ESO Unveils Most Detailed Map of the Milky Way

If during a clear night, we watch toward the center of the Milky Way (in the Sagittarius constellation area), we can notice that it is quite a dense region. Due to this richness, it is one of the most interesting areas for astronomers, but also one of the most difficult to study. With this aim, in recent years several projects have focused on this portion of the sky.

In mid-August 2010 ESO Photo Ambassador Yuri Beletsky snapped this amazing photo at ESO’s Paranal Observatory. A group of astronomers were observing the centre of the Milky Way using the laser guide star facility at Yepun, one of the four Unit Telescopes of the Very Large Telescope (VLT). Yepun’s laser beam crosses the majestic southern sky and creates an artificial star at an altitude of 90 km high in the Earth's mesosphere. The Laser Guide Star (LGS) is part of the VLT’s adaptive optics system and is used as a reference to correct the blurring effect of the atmosphere on images. The colour of the laser is precisely tuned to energise a layer of sodium atoms found in one of the upper layers of the atmosphere — one can recognise the familiar colour of sodium street lamps in the colour of the laser. This layer of sodium atoms is thought to be a leftover from meteorites entering the Earth’s atmosphere. When excited by the light from the laser, the atoms start glowing, forming a small bright spot that can be used as an artificial reference star for the adaptive optics. Using this technique, astronomers can obtain sharper observations. For example, when looking towards the centre of our Milky Way, researchers can better monitor the galactic core, where a central supermassive black hole, surrounded by closely orbiting stars, is swallowing gas and dust. The photo, which was chosen as Astronomy Picture of the Day for 6 September 2010 and Wikimedia Picture of the Year 2010, was taken with a wide-angle lens and covers about 180 degrees of the sky. This image is available as a mounted image in the ESOshop. #L — *A laser beam towards the Milky Way’s centre at ESO’s Paranal Observatory. Credits: ESO/Yuri Beletsky*

One of the most important is the European Southern Observatory (ESO) public survey VISTA (Visual and Infrared Survey Telescope for Astronomy) Variables in the Via Lactea (VVV) that, after its conclusion, then became VVV eXtended (VVVX).

Two surveys to discover the Milky Way

As we said, the internal structure of the inner regions of the Milky Way and the details of its formation and evolution are poorly understood, mainly because of the difficult observation conditions (especially in the visible spectrum of light). By using some reference objects of which we know the distance, and by observing in the near-infrared wavelengths, the VVV was able to map the 3D structure of our Galaxy.

In 2016, the legacy of the VVV survey was continued by the VVVX until its end in March 2023, when the VISTA InfraRed CAMera (VIRCAM) was decommissioned. Improving on its predecessor, VVVX was assigned a larger survey area, to provide a more complete picture of the inner Milky Way.

This image shows the regions of the Milky Way mapped by the VISTA Variables in the Vía Láctea (VVV) survey and its companion project, the VVV eXtended survey (VVVX). The total area covered is equivalent to 8600 full moons. The Milky Way comprises a central bulge — a dense, bright and puffed-up conglomeration of stars — and a flat disc around it. Red squares mark the central areas of our galaxy originally covered by VVV and later re-observed by VVVX: most of the bulge and part of the disc at one side of it. The other squares indicate regions observed only as part of the extended VVVX survey: even more regions of the disc at both sides (yellow and green), areas of the disc above and below the plane of the galaxy (dark blue) and above and below the bulge (light blue). The numbers indicate the galactic longitude and latitude, which astronomers use to chart objects in our galaxy. The names of various constellations are also shown. — *Regions of the Milky Way mapped by the VISTA Variables in the Vía Láctea (VVV) survey and the VVV eXtended survey (VVVX). Credits: ESO*

VISTA has dedicated more than 4000 hours to the two projects combined. During this period “only” 4% of the sky has been studied, however, most of the stars in our galaxy were observed. Currently, although there are no other projects like VVV and VVVX, a work complementary to them is done by the James Webb Space Telescope, even if it is not able to cover large areas with such precision.

From its dome on Cerro Paranal in Chile’s Atacama desert, the ESO-operated Visible and Infrared Survey Telescope for Astronomy (VISTA) enjoys a stunning view of not only the majestic southern heavens, but also of the nearby mountainous peak, home to ESO’s Very Large Telescope (VLT). Conceived and developed by a consortium of 18 universities in the United Kingdom, VISTA is the world’s largest telescope entirely devoted to mapping the sky in infrared light. It carries out six public surveys taking up the majority of its observing time. The goal is to discover objects such as brown dwarfs, variable stars and ancient quasars, shed new light on fields such as the nature of dark matter and galactic structures, and even help create a 3D map of about five percent of the entire observable Universe. But why observe in the infrared? Because it allows VISTA not only to admire the cool side of the cosmos — cold objects often shine brighter at these wavelengths — but also to peer at some of the most remote celestial bodies. These are so distant that, due to the expansion of the Universe, the visible light they emitted has stretched towards longer infrared wavelengths over its journey to us. Last but not least, infrared light is not blocked by the thick, giant clouds of dust scattered in space, allowing astronomers to study objects that would otherwise remain hidden. — The ESO-operated Visible and Infrared Survey Telescope for Astronomy (VISTA) enjoys a stunning view of the majestic southern heavens, and the nearby mountainous peak, home to ESO’s Very Large Telescope (VLT). Credits: ESO

The instruments: VISTA and VIRCAM

Both VVV and VVVx wouldn’t be feasible without the VISTA telescope and its VIRCAM instrument.

VISTA is an infra-red ESO telescope, located near the VLT at the Paranal Observatory, at an altitude of 2518 m. Its main mirror has a diameter of 4.1 m and is the most curved mirror ever made of this dimension. To reflect as much infrared light as possible, it is coated with a layer of silver, instead of aluminum. Thanks to its characteristics, VISTA is the ideal instrument to detect very faint objects and rapidly map large areas of the sky.

The 2.9-tonne VISTA camera awaiting its shipment for Paranal at the CCLRC Rutherford Appleton Laboratory, UK. Credit: ESO — *The 2.9-tonne VISTA camera awaiting its shipment for Paranal at the CCLRC Rutherford Appleton Laboratory, UK. Credits: ESO*

This is possible also thanks to the VIRCAM camera, an instrument made of 16 detectors, equating to 67 million pixels. Their pairing produces an average of 315 GB of data per observation; equal to a 7-hour-long video with a 4k resolution.

Time to analyze data

VVVX, at the end of its life, produced around 200,000 images of billions of objects. All these data are now available to the scientific community for further studies about our galaxy’s origin, variable stars and exoplanets. Given the amount of data, the researchers themselves defined the survey as a “goldmine for Milky Way studies”.