60 Million Stars: Inside Euclid’s Unprecedented Map of the Milky Way’s Core

Introduction to ESA’s Euclid space telescope

The European Space Agency’s Euclid space telescope, launched in July 2023, was fundamentally engineered to execute an unprecedented mapping of the dark universe¹. Its primary cosmological objective involves measuring the redshift and morphological distortions of billions of distant galaxies across more than a third of the sky, thereby constraining the parameters of dark energy and dark matter¹. However, the astronomical capabilities required for such a vast cosmological survey—namely, a wide field of view combined with exceptionally high angular resolution and photometric stability—render the observatory an immensely powerful tool for localized galactic astrophysics.

In March 2025, Euclid temporarily paused its extragalactic cosmology program to conduct the Euclid Galactic Bulge Survey, a focused, high-cadence observational campaign directed at the dense, star-rich core of the Milky Way galaxy⁶. The resulting dataset, formally released to the scientific community in June 2026 as part of Euclid's Quick Data Release 2, constitutes the largest and most detailed visible-light survey of the galactic center ever produced⁸. By resolving more than 60 million individual stars across a highly crowded field, the survey serves as a critical foundational dataset for gravitational microlensing studies¹. This report provides an in-depth academic analysis of the survey, examining the underlying spacecraft architecture, the technical challenges of crowded-field data processing, the physical mechanisms of microlensing, and the profound synergistic implications for the upcoming Nancy Grace Roman Space Telescope.

Spacecraft Architecture and Instrumental Capabilities

Euclid operates from a Lissajous orbit around the Sun-Earth Lagrange point 2, located approximately 1.5 million kilometers from Earth². The spacecraft payload centers on a 1.2-meter Korsch off-axis three-mirror anastigmat telescope constructed from silicon carbide³. Silicon carbide was chosen for its extreme thermal and mechanical stability, which is vital for preventing minute structural deformations that could compromise the telescope's point spread function¹¹. The incoming light collected by the primary mirror is divided by a dichroic filter located in the exit pupil, which reflects visible wavelengths to the visible imager and transmits near-infrared wavelengths to the near-infrared spectrometer and photometer⁴.

The Visible Imager

The visible instrument operates as a massive wide-field camera covering a exceptionally broad optical bandpass. It utilizes a focal plane array consisting of thirty-six charge-coupled devices, each featuring a grid of four thousand by four thousand pixels⁴. With a physical pixel size of twelve micrometers, the entire detector encompasses approximately 600 megapixels, making it one of the most expansive and sensitive cameras ever deployed in a space environment⁷. The visible imager achieves a spatial resolution of roughly 0.1 arcseconds¹³. While this sharpness is comparable to the wide-field instruments aboard the Hubble Space Telescope, Euclid captures an area 270 times larger in a single exposure⁶.

The broad bandwidth of the visible filter, spanning from 550 to 900 nanometers, maximizes photon collection efficiency¹³. This optimization is critical for the detection of faint, low-mass stars, which are the primary targets for gravitational microlensing observations⁵.

The Near-Infrared Spectrometer and Photometer

Complementing the visible light data, the near-infrared instrument provides critical infrared measurements that are capable of penetrating the thick clouds of interstellar dust that obscure the galactic plane⁴. The instrument contains an array of sixteen mercury cadmium telluride detectors¹¹. To ensure low thermal noise during infrared observations, the detectors are actively cooled to operating temperatures below 100 Kelvin, while the associated readout electronics are maintained at approximately 140 Kelvin¹¹.

The near-infrared payload operates in two primary modes. In photometer mode, the instrument routes light through three broad photometric filters corresponding to the Y, J, and H bands, allowing astronomers to measure the precise brightness and color of sources across the near-infrared spectrum⁴. In spectrometer mode, the light is dispersed by a series of grisms—three optimized for redder infrared wavelengths and one for bluer infrared wavelengths—to perform slitless spectroscopy¹¹. This spectroscopic capability is primarily utilized for determining the redshift of distant galaxies, though the photometric data is invaluable for characterizing stellar populations in the Milky Way⁴.

The primary capabilities of both instruments are summarized in the table below.

Table 1: Primary Specifications of Euclid's Scientific Instruments

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The Euclid Galactic Bulge Survey Design

The Euclid Galactic Bulge Survey was executed over approximately 26 hours spanning March 23 and 24, 2025⁶. Directing a telescope designed for deep extragalactic voids toward the brightest, most crowded region of the Milky Way required careful mission planning. Due to strict thermal stability requirements and stray-light constraints, Euclid must maintain a highly specific orientation relative to the Sun, utilizing its sunshield to keep the optical assembly in permanent shadow²⁰. Consequently, the fixed coordinates of the galactic bulge are only accessible to Euclid twice a year, during the equinoxes, making the timing of this survey highly constrained²⁰.

During this narrow observation window, Euclid completed nine overlapping pointings, creating a contiguous mosaic covering 4.8 square degrees of the sky—an area roughly equivalent to 22 times the angular size of the full Moon⁷.

The Sixteen-Dither Strategy

A critical deviation from Euclid’s standard cosmological survey strategy was the implementation of a high-density spatial dither pattern⁵. In typical extragalactic observations, the standard reference observation sequence involves a minimal number of positional shifts to cover the physical gaps between the charge-coupled devices while maximizing the total survey area covered per day²³.

For the galactic bulge, however, a specialized sixteen-dither strategy was employed⁵. Each of the nine survey fields was imaged using sixteen distinct exposures of 400 seconds each in the visible band, totaling 1.8 hours of integration time per field⁸. This repeated micro-shifting of the telescope's pointing serves multiple indispensable analytical purposes:

1. Sub-pixel Resolution Enhancement: By offsetting the sequential exposures by fractions of a pixel, researchers can reconstruct a highly accurate, oversampled point spread function for the instrument, which is necessary for distinguishing individual stars in tightly packed clusters⁵.

2. Cosmic Ray Mitigation: Solid-state detectors in deep space are frequently bombarded by high-energy particles that register as false point sources. Dense temporal sampling across sixteen exposures allows for the robust statistical identification and rejection of these transient artifacts²⁴.

3. Dynamic Range Optimization: The galactic bulge contains a high population of exceptionally bright giant stars. High-cadence, overlapping exposures prevent the complete loss of local data due to detector saturation, facilitating high signal-to-noise measurements of the faint background stars that act as microlensing sources⁵.

Table 2: Galactic Bulge Survey Field Coordinates

⁵

Data Processing Challenges in High-Density Fields

Processing the raw telemetry from the survey posed severe computational and algorithmic challenges for the Euclid Science Ground Segment²⁴. The standard data pipeline, which utilizes immense computational resources distributed across a dozen countries, was specifically engineered to detect, isolate, and measure the elliptical shapes of diffuse, distant galaxies against a relatively sparse stellar background²⁴. Subjecting this extragalactic pipeline to a field containing approximately 10 billion stars per steradian resulted in significant source confusion and processing bottlenecks²⁰.

Astrometric Calibration and Crowding

Astrometric calibration—the process of determining the precise celestial coordinates of every pixel in an image—relies on matching observed stellar positions against a known reference catalog, which for Euclid is primarily the Gaia mission database²⁴. In the central bulge, the extreme stellar density simply overwhelmed the standard pattern-matching algorithms²⁴. Engineers resolved this failure by drastically filtering the Gaia reference catalog, stripping out the vast majority of entries and utilizing only reference stars within a narrow brightness range (magnitudes 18.5 to 19.0)²⁴. This algorithmic compromise successfully reduced the data volume and processing time, allowing the astrometric solutions to converge²⁴.

Furthermore, standard cosmic ray detection algorithms function by identifying sharp, localized peaks in pixel charge that do not conform to an expected stellar profile²⁴. In the bulge, the density of legitimate, faint point sources is so high that the software routinely and falsely flagged thousands of actual stars as cosmic ray artifacts²⁴. Tuning these threshold parameters required delicate balancing by the engineering teams to preserve the photometric integrity of the 60 million confirmed stars in the final catalog⁷.

Extinction and Chromatic Artifacts

The line of sight toward the galactic center is heavily obscured by vast clouds of interstellar dust¹. This dust absorbs and scatters optical light—a process known as extinction—which severely attenuates the flux of background stars and shifts their apparent spectra toward longer wavelengths, creating interstellar reddening¹⁶.

Because Euclid's visible instrument captures images through a single, highly broad bandpass, the resulting raw scientific images are inherently monochromatic⁹. While excellent for capturing maximum light, this prevents direct color analysis. To generate color composites for detailed stellar population analysis and public release, researchers integrated archival data from the ground-based Canada-France-Hawaii Telescope's MegaCam, utilizing specific optical filters that overlap Euclid's broad visible band²².

However, merging space-based and ground-based optical systems introduces distinct artifacts. The appearance of the most luminous stars in the combined color images exhibits additional diffraction spikes and subtle halos that are absent in the pure Euclid data²². These visual differences are a direct consequence of combining the pristine, diffraction-limited sensitivity of Euclid's space environment with the atmospheric scattering and different secondary mirror support structures of the ground-based telescope²².

Gravitational Microlensing: The Core Scientific Driver

While the survey provides a wealth of general astrophysical data, its fundamental scientific motivation is to advance exoplanetary science through the phenomenon of gravitational microlensing⁵. First theorized as an extension of Einstein's general relativity, microlensing occurs due to the precise, chance alignment of a foreground mass (the lens) and a distant background star (the source) relative to an observer⁵.

As the lens star transits the line of sight, its gravitational field warps the surrounding spacetime, acting as a cosmic magnifying glass⁶. This curvature deflects the light of the background source, causing a smooth, transient, and mathematically predictable increase and subsequent decrease in the source star's apparent brightness over a period of days or weeks¹⁹.

If the foreground lens star hosts an orbiting planet, the planet's localized mass induces a secondary, smaller perturbation in the spacetime curvature⁶. As the magnified image of the background source star sweeps across this secondary curvature—known in optics as a caustic—it creates a brief, uneven spike or anomaly in the otherwise symmetrical light curve⁹. By analyzing the timing, duration, and amplitude of this spike, astronomers can deduce the mass ratio and orbital separation of the planet relative to its host star⁶.

The Advantages of the Microlensing Technique

Compared to the transit method (which heavily favors massive planets orbiting extremely close to their host stars) and the radial velocity method (which requires massive planets to induce a measurable wobble in the host star's spectrum), microlensing is remarkably unbiased regarding orbital distance and host star luminosity⁹. It is uniquely sensitive to low-mass, cold exoplanets situated at wide orbits beyond a star's "snow line"—analogous to the positions of Jupiter, Saturn, Uranus, and Neptune in our own solar system⁵.

Furthermore, microlensing is currently the only robust observational method for detecting free-floating planets³⁰. These are sub-stellar bodies that have been dynamically ejected from their host planetary systems and wander independently through the interstellar medium³⁰. When a free-floating planet crosses a background star, it acts as a solitary lens, producing a very brief microlensing event lasting only a few days or hours, completely devoid of a host star's primary magnification curve³¹.

Despite these advantages, the probability of the required stellar alignment is exceptionally low. Toward the dense galactic center, the chance of observing a microlensing event at any given moment is roughly one in a million; looking away from the center, the odds fall to one in a billion⁵. Consequently, surveys must continuously monitor hundreds of millions of stars simultaneously⁵. While ground-based observation networks have successfully identified over 300 microlensing planets over the past two decades, Earth's atmospheric turbulence limits their angular resolution⁵. Ground telescopes struggle to isolate faint source stars from their brighter neighbors, a phenomenon called blending, which dilutes the microlensing signal and limits ground-based sensitivity to planets larger than two or three Earth masses⁵. Space-based platforms like Euclid overcome this limitation entirely, extending detection sensitivity down to bodies as small as Mars or the Moon⁵.

Breaking the Mathematical Degeneracy: Resolving Historical Events

The most significant analytical limitation of standard microlensing observations is the inherent mathematical degeneracy of the primary observable parameter: the Einstein crossing time³³. The duration of a magnification event is a function of three variables: the lens's mass, its relative transverse velocity, and its distance from Earth³³. Without additional data, a standard light curve cannot distinguish between a massive, fast-moving lens located far away, and a low-mass, slow-moving lens located relatively close to Earth³³. Because the light curve only provides the mass ratio of the planet to the star, an unknown stellar mass results in an unknown planetary mass.

The primary objective of the Euclid Galactic Bulge Survey is not to discover new transient microlensing events—a single 26-hour observation window is far too brief to capture the weeks-long rise and fall of a light curve⁶. Instead, the objective is to conduct a high-resolution forensic analysis of past events⁸.

Over the past twenty years, ground-based survey networks—specifically the Optical Gravitational Lens Experiment (OGLE), Microlensing Observations in Astrophysics (MOA), and the Korea Microlensing Telescope Network (KMTNet)—have cataloged roughly 8,000 microlensing events within the footprint of the Euclid survey³⁰. By revisiting the precise coordinates of these historical events, Euclid's extreme resolving power can detect the foreground lens star and the background source star as two distinct, separated points of light⁸.

Over the intervening decades since the original events were recorded, the natural proper motion of the stars through the galaxy has caused them to physically drift apart from our line of sight⁸. By directly measuring the angular separation and the respective brightnesses of the now-separated stars in the Euclid images, astronomers can definitively calculate the relative proper motion and the lens flux³³. This direct observation shatters the mathematical degeneracy, allowing the exact mass and distance of the host star, and consequently its planet, to be calculated with an uncertainty margin often falling below ten percent¹⁸.

Case Studies: OGLE-2005-BLG-390 and OGLE-2013-BLG-341

The catalog generated by the Euclid survey encompasses 51 previously known planetary systems⁶. Among these are several landmark historical discoveries that have awaited definitive mass characterization.

One prominent target is OGLE-2005-BLG-390Lb, discovered roughly twenty years prior to the Euclid observation⁹. Initial ground-based models suggested it was a cold, icy super-Earth or Neptune-class planet orbiting a low-mass star, but precise mass validation was impossible at the time due to the blended light of the source and lens²⁹. By capturing this system two decades post-event, the new data isolates the lens and source, finally confirming the precise physical properties of the system²⁰.

Similarly, the event OGLE-2013-BLG-341 involved a highly complex binary star system hosting a circumprimary planet⁶. The orbital dynamics of binary stellar lenses create highly complex, overlapping caustic structures that are difficult to model from light curves alone³⁴. By combining the recent high-resolution astrometry with historical archival data from the Keck and Hubble observatories, researchers can trace the orbital mechanics of the binary pair and accurately determine the planetary mass⁶.

Strategic Synergies with the Nancy Grace Roman Space Telescope

The most profound scientific impact of the Euclid survey lies in its forward-looking synergy with NASA's Nancy Grace Roman Space Telescope, officially scheduled for launch on August 30, 2026¹. Operating from the same L2 Lagrange point as Euclid, Roman is specifically designed to conduct the Galactic Bulge Time-Domain Survey²⁸.

Over its five-year primary mission, the Roman telescope will repeatedly observe a roughly two-square-degree patch of the galactic bulge at fifteen-minute intervals, utilizing its Wide Field Instrument to detect thousands of new microlensing events in the near-infrared spectrum²⁸. NASA estimates this immense time-domain survey will discover approximately 1,400 cold exoplanets, probing the galactic demographics of planetary systems analogous to the outer regions of our own solar system²⁰.

Extending the Temporal Baseline

The 4.8-square-degree footprint of the Euclid survey was intentionally designed to envelop the entirety of Roman's future survey field⁶. Because Euclid executed its observation in early 2025, it effectively establishes a "time-zero" reference frame for the exact stellar coordinates Roman will monitor beginning in 2027⁹.

When the Roman observatory detects a new microlensing event, astronomers will immediately consult the Euclid archive to observe the exact state of the lens and source stars several years before the alignment occurred⁹. This pre-event baseline functions similarly to the post-event resolution of historical ground-based data. By comparing the stellar positions in the 2025 Euclid data against the positions during and after the Roman detections in the late 2020s, scientists will accurately measure the proper motion of the planetary hosts⁹.

This inter-mission collaboration effectively extends the temporal baseline of Roman's survey by more than two years²⁸. It guarantees that mass and distance parameters for a significant fraction of Roman's exoplanet discoveries can be derived almost immediately, rather than requiring astronomers to wait decades for the stars to separate²³. Furthermore, for free-floating planets, joint observations can measure satellite parallax—a slight difference in the light curve observed simultaneously from Roman at L2 and ground-based telescopes on Earth—which, combined with the extended temporal baseline, allows for the unambiguous confirmation of planetary mass objects unbound from any star³².

Table 3: Comparative Specifications of Euclid and Roman Surveys

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The synergy between the two observatories is structurally complementary. Euclid’s visible-light data offers supreme angular resolution at shorter wavelengths, which is ideal for precise astrometry and separating tight stellar pairs⁹. Conversely, Roman’s infrared capabilities will peer much deeper through the heavy dust lanes that obscure the very center of the galactic plane, detecting events that are entirely invisible in optical wavelengths²⁸.

Broader Astrophysical Implications

While exoplanetary microlensing dominates the strategic rationale for the galactic bulge survey, the sheer volume and depth of the dataset offer extensive utility for broader galactic astrophysics¹⁹. The deep photometric catalog of over 60 million well-characterized sources facilitates demographic studies of stellar evolution, galactic structure, and localized kinematics¹⁰.

Specific morphologies captured within the survey fields underscore this diverse scientific utility:

• NGC 6451: This is a dense open star cluster located approximately 8,700 light-years away in the constellation Scorpius¹⁹. Observing such clusters in the context of the surrounding bulge allows for precise calibration of stellar age and metallicity gradients against the broader galactic population, providing clues to the formation timeline of the Milky Way¹⁹.

• G000.583-00.870: An emission nebula composed of ionized hydrogen gas, indicating recent active star formation¹⁹. This ionization is driven by the intense ultraviolet radiation of massive, short-lived blue stars embedded within the Milky Way's inner spiral arms, highlighting regions of ongoing stellar genesis²².

• LDN 10: A dense molecular cloud¹⁹. These dark, dust-rich complexes absorb background starlight, appearing as irregular, ink-like voids in the visual spectrum²². Analyzing the differential extinction across these clouds provides researchers with a three-dimensional topographic map of interstellar dust distribution between Earth and the galactic core⁶.

By analyzing the comprehensive color-magnitude diagrams derived from the survey data, researchers can trace the formation history of the Milky Way, differentiating between the older, cooler stars that dominate the central spheroidal bulge and the younger stellar populations residing in the intersecting galactic disk¹⁶. Furthermore, the precise astrometry established by the intensive dither pattern will aid in mapping the complex orbital dynamics of the bulge, shedding light on the gravitational mechanisms that govern galactic evolution over billions of years⁵.

Conclusion

The Euclid Galactic Bulge Survey represents a brilliant exercise in cross-disciplinary astrophysics, proving that hardware optimized for extragalactic cosmology can yield transformative data for localized stellar and exoplanetary science. By navigating severe computational hurdles associated with extreme source crowding and variable dust extinction, the processing teams successfully produced an unprecedented catalog of over 60 million distinct stellar sources.

The strategic value of this survey is twofold. First, it provides a crucial observational endpoint for thousands of historical microlensing events, allowing astronomers to shatter mathematical degeneracies and definitively measure the mass and distance of exoplanets discovered decades ago. Second, it establishes an invaluable pre-event reference frame for the upcoming Nancy Grace Roman Space Telescope. By providing the precise coordinates and brightnesses of millions of source stars years before they undergo microlensing magnification, the Euclid dataset ensures that Roman’s discoveries can be rapidly and accurately characterized without decades of subsequent observation. In seamlessly linking the observational legacy of ground-based surveys with the future of space-based time-domain astronomy, the Euclid Galactic Bulge Survey firmly anchors a new era in the demographic study of cold exoplanets and the structural evolution of the Milky Way.

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