In the vast universe, stars generally belong to a certain galaxy or star cluster, but there are also some stars that have or are trying to break free and wander among the stars, so they are often extremely fast. These are high-speed stars, such as the one we are going to talk about today, J0731+3717. A research team led by the National Astronomical Observatory of the Chinese Academy of Sciences revealed the secrets of this hypervelocity star through data from the Gaia satellite and China's Guo Shoujing Telescope (LAMOST):
About 20 million years ago, it was a member of the globular star cluster M15. Under the pull of some strong gravity, it was violently thrown out at a speed of up to 550 kilometers per second.
This is a dynamic effect called the "Hills Mechanism": when a pair of tightly orbiting binary stars gets too close to a super-strong gravitational source, one of the stars will be captured, and the other will be accelerated and thrown out like a slingshot.
Globular cluster M15
According to calculation,The most likely gravitational source that can eject a star at such a high speed in such a short period of time is an intermediate-mass black hole with a mass of about 1,700-3,200 suns.
Intermediate-mass black holes are the missing link between stellar black holes and supermassive black holes. Their masses range from a hundred to hundreds of thousands of suns. Confirming their existence is crucial to explaining the origin of supermassive black holes.
The left is an image of a galaxy where an intermediate-mass black hole is located. The middle is an artistic imagination of an intermediate-mass black hole tearing apart stars. The right is the power density spectrum obtained by analyzing the light curve.
With the introduction of gravitational wave detection, there are new and powerful methods for searching for intermediate-mass black holes.
The American Laser Interferometer Gravitational Wave Observatory LIGO and the European Virgo detector have successively discovered more than 100 black hole merger events, most of which are stellar black holes ten times the mass of the sun.
The gravitational wave signal GW190521 discovered in May 2019 is very likely to come from an intermediate-mass black hole.
Analysis shows that this event was caused by the merger of two black holes with a mass of about 85 times and 66 times the mass of the sun. The new black hole has a mass of 142 times the mass of the sun.
In other words,At the moment when the two black holes merge, about 9 times the mass of the sun is released in the form of gravitational waves.
This is the first time that humans have directly observed a black hole with a mass exceeding a hundred times the mass of the sun in gravitational waves. It is also the clearest example of the detection of an intermediate-mass black hole so far.
Two black holes spiraling together in the GW190521 merger event (artist imagination)