Sun-like star orbiting a black hole in the Milky Way observed by astronomers

Proven to be a black hole in our galaxy, one of probably many
Stephen Vicinanza

Black holes have always been something that fascinated us, ever since Karl Schwarzchild theorized their existence in 1916. Back then it was an explanation to resolve Einstein’s theory of General Relativity. Later, in the middle of the 20th century, astronomers detected black holes by how they affected the celestial bodies around them. In the 1980s scientists studied supermassive black holes (SMBHs) which reside in the most massive galaxies in the universe.

By 2019 the Event Horizon Telescope (EHT) collaboration released the first ever image of an SMBH. The observations allow scientists to test the laws of physics under extreme conditions. These observations can lead to insights into how the universe formed.

In a recent study, a sun-like star had an odd orbital rotation revealed in data from ESAs Gaia Observatory. Due to the nature of the orbit, being somewhat unusual, the team concluded that it must be part of a black hole binary system.

This made it the nearest black hole to our solar system and implies the existence of many dormant black holes in our galaxy.

The study was led by Kareem El-Badry, a Harvard Society Fellow astrophysicist with the Harvard-Smithsonian Center for Astro Physics (CfA) and the Max Planck Institute for Astronomy (MPIA). He was joined by researchers from CfA, MPIA, Caltech, UC Berkely, the Flatiron Institutes Center for Computational Astrophysics (CCA), the Weizmann Institute of Science, The Observatoire de Paris, MIT’s Kavli Institute for Astrophysics, and Space Research and multiple universities. This paper lays out the findings and was published in the Monthly Notices of the Royal Astronomical Society.

The observations were part of a wider campaign to find dormant black hole companions to normal stars in our galaxy. For this study, El-Badry and the team relied on the data obtained from the ESA Gaia Observatory. The analysis they performed found a good candidate, A G-Type (yellow star) designated Gaia DR3 with a huge number after the DR3, for their purposes they designated it Gaia BH1. Based on the orbital solution, El-Badry and the team determined that this star must have a binary companion of a dormant black hole.

El-Badry said in a statement “the Gaia data constrain how the star moves in the sky, tracing out an ellipse as it orbits the black hole. The size of the orbit and its period give us a constraint on the mass of the unseen companion – about 10 solar masses. In order to confirm this was the correct information and not a non-black hole companion, the star was observed spectrographically with several other telescopes. This tightened our constraints on the companion's mass and proved it is really ‘dark’.” In other words, the companion was so dense it was absorbing light and matter around it.

Similar to the method used for hunting exoplanets (Doppler Spectroscopy), the spectra provided by these instruments allow the team to observe the gravitational forces acting on the orbit, and measure it. The follow-up observations confirmed that Gaia BH1 orbital solution was that a companion body of 10 solar masses was co-orbiting with it. This would confirm that the black hole orbiting Gaia BH1 was the first Milky Way galaxy black hole to be discovered without the use of X-ray emissions or other energetic releases.