In the cold, dark depths of space, giant clouds of gas and dust slowly give birth to the brightest beacons in the universe — massive stars. Now, an international team of astronomers has peered into one such stellar nursery with unprecedented detail, uncovering a surprising dual process of star formation that challenges long-held theories.
Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the researchers have, for the first time, observed two distinct modes of fragmentation — the breaking up of vast clouds into smaller, star-forming clumps — coexisting within a single molecular cloud. Their findings, published in Astronomy & Astrophysics, also reveal a complex, multi-scale “feeding” process that funnels material into growing stars.
A Molecular Cloud with a Hidden Story
The team focused on a structure known as a hub-filament system (HFS) — an intricate cosmic web where long filaments of gas converge into a dense central hub. These regions are thought to be the ideal breeding grounds for massive stars, but the precise details of how such giants form have remained elusive.
Their target, the HFS called I18308, sits in a high star-forming region and displays the classic features of this architecture. “It’s like a cosmic highway system,” explained one team member, “where the filaments are the roads and the hub is the bustling city center where new stars are assembled.”
Two Fragmentation Modes, One Stellar Nursery
When clouds fragment, they can break apart in different ways depending on the balance of forces at play — turbulence, gravity, and temperature. In the I18308 cloud, ALMA’s high-resolution observations (about 3,000 astronomical units, or AU — roughly 450 billion kilometers) revealed something never before seen: two fragmentation modes operating at the same time but in different parts of the cloud.
The two main filaments, dubbed F1 and F2, showed a cylinder-like fragmentation pattern, with star-forming cores spaced almost periodically along their length. This pattern matched predictions from turbulence-dominated fragmentation, where chaotic motions in the gas break it up like ripples on water.
In contrast, the central hub clump exhibited a spherical-like fragmentation pattern, consistent with gravity-dominated Jeans fragmentation, where the mutual pull of matter overcomes all other forces and causes the gas to collapse inward.
This dual-mode behavior overturns simplified models that predict a single fragmentation process across all density scales — such as the global gravitational collapse model — and paints a more nuanced picture of stellar nursery physics.
Growing Stars Without Giant Seeds
One of the most intriguing results came from what the astronomers didn’t find. The data revealed no massive “seed” cores — the kind of high-mass, prestellar clumps larger than 30 times the mass of the Sun that some theories suggest are needed to make massive stars.
Instead, all the cores detected were of relatively low mass, but they grew heavier and denser over time. This pattern supports a multi-scale accretion scenario: small cores form first, then steadily feed on a continuous flow of material from their surroundings — from the filaments to the hub to the core itself — until they become massive stars.
“It’s a bit like building a skyscraper floor by floor, instead of assembling the whole thing in one go,” said a lead scientist on the project.
A Window into the Universe’s Most Powerful Engines
Massive stars are cosmic powerhouses, driving galactic evolution through their intense radiation, stellar winds, and explosive deaths as supernovae. Understanding how they form is not just an academic question — it’s key to understanding the life cycle of galaxies, including our own Milky Way.
By combining ALMA’s sharp vision with careful analysis, the team has offered a rare glimpse into the messy, dynamic process that creates these giants. And by revealing that star formation can proceed through different fragmentation modes within the same cloud, they’ve opened new directions for both observation and theory.
“Every time we look deeper into these stellar nurseries, the universe surprises us,” one co-author reflected. “The more we understand, the more complex and beautiful the picture becomes.”
More information: L. M. Zhen et al, Hierarchical fragmentation in hub-filament-system I18308 observed as part of the INFANT survey, Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202554634