Last week, four lasers were projected into the sky above the European Southern Observatory (ESO) Paranal site in Chile. The lasers successfully created an ‘artificial star’ which astronomers can use to measure and then correct the blur caused by Earth's atmosphere, ESO announced today.

The striking launch of these lasers from each of the eight-metre telescopes at Paranal is a significant milestone of the GRAVITY+ project – a complex upgrade to ESO’s Very Large Telescope Interferometer (VLTI).

GRAVITY+ unlocks a greater observing power and much wider sky coverage for the VLTI than previously possible, enabling the study of even fainter and more distant objects.

The first target for the GRAVITY+ and ESO teams at Paranal performing test observations using the new lasers was a cluster of massive stars at the centre of the Tarantula Nebula – a star-forming region in our neighbouring galaxy, the Large Magellanic Cloud. These first observations revealed that a bright object in the nebula, thought to be an extremely massive single star, is actually a binary of two stars close together, which showcases the capabilities and scientific potential of the upgraded VLTI.

Dr Rebeca Garcia Lopez, who is an expert in star and planet formation at UCD School of Physics, is an associate partner in the GRAVITY+ consortium, in charge of the instrument spectrograph upgrade. She said: “This opens a new era in optical interferometry and it will allow us to understand how solar systems similar to our own form with unprecedented detail.”

The VLTI combines light from several individual telescopes using interferometry. GRAVITY is a very successful VLTI instrument that has been used to generate images of exoplanets, observe near and far stars and perform detailed observations of faint objects orbiting the Milky Way’s supermassive black hole.

GRAVITY+ is implementing infrastructural changes to the telescopes and upgrades to the VLTI underground tunnels, where the light beams are brought together. The installation of a laser at each of the previously unequipped telescopes is a key achievement of this long-term project, transforming the VLTI into the most powerful optical interferometer in the world.

Principal Investigator Professor Frank Einsenhauer, of Max-Planck Institute for Extraterrestrial Physics (MPE), Germany, which led the consortium, said: “The VLTI with GRAVITY has already enabled so many unpredicted discoveries, we are excited to see how GRAVITY+ will push the boundaries even further.”

Dr Taro Shimizu, MPE astronomer and consortium member, said: “This opens up the instrument to observations of objects in the early distant Universe, less than a few hundred million years after the Big Bang.”

The series of upgrades has been ongoing for a few years and includes revised adaptive-optics technology – a system to correct the blur caused by the Earth’s atmosphere – with advanced state-of-the-art sensors and deformable mirrors.

Until now, for the VLTI, adaptive-optics corrections have been done by pointing to bright reference stars that need to be close to the target, limiting the number of objects we can observe. With the installation of a laser at each of the telescopes, a bright artificial star is created 90km above Earth’s surface, enabling the correction of atmospheric blur anywhere in the sky. This unlocks the whole southern sky to the VLTI and dramatically enhances its observing power.

With the addition of these lasers, astronomers will be able to study distant active galaxies and directly measure the mass of the supermassive black holes that power them, as well as observe young stars and the planet-forming discs around them.

GRAVITY has made significant breakthroughs in astrophysics in the last decade. It successfully tested Einstein’s Theory of General Relativity (through the measurement of gravitational redshift), earning Professor Reinhard Genzel, MPE, and Professor Andrea Ghez, University of California, the Nobel Prize for Physics in 2020. UCD’s Dr Garcia Lopez is co-author on the paper ‘Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole’ (Astronomy & Astrophysics, 2018; 615L..15G).

In addition, astronomers also used GRAVITY to find the first observational evidence of magnetospheric accretion, the process by which matter is ‘fed’ into newborn stars. The results were published in 'A measure of the size of the magnetospheric accretion region in TW Hydrae,' (Nature, 2020; 584..547G) with Dr Garcia Lopez as first author.
Through Dr Garcia Lopez, UCD is involved in the upgrade of the GRAVITY spectrograph, in charge of the spectral-resolution upgrade. In collaboration with Universidad National Autonoma de Mexico (UNAM), they have designed a holographic grating prism to be installed in the spectrograph and are also responsible for testing and installation at the VTLI.