RadioStars allows users to perform remote radio observations of the 21 cm line of neutral hydrogen, HI. This line, centred near 1420 MHz, is a fundamental tool for studying cold interstellar gas, the kinematics of the Galactic disk and the spiral structure of the Milky Way.

The educational goal is to understand how a radio spectrum measured in a given direction can be converted into physical information: radial velocity, rotation curve, dynamical mass and approximate gas location within the Galactic disk.

To use the service, users log in to the RadioStars interface and reserve an observing slot. Once a slot is available, they can remotely control a radio antenna and select an observing direction on the sky, in particular using Galactic coordinates.

A typical protocol consists of observing close to the Galactic plane, with Galactic latitude near b = 0°, because most of the HI gas is concentrated in the Milky Way disk. The Galactic longitude is then varied in order to obtain several HI spectra in different directions.

The antenna is controlled from the Observer page, where observing slots can also be booked via Booking.

Each observation provides a radio spectrum around the 21 cm line. The spectral peaks correspond to neutral hydrogen clouds located at different distances along the line of sight. Their Doppler shift provides their radial velocity.

To build the rotation curve, one generally selects the most velocity-shifted peak, associated with the tangent point in the inner Galactic quadrants. This peak gives the maximum radial velocity, Vr,max. From the Galactic longitude l, one can then compute the Galactocentric radius and the corresponding rotation velocity.

RadioStars also provides access to archive data, including professional HI surveys such as LAB. These data make it possible to simulate observations when no telescope slot is available, or to compare remote antenna measurements with reference observations.

Archive data can be used to test the experimental protocol under stable conditions, improve longitude coverage and compare rotation curves obtained from different data sets.

The full observation archive is available on the Archive page.

The tangent-point method converts spectral measurements into rotation-curve points. Using the geometry of the Milky Way, one obtains a pair R, V(R) for each observed direction.

The resulting curve usually rises at small radii and then reaches a plateau around 220 km/s. This behaviour is important: unlike a simple Keplerian law, the velocity does not rapidly decrease with radius. This is one of the historical arguments for the presence of a non-luminous mass component, associated with dark matter.

The rotation-curve tool is available on the Archive page.

Beyond the maximum velocity alone, all Doppler components present in a spectrum can be used. Each component corresponds to gas located at a different position along the line of sight. By combining measured velocities, Galactic longitude and an assumed rotation curve, one can estimate the position of HI clouds in the Galactic plane.

This method provides a qualitative reconstruction of the neutral-gas distribution in the Milky Way and can reveal structures compatible with spiral arms. The resulting map remains simplified, limited by distance ambiguities, angular resolution, instrumental noise and incomplete coverage of the Galactic plane.

The HI-distribution map reconstruction tool is also available on the Archive page.