Radio waves could detect hidden stars

Galactic dust and fog prevents our telescopes from detecting stars hiding at the centre of the Milky Way. But a new detection method using radio waves has been devised that could reveal stars no one has ever seen before.

The central parts of the Milky Way, seen using the near-infrared NACO instrument on ESO’s Very Large telescope.
Credit: ESO/S. Gillessen et al

The central parts of the Milky Way, seen using the near-infrared NACO instrument on ESO’s Very Large telescope. Image Credit: ESO/S. Gillessen et al

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Radio telescopes could be used to peer beyond cosmic fog and dust and observe the stars hiding at the centre of the Milky Way, according to a team of Harvard researchers.

The path from the middle of the Galaxy to Earth is paved with space dust, to the extent that out of every trillion photons of visible light sent in our direction, only one will reach our telescopes.

However, radio waves have lower energies and longer wavelengths because they are from a different part of the electromagnetic spectrum.

As a result, the radio waves emanating from supersonic stars could theoretically be spotted through the dense dust at the centre of the Galaxy and allow astronomers to detect stars that have never been seen before.

Not all stars could be detected by this method: just the ones at the middle of the Milky Way.

Stars themselves do not emit enough radio waves to be detected from Earth, but when they travel through gas faster than the speed of sound, material blowing off the star as a stellar wind hits interstellar gases and creates a shock wave.

This shock wave then produces radio emissions that could be detected by radio telescopes on Earth.

The speed required for this process to occur can only be reached at the galactic centre, where stars are pulled by the gravity of the supermassive black hole.

As the star gets closer to the middle of the Galaxy, it reaches the speeds of thousands of kilometres per second needed to generate the stellar winds that lead to its detection on Earth.

“There’s a lot we don’t know about the galactic centre, and a lot we want to learn,” says lead author Idan Ginsburg of the Harvard-Smithsonian Center for Astrophysics (CfA).

“Using this technique, we think we can find stars that no one has seen before.

In a sense, we’re looking for the cosmic equivalent of a sonic boom from an airplane.”

The team has already suggested using this method to detect star S2, which is already known to orbit the Milky Way’s centre because it is hot and bright enough to be seen in infrared through the cosmic dust that might otherwise block its detection from Earth.

S2 will make its closest approach to the centre of the Galaxy in late 2017 or early 2018 and, when it does, astronomers can attempt to look for radio emissions generated by its shock wave.

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This should provide the perfect test to try out the new detection method and, if the shock wave can be seen in the radio, could pave the way for detecting smaller and fainter stars that do not appear in infrared.