Michal Levinstein, Israel Astronomical Society
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Black holes are known for tearing apart stars, but new observations of the center of our Milky Way galaxy prove that they actually help them form.
Until now, scientists have disagreed about the origin of supermassive stars orbiting our galaxy's central black hole (called Sagittarius A*) no more than one light-year away. The stars were first discovered by infrared telescopes.
The new findings - based on observations made at the Chandra X-ray Observatory - confirm the theory that blacks can help create massive stars and even make a significant contribution to the formation of galaxies.
"In one of the least hospitable places in our galaxy, the stars managed to win," says study director Sergei Nayakshin of the University of Leicester. "It turns out that star formation is much more stubborn and powerful than we thought."
The mystery remains
Astronomers still need to understand how the process occurs. Many of them thought that the strong motions of matter near the black hole would prevent star formation. It could be that the massive disk of gas surrounding the black hole provides the fuel for star formation - what scientists call "The Disk Model", or that it provides a kind of habitat for a cluster of lost stars - what is known as "The Cluster Migration Model" ).
In the disk model, gravity in the compressed gas disk surrounding the black hole diverts the massive gravitational force from the black hole itself and allows stars to form. When massive bursts of gas jets emerge from the black hole, they send supersonic shock waves through the gas cloud, which compress and heat the gas. They also ionize the gas, by taking electrons from it.
After the shock passes, the cloud shrinks back and the electrons connect and create radiation and energy that exits it. The cooling causes the cloud to contract more - and when a ball of gas becomes compressed enough, it can collapse and form a star.
In this model, the food for new star formation is actually stolen from the black hole - an idea that defies conservative models of black holes that say an accretion disk is the engine that feeds the black hole.
No wandering
In the migration model, the stars form in a cluster away from the black hole and migrate until they form a ring around it. The scenario predicts that a million low-mass stars may be found accompanying the massive stars.
"The only apparent problem here is that a star cluster has to be very heavy - about the mass of a million stars," says Naikshin. "What we discovered is that you can't hide more than 10,000 young, low-mass stars instead of a million - then it's clear that the cluster model is ruled out."
Relying on the disk model also raises a set of problems. In most star clusters, low-mass stars make up 90% of the cluster's mass, with thousands of bright young stars surrounding a few rare massive stars. Since the cluster around our hole lacks such small stars, scientists will have to rethink theories of star cluster formation.
"Usually, supermassive stars are very rare, like whales in the ocean - compared to low-mass stars, which are like tuna in the ocean and there are many more of them. What is interesting here - that you can clearly see the whales, because they are very bright, but not so visible A lot of tuna as we would expect" says Nikeshin. "Therefore, any theory you want to build to explain the mechanism of star formation will have to be one that predicts the formation of fewer low-mass stars near supermassive stars."
They knew the black holes
For NASA's announcement as published on the Universe Today website
The Israeli Astronomical Society
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