11 May 2026

On 11 May 1951, the predecessor of ASTRON (the Netherlands Institute for Radio Astronomy) observed the 21-centimeter radiation of neutral hydrogen for the first time in the Netherlands. Since then, this radio emission has become one of the most important tools in modern astronomy.

The theoretical prediction of the hydrogen line had already been made in 1944 by the Dutch astronomer Henk van de Hulst. The Netherlands Foundation for Radio Astronomy (NFRA or SRZM, the predecessor of ASTRON) subsequently worked on detecting this signal, under the leadership of Leiden astronomer Jan Oort. This was done with converted radar equipment in Kootwijk. The detection marked the beginning of a Dutch tradition in radio astronomy that would become world-leading.

The discovery of the hydrogen line made it possible to map the cold hydrogen gas — the raw material from which stars are formed — in the Milky Way. To study this gas in detail, the 25-metre Dwingeloo radio telescope was built in 1956, at the time the largest fully steerable radio telescope in the world. Using the Dwingeloo telescope, Dutch astronomers systematically mapped the hydrogen gas in our Galaxy, leading to one of the first clear maps of the spiral structure of the Milky Way.

The next major step came with the inauguration of the Westerbork Synthesis Radio Telescope (WSRT) in 1970. The telescope consists of 14 dishes spread over 2.7 kilometres and grew into one of the world’s leading radio telescopes.

With the WSRT, ASTRON  made groundbreaking contributions to the study of hydrogen in galaxies. Together with astronomers from Groningen, Westerbork provided important evidence for the existence of dark matter through the motion of hydrogen gas in galaxies.

Technically as well, Westerbork was a pioneer. The exceptional stability and precision of the telescope enabled extremely deep observations and helped lay the foundation for later flagship instruments such as LOFAR and the international Square Kilometre Array (SKA).

In recent years, this tradition was continued with Apertif, an innovative “camera system” that used phased-array feeds to observe a much larger area of the sky at once, allowing many more galaxies to be mapped simultaneously.

Seventy-five years after the first Dutch detection of the hydrogen line, ASTRON is currently building new state-of-the-art, low-power receivers to focus once more on the neutral hydrogen line, with observations aimed at sensitive imaging of the dynamic radio sky.

With new instruments, international collaborations, and preparations for the SKA, the Netherlands continues to play a leading role in the study of neutral hydrogen in the Universe, from the structure of our Milky Way to the evolution of galaxies.