Instrument will survey 20 million stars and galaxies
The hexagonal field of view of 4MOST and the observed objects (with different colors for different types of objects). 4MOST collected a spectrum for each of these objects, making it possible to study their properties. Image: AIP/Background: Harshwardhan Pathak/Telescope Live
The instrument 4MOST on ESO's VISTA telescope has received first light. Over the next five years, researchers from Uppsala University, together with researchers from across Europe, will collect spectra from around 20 million stars and thousands of galaxies. This will enable study of the different components and structure of the Milky Way, and how our galaxy was formed in detail.
On October 18, 2025, first light reached the 4MOST multi-object spectrograph, installed on the VISTA telescope at the European Southern Observatory (ESO) Paranal Observatory in the Atacama Desert in Chile.
“With this new instrument, we will gain a better understanding of the different components and structure of the Milky Way, and how our galaxy was formed in detail,” says Paul Barklem, professor at the Department of Physics and Astronomy and one of the researchers involved in the project.
Observe 2,400 objects simultaneously
4MOST is one of the most effective instruments for large-scale surveys, but 4MOST differs from conventional telescopes, by recording spectra instead of taking images of the night sky.
The instrument has a network of optical fibers directed at specific celestial bodies, that makes it possible to observe 2,400 objects simultaneously. Each fiber collects light from an object and sends it to a spectrograph that splits the light into up to 18,000 color components, which are analyzed with spectral analysis.
“By analyzing these spectra, we can study the detailed chemical composition and physical properties of stars and galaxies. We will be able to create catalogs of, among other things, temperatures, chemical compositions, velocities, and ages for tens of millions of objects scattered across the entire southern sky,” says Paul Barklem.
The construction of the 4MOST instrument began in 2010, and the facility is now the largest spectroscopic mapping facility in the southern hemisphere. The project involves over 330 researchers from 15 European countries and Australia.
Sweden has played a central role
The observations are carried out over a long period of time by local staff at the telescope in Chile, but many of the observations are also automated. Sweden has, from the beginning of the project, played a central role, and in addition to researchers from Uppsala University, researchers from Stockholm University and Lund University are also participating.
“Our part of the project is largely about developing software and models, which we have extensive knowledge and experience of at Uppsala University. The software is used to classify, model, and analyze spectra from the enormous amounts of data. Our part also involves interpreting data, such as chemical composition, age, and motion, to understand how different components of the galaxy were formed and the physical processes that influenced this,” says Paul Barklem.
Camilla Thulin
4MOST
4MOST stands for 4-metre Multi-Object Spectroscopic Telescope and is a multi-object spectrograph installed on the VISTA telescope at the European Southern Observatory (ESO) Paranal Observatory in the Atacama Desert in Chile. The instrument is designed to operate for up to 15 years, enabling a long-term survey of the southern sky.
The first light received by the telescope came from the center of the Sculptor Galaxy, from stars in the globular cluster NGC288, and the core of a distant active galaxy.
The 4MOST project is a European collaboration led by the Leibniz Institute for Astrophysics Potsdam (AIP). The project involves researchers from 15 institutes in Germany, Australia, France, the Netherlands, Sweden, Switzerland, and the United Kingdom, and is led by the Leibniz Institute for Astrophysics Potsdam (AIP). In addition to Uppsala University, Lund University and Stockholm University are also participating from Sweden.
4MOST is a central part of the Swedish research project “The New Milky Way,” funded by the Knut and Alice Wallenberg Foundation between 2013 and 2018 and funding for construction of the instrument The project is led by Lund University in collaboration with Uppsala University. The aim is to use 4MOST to create the most detailed survey of the Milky Way to date and thereby understand how our galaxy was formed and has evolved.
From the Department of Physics and Astronomy at Uppsala University Anish Amarsi, Paul Barklem, Simon Barton, Sema Caliskan, Danielle de Brito Silva, Bengt Edvardsson, Diane Feuillet, Ulrike Heiter, Andreas Korn, Katherine Lee, Thomas Nordlander, Nikolai Piskunov, and Martin Sahlén are working on the project.