Record-sized black hole discovered

Video: The record eclipse (red orbit) gave itself away through the wobbling movement of its companion star (blue orbit). © ESO/L. Calcada

The dark remnant of a collapsed giant star is hidden in our cosmic neighborhood: Researchers have identified the most massive stellar black hole yet found in the Milky Way. It became noticeable through the wobbling motion of its companion star and could be calculated to be 33 times the mass of the Sun. The characteristics of the partner star also provided clues to the history of the black hole: Apparently it emerged from a massive star with particularly few heavy elements, the researchers report.

Their gigantic gravitational force bends space so strongly that not even light can escape: their fascinating features make black holes the focus of astronomical research and public interest. When it comes to mass records for dark astro celebrities, you have to differentiate between the different categories. The absolute peak in our Milky Way is Sagittarius A*. This is the supermassive black hole at the center of our galaxy, which has around four million times as much mass as the Sun. The current discovery, however, is an example of the so-called stellar black holes. They are formed when the material of a massive star collapses into an ultra-dense celestial body after its supernova explosion.

On the trail of darklings

Given the countless massive stars in the universe, there must also be a corresponding number of stellar black holes. However, their “sinister” nature makes them difficult to detect. Nevertheless, some have already been detected in the Milky Way and also in distant galaxies. Some specimens have become noticeable by the glow of matter that they are sucking out from a companion star. Others were discovered based on gravitational waves that arise when black holes merge with each other or with neutron stars. However, with the third detection option, inactive stellar black holes can also be detected: the existence of the darkling is reflected in the movement of a companion star and conclusions about the mass of the black hole are also possible based on the influence.

This is how the new record-holder for the mass of stellar black holes in our Milky Way was discovered. The discovery is thanks to the European Space Agency’s Gaia mission. The international research team evaluated data from the Gaia space telescope, which scans the entire night sky – among other things to show unusual star movements. As the team reports, Gaia came across a star with an unusual movement pattern in the constellation Aquila. It was then targeted and examined more closely by ground-based observatories – including the Very Large Telescope of the European Southern Observatory in the Chilean Atacama Desert.

As the researchers report, the star’s wobbling motion clearly suggested the existence of an invisible partner. The companion star therefore moves around a stellar black hole, which also performs a smaller wobbling motion. Using the observation data, the researchers were then able to determine the mass of the black hole: they found it to be 33 times that of the Sun. Similar massive specimens have only been discovered in distant galaxies.

Comparative view of stellar black holes in our galaxy: Gaia BH1, Cygnus X-1 and Gaia BH3. © ESO/M. grain knife

The black hole known as Gaia BH3 now overshadows the previous record holder in our galaxy: Cygnus X-1, which only reaches 21 solar masses. Compared to this, Gaia BH3 is also very close to us – at a distance of 2000 light years, it is now the second closest known black hole to Earth. It is surpassed only by BH1, which lies 1,500 light-years away from us but has only ten times the mass of the Sun.

In addition to the new record, another interesting aspect emerges from the observation data: based on the characteristics of the companion star, conclusions about the black hole’s predecessor star were possible. As the researchers explain, it can be assumed that the partners were once formed from the same material – they were therefore similar. The spectral data from the companion star now showed that it is unusually poor in heavy elements such as metals. This suggests that the star that collapsed into black hole BH3 was also metal-poor. According to the team, this confirms the previous assumption that the particularly massive stellar black holes arise from metal-poor stars. They lose comparatively little mass over the course of their lives, leaving more material to form a black hole after their death, according to the explanation.

The exciting BH3 system should now continue to be the focus of astronomy: With the publication, the Gaia team wants to encourage other research groups to examine it in more detail. Future observations may soon reveal more about the history of the system and the black hole itself, say the authors.

Source: European Southern Observatory, specialist article: Astronomy & Astrophysics, doi: 10.1051/0004-6361/202449763

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