The Milky Way must be full of tiny black holes. A black hole with a mass of between 10 million and 1 billion stars is believed to be lurking darkly and mysteriously in the far corners of the galaxy. Unless they are active, we cannot do a census because we usually cannot see them. We also don’t know where they are in the Milky Way. We know of only 20 stellar-mass black holes in our galaxy, and the closest candidate to Earth is about 1,565 light-years away.
But a new study suggests they may be much closer than we think, even on our cosmic doorstep. By analyzing and modeling the Hyades cluster, a group of stars located 150 light-years away, a team of astronomers discovered that this cluster may host two or three stellar-mass black holes.
“Our simulations can only match the mass and size of the Hyades if some black holes were present today (or until recently) at the center of the cluster,” says astrophysicist Stefano Torniamenti from the University of Padua in Italy. Visible to the naked eye in the night sky in the constellation Taurus, the Hyades are a group of stars known as an open cluster that share similar properties, moving through space as a gravitationally bound piece.
Each of the scattered clusters is actually a family of sister stars born from the same vast molecular cloud, swinging together before eventually going their separate ways. The Hyades is thought to be about 625 million years old and contains hundreds of stars; those furthest from the center are apparently starting to break away, and those in the center are the most tightly packed.
In such densely populated environments, stars are expected to repel each other at a higher rate than in less populated stellar environments, leading to a higher frequency of collisions and mergers. Astronomers predict that black holes, the end product of these interactions, may be found here at the heart of star clusters. We’ve seen hints of their existence in other types of clusters, but they’re pretty hard to find because black holes don’t emit any light unless they’re actively eating star flesh.
Torniamenti and his colleagues looked for the Hyades in a more indirect way. They modeled the mass and motion of stars in the cluster using data from Gaia, a satellite that maps the three-dimensional positions and velocities of stars in the Milky Way.
They then ran simulations to try to reproduce these observations. They found that their simulations most closely matched the observed cluster when they included two or three stellar-mass black holes in the mix.
These black holes are either still present in the cluster or were ejected less than 150 million years ago, meaning they are currently hovering around the cluster’s outskirts. This latest ejection will mean that traces of the black holes’ gravitational influence will still be present in the core of the cluster.
As for the black holes themselves, researchers have not been able to pinpoint their exact location. However, this finding strongly suggests that the Hyades contains the closest candidate black holes to the Solar System, 10 times closer than previous candidates, according to the researchers.
To be clear, we are not in danger from them; The researchers found that the fastest any of these black holes can go is 3 kilometers per second; Even if they went in our direction, it would take a long, long time to get here.
And in any case, black holes have no greater gravitational pull than any star of equivalent mass. Therefore, we are no more in danger from black holes with the wrong stellar mass than from stray stars with the same mass. Of course we won’t be able to see it coming, but what will we do anyway?
Rather, this discovery helps us better understand the Milky Way’s invisible population of stellar-mass black holes.
“This observation helps us understand how the presence of black holes affects the evolution of star clusters and how star clusters contribute to the sources of gravitational waves,” says astrophysicist Marc Giles from the University of Barcelona.
“These results also give us insight into how these mysterious objects are distributed across the galaxy.”
The study was published on: Monthly Notices of the Royal Astronomical Society.
Source: Port Altele
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