Sagittarius A*

Sagittarius A* is the supermassive black hole at the heart of the Milky Way — the still point around which the entire galaxy, including our Sun, slowly turns. It holds about 4.3 million times the mass of the Sun, yet all of that is crushed into a region no wider than the orbit of Mercury, some 27,000 light-years away in the direction of the constellation Sagittarius.

Its name (pronounced "Sagittarius A-star") is unlike any planet's: the asterisk marks the compact, brilliant radio source buried inside the larger Sagittarius A complex. In 2022 it became only the second black hole ever imaged directly, joining the giant in galaxy M87 — but this one lies in our own cosmic backyard.

Not a place, but a boundary

A black hole has no surface. What we call its "size" is the event horizon — the one-way boundary from which not even light can escape. For a mass of 4.3 million Suns, that horizon is about 12.7 million km in radius, roughly eighteen times the width of the Sun and small enough to fit comfortably inside Mercury's orbit. Everything inside is hidden from us forever.

Sagittarius A* is a rotating, or Kerr, black hole. Modelling of its accretion flow and X-ray flares suggests it may spin fairly fast — estimates cluster in the range of about 0.5 to 0.9 out of a theoretical maximum of 1 — meaning it drags the very fabric of space around with it, though the exact figure remains uncertain. It has no "temperature" in the ordinary sense, but the thin plasma spiralling in around it is heated to some ten billion degrees, radiating the light by which we see it at all.

The photograph that took a planet to make

On 12 May 2022 the Event Horizon Telescope collaboration unveiled the first direct image of Sagittarius A*: a luminous orange ring of superheated gas encircling a dark central shadow. The shadow spans just 51.8 microarcseconds on the sky — the collaboration likened the challenge to spotting a doughnut on the surface of the Moon.

No single dish could resolve something so tiny. Instead, eight radio observatories scattered from Chile's Atacama Desert to Spain, Hawai'i, Arizona, Mexico and the South Pole were linked into a virtual telescope the size of the whole Earth, all trained on the galactic centre during April 2017. The gas around Sgr A* races at nearly the speed of light and completes an orbit in mere minutes, so its appearance flickered even as the data were gathered — one reason it took five more years of analysis than the earlier M87 image.

Stars that gave it away

Long before any photograph, the black hole revealed itself through gravity. For decades, two teams — led by Reinhard Genzel in Germany and Andrea Ghez in the United States — used the world's largest telescopes to track individual stars whipping around an invisible point at the galactic centre. One star, catalogued S2, races around Sgr A* on a tight 16-year orbit, and in May 2018 swung within about 18 billion km of it at nearly three percent of light-speed.

The starlight was stretched to redder wavelengths as it climbed out of the black hole's gravity — a gravitational redshift exactly as Einstein's general relativity predicts. These decades-long stellar orbits pinned down the mass to within a fraction of a percent and earned Genzel and Ghez a share of the 2020 Nobel Prize in Physics, alongside Roger Penrose.

A surprisingly quiet monster

For its bulk, Sagittarius A* is remarkably calm. It is not shredding stars or blasting out quasar-scale radiation; it swallows only about a hundred-thousandth of a solar mass of material each year, and much of even that is flung back out before it can fall in. Compared with the actively feeding black holes in distant galaxies, our own is practically dormant.

It is not entirely silent, though. Space telescopes such as NASA's Chandra and NuSTAR regularly catch it flaring in X-rays — brief brightenings thought to come from magnetic activity in the infalling gas. There are also hints in ancient light echoes that Sgr A* was far more active roughly two centuries ago.

From radio blip to galactic anchor

Sgr A* was discovered on 13 and 15 February 1974 by Bruce Balick and Robert Brown, who detected an unusually compact radio source at the galactic centre using a radio-link interferometer at Green Bank. Brown later coined the name — adding the asterisk to signal that the source was "exciting", by analogy with the notation for excited atoms — and it stuck.

That faint radio point is now understood to mark the gravitational anchor of the Milky Way: a hub around which several hundred billion stars orbit, our Sun among them, taking roughly 230 million years to complete a single lap.

Why you cannot see it

Unlike a planet or a comet, Sagittarius A* is impossible to spot by eye. It lies behind thick clouds of interstellar dust that block visible light almost entirely, which is why it was found in radio waves and is studied today in radio, infrared and X-rays rather than the optical band. What you can do is look toward the constellation Sagittarius, low in the summer sky from the northern hemisphere, and know that the Milky Way's densest star clouds there conceal, at their very centre, a four-million-solar-mass shadow that took a planet-sized telescope to photograph.