NASA’s James Webb Space Telescope (JWST) has uncovered hundreds of galaxies from the first 600 million years of the universe, revealing that early star formation occurred in intense, erratic bursts. These findings, driven by the JADES program and studies of local relic galaxies, suggest that hot, massive stars were the primary drivers of cosmic reionization.
For decades, the earliest reaches of the cosmos were a blur. Astronomers could see only a few dozen galaxies from the era when the universe was younger than 650 million years, and even those appeared as mere little smudges
, according to Kevin Hainline of the University of Arizona. Now, the James Webb Space Telescope is replacing those smudges with high-resolution data, revealing that the early universe was far more crowded and active than previously predicted.
The JADES Program and the Epoch of Reionization
One of the most ambitious efforts in the telescope’s first year is the JWST Advanced Deep Extragalactic Survey (JADES). By dedicating roughly 32 days of observation time, the program aims to characterize faint, distant galaxies to understand how the first stellar structures assembled and why some eventually stopped forming stars.
The data is shedding light on the Epoch of Reionization, a period between 500 and 850 million years after the big bang. During this window, the universe transitioned from an opaque, gaseous fog to a transparent state. While scientists previously debated whether supermassive black holes or star-filled galaxies caused this shift, JADES data points toward the latter.
Almost every single galaxy that we are finding shows these unusually strong emission line signatures indicating intense recent star formation.
Ryan Endsley, University of Texas at Austin
These massive stars emitted torrents of ultraviolet light that ionized surrounding atoms, stripping electrons from their nuclei and clearing the cosmic fog. The subsequent reunion of these electrons and nuclei created the strong emission lines that Endsley and his team detected using the telescope’s NIRSpec (Near-Infrared Spectrograph) instrument.
Erratic Bursts of Early Star Formation
The process of building a galaxy wasn’t a steady climb. Evidence shows that these early systems experienced rapid periods of star formation interspersed with quiet intervals. This fits and starts
rhythm may have been caused by galaxies capturing clumps of gaseous raw materials.
Alternatively, the very stars that defined these galaxies may have hindered their growth. Because massive stars explode quickly, they likely injected bursts of energy into their surroundings, periodically preventing gas from condensing into new stars. This creates a cycle of growth and stagnation that differs from the more stable star formation seen in the modern universe.
Measuring Redshift and the 400-Million-Year Mark
To map this timeline, astronomers rely on redshift—the stretching of light to longer, redder wavelengths as the universe expands. By measuring this shift, researchers can determine exactly when a galaxy existed. While the gold standard for this is spectral measurement, researchers have used photometric redshifts—taking photos through narrow color filters—to estimate distances for thousands of galaxies simultaneously.
Using the NIRCam (Near-Infrared Camera), Kevin Hainline and colleagues identified more than 700 candidate galaxies that existed between 370 million and 650 million years after the big bang. The sheer volume of these detections far exceeds what was predicted before the telescope’s launch, suggesting the early universe was significantly more efficient at producing galaxies than previous models assumed.
Boötes I and the Stellar Initial Mass Function
While JADES looks at the distant past, other researchers are using “local relics” to understand the same era. Ultra-faint dwarf galaxies (UFDs) are described as incredibly funky little galaxies
that are roughly 10,000 times less massive than the Milky Way. Because they are metal-poor and composed of very old stars, they act as fossils of the early universe.
Researchers are focusing on Boötes I, a luminous UFD orbiting the Milky Way, to measure the Stellar Initial Mass Function (IMF). The IMF describes the distribution of stellar masses born from a single cloud; in the Milky Way, this typically follows a negative slope, meaning low-mass stars are far more common than high-mass ones.
Measuring the IMF in the early universe is notoriously difficult because most measurements rely on models that already assume a specific IMF. By using the NIRCam instrument to extract data from roughly 10,000 individual stars in Boötes I, and applying a Bayesian inference tool called Starwave, astronomers can test whether the IMF of these “fossils” matches the broken power law or log-normal distribution seen in the Milky Way.
The Stakes of Cosmic Modeling
The ability to verify the IMF in a relic like Boötes I is more than an academic exercise; it is a calibration tool for all of extragalactic astronomy. Because low-mass stars are dimmer, astronomers use the IMF to convert the total light of a distant galaxy into an estimated number of stars. If the IMF in the early universe differed from the Milky Way’s, current measurements of early galaxy masses could be fundamentally wrong.
Between the direct observations of JADES and the forensic analysis of UFDs, a clearer picture is emerging: the early universe was a place of intense, high-mass stellar production and violent energy cycles. The remaining uncertainty lies in whether this “massive star” preference was a universal rule for the first billion years or a characteristic of specific types of early galaxies.
