The Sombrero Galaxy (M104)

Image: NASA/ESA and the Hubble Heritage Team (STScI/AURA). The Sombrero Galaxy (M104) in Virgo, about 30 million light-years away. We view it just 6 degrees off its equator, which is why the dust lane cuts across as a crisp dark brim.

A hat made of a hundred billion suns

The Sombrero is a spiral galaxy we happen to see nearly edge-on, so its flat disk appears as a thin line and its central bulge as the glowing dome above and below. That "brim" is a ring of dark dust circling the galaxy in its equatorial plane — the same kind of dust that laces the arms of any spiral, but seen here in silhouette, from the side. The bright, oversized bulge is packed with old stars and studded with an unusually large swarm of globular star clusters, nearly 2,000 of them. Spanning about 50,000 light-years and holding the equivalent of 800 billion Suns, it's one of the most massive galaxies in the nearby Virgo region.

At its heart sits a supermassive black hole of roughly a billion solar masses — one of the more massive central black holes known in a nearby galaxy, hundreds of times heavier than our own Milky Way's. And because it's 30 million light-years away, the light in that photo left the Sombrero 30 million years ago: you're seeing it as it was when the first primates were appearing on Earth.

Spiral, elliptical, or both?

For most of its history the Sombrero was filed as a straightforward spiral, but modern infrared eyes have complicated that tidy label. In 2012, NASA's Spitzer Space Telescope — which sees the heat of stars through obscuring dust — revealed a surprise: the Sombrero is wrapped in a vast, rounded halo of old stars far larger and heavier than a normal spiral's, more like the body of a giant elliptical. In effect, a disk galaxy and an elliptical galaxy seem to occupy the same space. That hybrid nature hints at a dramatic past — quite possibly an ancient major merger that built the swollen bulge and its rich swarm of globular clusters.

In November 2024, the James Webb Space Telescope trained its mid-infrared MIRI camera on the galaxy, and the familiar hat transformed. The brilliant visible core simply vanished — old stars barely glow at those wavelengths — leaving a smooth inner disk on view, while the dark brim lit up as an intricate, clumpy ring. Those clumps hold carbon-rich molecules (polycyclic aromatic hydrocarbons) that often mark pockets of new star formation. Yet for all that raw material, the Sombrero is a placid galaxy: it forms stars only sluggishly, nothing like a vigorous starburst such as Messier 82. Webb's image rounded out a century-old portrait, mapping exactly how the dust that makes the "brim" is arranged.

A galaxy found before galaxies were known

The Sombrero was discovered in 1781 by Pierre Méchain, a friend and collaborator of Charles Messier — though it was catalogued as M104 only later. For over a century it was just another "spiral nebula" of unknown nature. It played a quiet but real role in the story of how we learned what galaxies are: in 1912, the astronomer Vesto Slipher measured the Sombrero's light and found it was rushing away from us at enormous speed — one of the very first hints that these "nebulae" were distant objects with their own huge motions, groundwork for the Great Debate and Hubble's discovery a decade later. The friendly hat was, quietly, one of the clues that cracked open the universe.

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