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Fortunately, astronomers have many different wavelengths to study and it turns out that a specific type of star can highlight its presence like a flare in the night through its interaction with the interstellar medium.
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“There’s a lot we don’t know about the galactic center, and a lot we want to learn,” said Idan Ginsburg, of the Harvard-Smithsonian Center for Astrophysics (CfA) and lead author of a study accepted for publication in the Monthly Notices of the Royal Astronomical Society. “Using this technique, we think we can find stars that no one has seen before.”
Although most stars in the galactic core will forever remain hidden, stars traveling faster than the speed of sound in that medium generate powerful shock waves through the gas and dust. This interaction can accelerate electrons that, in turn, generate a specific type of emission — called synchrotron radiation — which could possibly be detected by sensitive radio observatories on Earth.
“In a sense, we’re looking for the cosmic equivalent of a sonic boom from an airplane,” said Ginsburg.
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Indeed, a supersonic airplane travels faster than the speed of sound in atmospheric gases. The sonic “boom” is the sound of the atmospheric shock wave racing past your location. In the case of supersonic stars, the shock wave generated is a dense region generating radio emissions, highlighting the star’s location, but the star is moving at a much higher speed than a supersonic aircraft.
To produce a stellar shock wave, a star needs to move at a speed of thousands of miles per second (as a comparison, a supersonic aircraft breaks the speed of sound at 768 miles per hour). Usually, for the majority of our galaxy, acquiring this speed is rare, but in the galactic core, where the supermassive black hole (called Sagittarius A*, or Sgr A*) resides, stars are accelerated to these mind-boggling speeds.
Like a swarm of bees flying around an invisible point, when stars orbiting Sgr A* make close approach, the black hole’s powerful gravity will accelerate these stars to thousands of miles per second, likely generating powerful “sonic booms” that could be detected. And Ginsburg’s team already have a known candidate star to test their detection method out on.
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A star known only as “S2″ is hot and bright and generates a strong infrared signal despite the thick clouds of dust in the core. S2 is predicted to make close approach with Sgr A* somewhere around late 2017 or early 2018, so radio astronomers will be on the lookout for its shock wave.
“S2 will be our litmus test. If it’s seen in the radio, then potentially we can use this method to find smaller and fainter stars — stars that can’t be seen any other way,” added co-author Avi Loeb, also from the CfA.
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