A team of astronomers led by Dr István Dékány from Millennium Institute of Astrophysics and the Universidad Católica de Chile, Santiago, Chile, has used ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA) to observe the Trifid Nebula – a unique combination of a reflection nebula, an absorption nebula and an emission nebula – in the infrared wavelengths, and to discover a pair of extremely reddened classical Cepheid variable stars that lies almost directly behind the nebula.
This image shows the famous Trifid Nebula to the right of center; it appears as faint and ghostly at these infrared wavelengths when compared to the familiar view at visible wavelengths; this transparency has brought its own benefits – many previously hidden background objects can now be seen clearly; among these are two newly discovered Cepheid variable stars, the first ever spotted on the far side of our Milky Way Galaxy near its central plane. Image credit: ESO / VVV consortium / D. Minniti.
The Trifid Nebula, also known as Messier 20 and NGC 6514, was discovered by Charles Messier on June 5, 1764.
The distance to this massive region of star formation is debatable, but estimates range broadly from 2,200 to 9,000 light-years.
The nebula is part of our Milky Way Galaxy and is located in the constellation of Sagittarius. It is estimated to have a magnitude of between 6.8 and 9.0.
Familiar pictures of the nebula show it in visible light, where it glows brightly in both the pink emission from ionized hydrogen and the blue haze of scattered light from hot young stars.
But the view in the VISTA infrared picture is very different. The Trifid Nebula is just a ghost of its usual visible-light self.
The dust clouds are far less prominent and the bright glow from the hydrogen clouds is barely visible at all. The three-part structure is almost invisible.
In the VISTA image, a spectacular new panorama comes into view. The thick dust clouds in the disc of the Milky Way that absorb visible light allow through most of the infrared light that the telescope can see.
Rather than the view being blocked, the telescope can see far beyond the nebula and detect objects on the other side of the galaxy that have never been seen before.
By chance this picture shows a perfect example of the surprises that can be revealed when imaging in the infrared.
Apparently close to the nebula, but in reality about 37,000 light-years away, two Cepheid variable stars have been discovered by Dr Dékány and his colleagues.
These stars are the only Cepheid variables detected so far that are close to the central plane, but on the far side of our Milky Way Galaxy. They brighten and fade over a period of 11 days.
The discovery is described in a paper published in the Astrophysical Journal Letters (arXiv.org preprint).
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I. Dékány et al. 2015. Discovery of a Pair of Classical Cepheids in an Invisible Cluster Beyond the Galactic Bulge. ApJ 799, L11; doi: 10.1088/2041-8205/799/1/L11
