JWST measurements of individual clouds around Sagittarius A* found a composition resembling the stars that orbit it, which bears on why our black hole eats so little compared with others.
The supermassive black hole at the centre of the Milky Way is a strange thing to have nearby. It weighs some four million suns, it sits in a region crowded with stars and gas, and by the standards of its counterparts in other galaxies it is barely eating.
Working out why has been difficult, and a study in The Astronomical Journal from McGill University approaches it from an unusual direction: not by measuring how much gas is there, but what the gas is made of.
The answer is that the gas orbiting Sagittarius A* is rich in metals — which, in astronomical usage, means everything heavier than hydrogen and helium.
Why composition bears on appetite
“We know there’s a black hole at the centre of our galaxy pulling material into itself, but we see that it’s eating a lot less material than other supermassive black holes, and more slowly,” said Dr Nicole Ford, a PhD student in Professor Daryl Haggard’s group at McGill and the study’s lead author.
“Part of understanding why this is happening means looking at the properties of the materials available for it to consume, including gas, dust and the stars orbiting around it.”
The physical link runs through cooling. Metals affect how quickly gas sheds heat, and that governs how readily it can be drawn inward.
The reason is spectroscopic. Hydrogen and helium are simple atoms with few ways to radiate energy away at the temperatures involved, while heavier elements have many closely spaced electronic transitions and so many channels for converting thermal energy into escaping photons. Gas laced with metals therefore cools far faster than primordial gas at the same temperature and density.
Cooling matters because hot gas resists falling in. Thermal pressure holds it up against gravity, so gas that cannot cool tends to form a puffed-up, inefficient flow rather than settling into a dense disc that feeds the black hole steadily. Metallicity is accordingly one of the knobs that sets how a black hole is fed, which is why measuring it is not merely a chemical inventory.
Separating the clouds
The measurement required distinguishing individual gas clouds, and that is what the James Webb Space Telescope made possible.
JWST, launched five years ago, measures infrared light across a wide range of wavelengths, and the team used it to pick out the heat signatures of multiple gas clouds and determine each one’s metal composition separately.
Infrared is the only practical option here. The galactic centre sits behind the full thickness of the Milky Way’s dust, which blocks visible light almost completely; infrared passes through. Measuring each cloud separately rather than averaging over the region is the part that required a telescope of JWST’s sensitivity, and it matters because an average over clouds of different composition describes nothing in particular.
The galactic centre also has one advantage no other supermassive black hole offers: Sagittarius A* is closer to Earth than any of them, by a wide margin. It is the only such object whose immediate surroundings can be resolved into individual clouds and individual stars at all, which is why it carries a disproportionate share of what is known about how these objects feed.
Gas and stars from the same material
The composition turned out to resemble that of many stars orbiting the black hole, which suggests the gas and the stars may have formed together.
Where the metals came from has a straightforward answer: stars that exploded near the black hole and released their processed material over time. Heavy elements are manufactured in stellar interiors and distributed by supernovae, so an environment that has hosted generations of massive stars accumulates them.
The shared composition is a genealogical claim as much as a chemical one. If the gas now circling the black hole and the stars now orbiting it carry the same fingerprint, they are plausibly drawn from one enriched reservoir rather than arriving separately — the gas being leftover material from the same episode of star formation that produced the stars.
The companion study takes something away
Published alongside it in The Astrophysical Journal, a companion study led by Mayura Balakrishnan, a Banting Postdoctoral Fellow, delivers a less convenient result.
It finds that the gas emission is shaped by local conditions, and therefore cannot reliably reveal the black hole’s past activity.
That closes off an appealing line of reasoning. Sagittarius A* is quiet now, but there is evidence it has not always been — X-ray echoes from surrounding clouds have been read as the light of past outbursts, and the idea that the gas retains a record of earlier feeding has been attractive. If what the gas radiates is governed by where it happens to be and what is happening to it locally, then it is reporting on its present circumstances rather than on history.
The two papers together are a useful pairing: one establishes what the fuel is made of, the other narrows what can be inferred from watching it. Understanding how the black hole feeds, the researchers conclude, will require comparing observations against simulations that follow gas evolution over much longer timescales than any observation covers.
No numbers
The release gives no quantitative abundance values and no other measurements. Metal-rich is a comparative statement, and without figures there is no way to judge from the release how rich — whether this is modestly above solar composition or substantially so — which is precisely the quantity that would determine how much the cooling argument actually explains.
Co-authors on the primary study are Sebastiano D. von Fellenberg, Joseph M. Michail, Yuzhu Cui, Joseph L. Hora, Joey Neilsen, Giacomo Principe, Tamojeet Roychowdhury, Nadeen B. Sabha, Howard A. Smith, Zach Sumners and S. P. Willner; the companion study adds Laurent Loinard, Sera Markoff and Shuo Zhang.
The work was supported by the Natural Sciences and Engineering Research Council of Canada, the Canadian Space Agency, the Canada Research Chairs program, the Fonds de recherche du Québec – Nature et technologies, the Centre de recherche en astrophysique du Québec, the Trottier Space Institute of McGill University and the International Space Science Institute.
Notes
- Ford, N. M., et al. The Astronomical Journal, 2026. DOI: 10.3847/1538-3881/ae994d.
- Balakrishnan, M., et al. The Astrophysical Journal, 2026. DOI: 10.3847/1538-4357/ae5dbe.
- McGill University. “The gas around Milky Way’s central black hole is rich in metals, study finds.” EurekAlert!, 19 October 2026. https://www.eurekalert.org/news-releases/1148340.
Credit: European Space Agency (ESA)
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