The dark heart of the Milky Way

Information about the closest supermassive black hole – the central object of our own galaxy

An article by Markus Pössel

To the best of our astrophysical knowledge, supermassive black holes, with millions or even billions solar masses are everywhere in the universe, with the core regions of most galaxies containing one such black hole. In the case of active galactic nuclei, these black holes are the cause of spectacular phenomena such as the jets of radio galaxies with their huge radio bubbles. But also in more quiet galaxies, such as our very own Milky Way, lurks a black hole.

Our very own black hole

A supermassive black hole on our own doorstep, a mere 25000 or so light years away, might sound a bit scary. For astronomers, it is a veritable boon: the most workable way of identifying an object as a black hole is a determination of its mass and an estimate of its size. Determining the mass of an astronomical object is, in general, no easy task. It is the least difficult for several objects orbiting each other, following their mutual gravitational pull: For such movements, the laws of celestial mechanics tie the masses of the objects in questions to the details of their orbital motion. Once the orbits are known, this allows conclusions about the masses.

Stars orbiting the central black hole

In the case of the central black hole in the Milky Way galaxy, astronomers have followed the motion of several stars in the centre of the galactic core for years. The following animation is based on observations made by researchers from the Max-Planck-Institute for Extraterrestrial Physics in the course of six years, using the New Technology Telescope of the European Southern Observatory.

[Simulation: Max-Planck-Institute for extraterrestrial physics]

The animation shows a region a few light days across, in the core region of the Milky Way that, as viewed from Earth, is in the constellation Sagittarius. As mentioned before, that region is roughly 25000 light years away from Earth. The blue cross in the centre marks the position of “Sagittarius A*”, a compact radio source. The astronomers have developped a detailed three-dimensional model to reconstruct the stars’ motions; the orbits thus reconstructed are shown in purple.

From the newest calculations, based on a full decade of high resolution observations of three dozen individual stars around Sagittarius A* by several independent groups of observers, it is clear that this object has the mass of 3.3 million suns. From the orbital data for one of the stars, one can also derive a very good upper limit for the central object’s size – in order to fit into that orbit, the radius of Sagittarius A* cannot be larger than 17 light-hours. More recent radio-astronomical measurements have yielded an even smaller size of a mere 20 light-minutes, comparable to size of the Earth’s orbit around the sun.

With this mass and size, there can only be one conclusion: the object is a supermassive black hole in the centre of our own galaxy.

For its discovery, the Nobel Prize in Physics was awarded to Andrea Ghez (University of California in Los Angeles, USA) and Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and University of California in Berkeley, USA) in 2020.

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