light from many stars, similar to globular clusters observed in the local universe. “What’s reassuring about this work is that if Earendel really is a star cluster, it isn’t unexpected!” Pascale said.
NIRSpec, an instrument that can resolve light between 0.6 and 5.2 microns at spectral powers to R≈2,700, provides a clearer picture than NIRCam photometry. However, as Brian Welch of the University of Maryland, who spearheaded the discovery of Earendel but was not a member of the new research team, warned, “At the spectral resolution of the NIRSpec [instrument], the spectrum of a lensed star and a star cluster can be very similar. It is therefore important to consider all available data when attempting to classify these highly magnified objects.”
If Earendel is actually a cluster, it could be an early instance of the type of tightly packed stellar systems that coalesce to become our modern globular clusters. Simulations of supersonically created gas objects dark-matter-free regions of original primordial gas have them creating massive, densely bound clusters in the early universe and making high-mass stars in large quantities. Other lensed systems, including the arc of the Cosmic Gems, have been found to contain proto-globular clusters younger than 50 million years, with star densities several orders of magnitude greater than in typical local clusters.
The derived metallicity of Earendel, lower than 10 percent of the Sun’s, and its age estimate of more than 30 million years fit in with these ancient cluster populations. Verifying the object’s true nature will probably rely on the detection of microlensing events short, subtle variations in brightness produced when individual stars in the foreground lens pass in front of the background source. These events are more noticeable for compact sources such as single stars than for extended clusters. Microlensing has been applied to map the shape of remote stars and to make otherwise invisible bodies visible, but the necessary alignments are transient and rare.
Ongoing monitoring of Earendel’s light curve might show whether its brightness has a characteristic that betrays it as a solitary star. For the time being, Earendel is a mystery: either the farthest star ever observed or an unprecedented glimpse of a primordial galaxy cluster. Either way, it provides a glimpse of the rarefied process by which the first shining structures were formed, when gravity, the dynamics of gas, and cosmic growth orchestrated the galaxies that exist today.
