- The 4MOST spectrograph has begun mapping the sky at record speed, simultaneously analyzing the light emitted by 2,400 celestial objects.
- With significant French involvement in the construction of two of its spectrographs, it is the largest instrument of its kind in the southern hemisphere.
- Over the next 15 years, 4MOST will determine the characteristics of distant stars and galaxies by mapping large regions of the sky.
A tool for mapping the universe at record speed
On October 18, 2025, the 4MOST (4-Meter Multi-Object Spectroscopic Telescope) instrument, installed on the European Southern Observatory (ESO) VISTA telescope in the Atacama Desert in Chile, obtained its first light in the sky. This first light is not just a simple image of space: 4MOST records spectra, meaning it captures light from celestial objects in all wavelengths-colors-from ultraviolet to infrared. Thanks to independent optical fibers, it can simultaneously observe 2,436 celestial objects in 18,000 wavelengths.Under development since 2010 and expected to be operational for at least fifteen years, the three 4MOST spectrographs will enable astronomers to study numerous fields of research ranging from the composition of stars to dark matter and dark energy. By analyzing the spectra of thousands of objects every 10 to 20 minutes, it will be possible to catalog the temperatures, chemical compositions, movements, and distances of tens of millions of stars in the southern hemisphere sky.
4MOST is the largest multi-object spectrograph in the Southern Hemisphere. Its wide field of view (five times larger than the Moon), its number of simultaneous sources, and the level of detail in its spectra make it a unique instrument in the world.
A significant French contribution
France and the CNRS played an important role in the development of this European instrument: two of its three spectrographs were built by the Lyon Astrophysics Research Center (CNRS/ Université Claude Bernard Lyon 1 /ENS de Lyon).These two identical spectrographs, known as "low resolution" (LRS) spectrographs, are essential to the 4MOST mission, providing continuous coverage of wavelengths from 370 to 950 nm. The third, known as the "high resolution" spectrograph, can produce detailed spectra in three narrower spectral bands. These spectrographs, each connected to 812 hair-thin optical fibers, will enable large-scale surveys of the sky, making VISTA-with its 4-meter mirror-the world’s largest telescope dedicated to this field of research.
The French team comprises around fifteen engineers, technicians, and researchers who have been involved since 2014 in designing, assembling, and testing the spectrographs, right up to their installation on the telescope this summer. "4MOST has been an exceptional human and technical adventure," says Florence Laurent, an optical engineer at the CNRS. Supported by international teams with complementary talents spread across several sites, this project illustrates the power of a collective effort united by a common ambition: science. The instrument is eagerly awaited by the entire astrophysics community, as Johan Richard, an astronomer at the same laboratory, explains: "It will increase our capacity to map large regions of the sky tenfold. The amount of physical measurements we will obtain on such distant objects is truly impressive!"
A first light full of promise
The first light from 4MOST shows the full potential of the instrument, which is capable of observing a wide field while simultaneously analyzing several thousand astronomical objects.Among the most remarkable observations is the Sculptor Galaxy (NGC253), the largest galaxy in the southern sky after the two Magellanic Clouds. Its apparent diameter is equivalent to that of the Moon, but its dimmer brightness makes it much less visible.
Another object observed: the globular cluster NGC 288, which is over 13 billion years old, dating back to the birth of the Milky Way, is composed of approximately 100,000 stars poor in elements heavier than hydrogen or helium.
In total, the spectra of more than 2,000 other sources were also analyzed. These included stars in our galaxy whose temperature, mass, diameter, velocity, age, and chemical composition are now known. 4MOST simultaneously produced spectra from more than 100 galaxies, up to 10 billion light-years away, to assess their distance, movement, evolution, and central black holes.
More than 700 people from universities and research organizations around the world will now be working with 4MOST. Twenty-five scientific programs, involving several research laboratories from the CNRS and its partners, are already set up for the next five years.
Contacts:
CNRS research associate florence.laurent [at] univ-lyon1.fr (florence[dot]laurent[at]univ-lyon1[dot]fr)
Researcher Université Claude Bernard Lyon 1 johan.richard [at] univ-lyon1.fr (johan[dot]richard[at]univ-lyon1[dot]fr)
CNRS Press