Observing black holes is difficult by definition: mass is concentrated in such a small diameter region that extremely strong gravity does not allow anything, not even light, to escape. With all this, these objects have long found their place in astrophysics. In particular, so-called stellar black holes of several solar masses are the final form of very massive stars. A group of astronomers now led by Karim El-Badri (Max Planck Institute for Astronomy) [MPIA] and the Harvard-Smithsonian Center for Astrophysics) used a new method to detect the closest known black hole. The discovery also reveals gaps in modern astronomy knowledge, namely the formation of binary star systems.

We are approaching the Gaia BH1 black hole. Background: region of the Milky Way galaxy; Panel 1: image of a star orbiting a black hole; Panel 2: reconstructed orbit of the star; Panel 3: Relative light bending effects that would be visible if we could see the star and black hole up close. Image source: T. Müller (MPIA), PanSTARRS DR1 (KC Chambers et al. 2016), ESA/Gaia/DPAC (CC BY-SA 3.0 IGO)

There are about a hundred million stellar black holes in our galaxy, the Milky Way, but only a small fraction of them have been discovered so far. Some have been detected by gravitational wave detectors measuring nearly a hundred stellar black hole mergers, providing additional data on the masses of black holes.

Of the several dozen stellar black holes detected by telescope observations, most orbit a companion star close enough for the black hole’s gravity to pull the hydrogen from the companion star into a so-called accretion disk surrounding the black hole. The gas then heats up enough to emit a significant amount of X-rays. 20 such “X-ray binaries” are known, with an additional 50 candidate objects.

There have also been several attempts to find “silent” black holes – black holes without an X-ray disk – in binary systems. Preferred tool: stellar spectra, which are iridescent decompositions of starlight that contain information about the motion of the star. We know this from everyday life thanks to the “Doppler effect” for sound: an ambulance with a loud siren sounds louder when it’s coming towards us, and lower when it’s passing by. Likewise, the light in the stellar spectrum tells us the star’s movement directly toward or away from us.

This video zooms the Milky Way closer to the location of the Gaia BH1 black hole, which is currently the closest black hole to Earth. Upon arriving at the location, we see the orbit of a sun-like star around Gaia BH1.

There have been several claims over the past few years that silent black holes have been discovered, trying to determine the orbit of a binary system and the mass of an invisible companion from the spectrum of stars alone. However, all but one of them (the June 2022 discovery of the binary system VFTS 243, co-authored by El-Badri) have been questioned or completely rejected by subsequent research. The main problem: Spectra provide only part of the information about the motion of the stars and therefore the orbit and mass of the satellite. Missing information is a source of great uncertainty – and this is also where ESA’s Gaia mission promises to help!