Astronomers have discovered a faint population of young stars extending far beyond the known edge of the nearby spiral galaxy Messier 74. The finding suggests the galaxy is nearly twice as large as previously measured. The new study was published Sept. 4 in Astronomy & Astrophysics.
How galaxies grow
Over time, galaxies grow by accumulating gas from their surroundings. Because this newer gas spins faster relative to the center, it cannot collapse all the way to the core. Instead, it settles into the outer disk, where it eventually sparks the formation of new stars. Astronomers have observed this growth, but whether it is smooth and gradual or occurs in fast, episodic bursts remains unclear. Mergers or interactions with satellite galaxies can also trigger bursts of star formation in the outskirts of galactic disks, making them grow bigger.
One way to identify such growth is the “extended ultraviolet disk” (XUV disk): UV light tracing young stars beyond a galaxy’s traditional boundary as seen in optical light. But astronomers are still debating how to define this “optical boundary,” making XUV disks difficult to classify.
Messier 74 (M74), located roughly 30 million light-years from Earth, was already known to be an interesting case. It has a grand-design spiral structure but shows several signs of stellar population buildup. Its unusually messy, warped and extended HI (neutral hydrogen gas) disk points to a recent accumulation of gas. Four supernovae have been detected in the galaxy, and a 2007 study flagged it as hosting an XUV disk.
In the new study, researchers led by Ignacio Ruiz Cejudo of the Institute of Astrophysics of the Canary Islands used deep optical imaging to spot a faint stellar population on the outskirts of M74. Using extremely long-exposure images (5.4–6.5 hours per filter) from a new telescope called the Transient Survey Telescope (TST) at Teide Observatory, they were able to detect much fainter light than standard optical surveys like the Sloan Digital Sky Survey (SDSS).
Explosive growth
Their work revealed an extremely faint, distinctly blue stellar structure extending 30 kiloparsecs (around 97,800 light-years) from the galaxy’s center, far beyond the previously known disk radius of about 14 kiloparsecs (around 45,600 light-years)—nearly doubling the galaxy’s size.
Comparing their findings with archival HI (THINGS survey) and UV (GALEX satellite) data, they found that the new optical structure aligns with the outer HI contours and the previously known UV-extended disk. “Now, our ultra-deep optical data reconcile the galaxy’s apparent size with the observed HI and UV extents,” the team wrote in its paper.
An analysis of the stars’ colors using a stellar population model suggests they formed roughly 640 million years ago. The estimate depends on metallicity, with possible ages ranging from about 260 million years to just over 1 billion years—extremely recent on galactic timescales. Based on the stars’ age, the team favors a close gravitational encounter with an irregular, blue satellite galaxy as the cause. “Given this and the visual inspection of the field, we propose a recent flyby of UGC 1176 at least 1 Gyr ago as the trigger for the new star formation in the outer regions,” they conclude.
The team arrived at the gravitational encounter scenario by comparing the estimated age of the new stars with how long it would take such an encounter to trigger star formation in the outskirts. Whether this kind of rapid, explosive, flyby-triggered disk growth is common in the universe or M74 is unusual remains a question for future work.
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Publication details
Ignacio Ruiz Cejudo et al, The explosive growth of the Messier 74 galaxy, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202659957
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Faint young stars reveal spiral galaxy Messier 74 may be twice as large as thought (2026, September 24)
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