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Image Astro du jour
Nothing is being attacked with space lasers in this Picture of the Week, although it may look like those four bright yellow streaks are menacing the galaxies to the right of the image. These streaks are, in fact, four artificial stars. As starlight passes through our restless atmosphere it gets blurred, distorting astronomical images. Artificial stars allow us to correct for this, but why do the ones in this picture look elongated rather than point sources, and how are they generated?
The artificial stars you see in this image were created by laser beams rising from ESO’s Very Large Telescope (VLT) in Chile. Each beam comes from one of the VLT’s four Unit Telescopes. The beams themselves show faintly at the lower left, scattering off the air on their way up. As they travel upwards, the atmosphere becomes less dense, and the beams appear progressively fainter. But when they reach 90 km above the ground they encounter a layer rich in sodium atoms that glow brightly as they are excited by the lasers.
Because these atoms are spread across a layer about 5-10 km thick, they appear as streaks when photographed from an angle, like in this picture taken 2.5 km away from the telescopes. But they look like point sources when seen from the telescopes where the lasers are coming from. An adaptive optics system monitors how these artificial points twinkle, and controls a thin flexible mirror that reshapes itself hundreds of times per second. This in turn unblurs the incoming light from the astronomical target one is observing.
The target in this case is the supermassive black hole at the centre of NGC 3227, one of the galaxies to the right of the image. It is what astronomers call an active galaxy, whose centre shines brightly as the black hole pulls in the gas around it. ESO astronomer James Leftley is studying several such galaxies with the GRAVITY and MATISSE instruments at the VLT Interferometer (VLTI), which links all four Unit Telescopes so that they work together like a single giant telescope up to 130 metres across. The facility was recently upgraded through the GRAVITY+ project, which saw lasers added to VLT telescopes. With these lasers, the VLTI and its instruments can unblur incoming light almost anywhere in the southern sky, creating stars on demand wherever they are needed.