GRAVITY (Very Large Telescope)

GRAVITY is an interferometric instrument installed at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) on Cerro Paranal in Chile. It combines the light from four 8.2-meter Unit Telescopes (UTs) using a technique called optical interferometry, effectively creating a telescope with a diameter of up to 130 meters. This provides an angular resolution of a few milliarcseconds, far surpassing that of any single telescope. The instrument operates in the near-infrared K-band (2.0–2.4 μm) and is designed for high-precision astrometry and imaging of faint objects, particularly the Galactic Center and exoplanets.¹

GRAVITY uses a combination of adaptive optics, fringe tracking, and a metrology system to stabilize the interference fringes and measure the positions of objects with extreme accuracy. It can achieve astrometric precision of 10 microarcseconds for bright sources, enabling the detection of minute motions of stars. The instrument has two main observing modes: a single-field mode for imaging and a dual-field mode for measuring the separation between two objects (e.g., a star and a planet). The dual-field mode is crucial for exoplanet studies, as it can measure the reflex motion of a star caused by an orbiting planet.

GRAVITY has delivered groundbreaking results since its first light in 2016. Most notably, it observed the star S2 (also known as S0-2) making a close approach to the supermassive black hole Sagittarius A* at the center of the Milky Way. These observations tested Einstein's general relativity in a strong gravitational field, detecting a gravitational redshift and a Schwarzschild precession in S2's orbit, consistent with predictions.² In 2018, GRAVITY detected the first direct observation of the relativistic beaming effect in S2's motion. Additionally, GRAVITY has been used to study the accretion disk around Sagittarius A*, resolving its flaring activity and providing insights into the black hole's environment.

Beyond the Galactic Center, GRAVITY has made significant contributions to exoplanet research. It directly detected the exoplanet HR 8799 e, resolving its spectrum and measuring its orbit. It also observed the young star HD 163296, revealing a protoplanetary disk with a gap likely carved by a forming planet. These observations demonstrate GRAVITY's ability to probe the formation and properties of planetary systems.³

While GRAVITY is famous for its black hole observations, it has also been used for more niche studies. For instance, it measured the diameter of the star Proxima Centauri, the closest star to the Sun, with unprecedented precision. It also observed the binary system Eta Carinae, resolving its wind-wind collision zone. Additionally, GRAVITY has been used to study active galactic nuclei (AGN) in other galaxies, such as NGC 1068, to measure the size of the broad-line region and estimate the mass of the central black hole. Another lesser-known fact is that GRAVITY's metrology system uses laser beams to track the positions of the telescopes, and it can operate in a 'self-calibrating' mode that corrects for atmospheric turbulence in real time.

GRAVITY is continuously upgraded. A planned upgrade, GRAVITY+, will enhance its sensitivity and allow for observations of fainter objects, potentially enabling the detection of exoplanets in the habitable zone of nearby stars. It will also improve the capability to observe the black hole photon ring, a prediction of general relativity that has not yet been directly imaged. GRAVITY+ is expected to be operational in the mid-2020s.⁴

Interferometry

A technique that combines light from multiple telescopes to achieve high angular resolution.

Adaptive optics

A system that corrects for atmospheric distortion in real time.

Fringe tracking

A method to stabilize the interference pattern by continuously measuring and correcting phase variations.

Astrometry

The precise measurement of the positions and motions of celestial objects.

Sagittarius A*

The supermassive black hole at the center of the Milky Way.