More than a decade of exceptionally precise observations has revealed two very different magnetic fields on Vega, one of the best-known stars in the night sky. The discovery could change our understanding of how intermediate-mass stars evolve.

For more than 150 years, Vega, the brightest star in the constellation Lyra, has served astronomers as a benchmark of stellar stability.

Yet Vega is proving to be considerably less simple than it appears.

After analysing more than a decade of observations, an international team led by researchers from CNRS Terre & Univers has discovered that Vega possesses a surprisingly complex magnetic structure.

Two different types of magnetic fields appear to coexist on its surface: a stable, large-scale fossil magnetic field and a smaller, variable field that may be generated by a dynamo mechanism.

The discovery opens a new window on one of the longstanding mysteries of stellar evolution.

A star that should not behave this way

Vega is an intermediate-mass star with approximately twice the mass of the Sun. It belongs to spectral class A and rotates extremely rapidly, reaching an equatorial velocity of almost 200 kilometres per second.

In 2009, astronomers made a surprising discovery: Vega possesses a very weak magnetic field.

That finding was unexpected.

According to conventional models of stellar evolution, stars of Vega’s type should not easily generate magnetic fields. Unlike cooler stars, they lack the deep convective layer near the surface that normally helps produce stellar magnetic activity.

Astronomers therefore faced a puzzle: Where was Vega’s magnetism coming from?

A magnetic fossil or an active dynamo?

There were two main possibilities.

The first was that Vega’s magnetic field is a fossil field — a remnant created during an earlier stage in the star’s life and preserved ever since.

The second possibility was much more dynamic: perhaps some mechanism inside Vega is still actively generating magnetism.

Evidence supporting the second possibility began accumulating.

In 2015, detailed analysis of large numbers of high-resolution spectra revealed bright and/or dark spots rotating together with Vega. Such features are commonly associated with dynamo-generated magnetic fields.

By 2017, astronomers also suspected that these spots were changing over relatively short periods.

A much larger observational campaign was needed to understand what was happening.

More than 13,000 observations

The international research team analysed more than 13,000 high-resolution spectra collected over more than a decade.

The observations came from two major French astronomical facilities.

At the Observatoire de Haute-Provence, researchers used the SOPHIE spectrograph. At the Pic du Midi Observatory, they employed the Narval and later Neo-Narval instruments mounted on the Bernard Lyot Telescope.

These instruments allowed scientists not only to analyse Vega’s light with exceptional precision but also to study its magnetic properties through spectropolarimetry, a technique that extracts information about magnetic fields from polarised light.

Combined with extremely precise data processing, the enormous dataset revealed something astronomers had not expected.

Vega has two magnetic faces

The researchers found evidence that two fundamentally different magnetic structures coexist on Vega.

The first is a stable, large-scale fossil magnetic field. It takes the form of an inclined dipole, meaning that the star’s magnetic poles are not perfectly aligned with its axis of rotation. This configuration had already been detected in 2022.

But alongside this stable structure exists a second magnetic component.

This field operates on much smaller spatial scales and changes over time. The researchers believe it is probably being produced by a dynamo effect.

For the first time, a prototype intermediate-mass star has therefore revealed both a persistent fossil magnetic field and a changing magnetic field that appears to be actively generated.

That combination challenges the traditional picture of stars such as Vega.

Why Vega matters

The discovery is important for more than understanding one famous star.

Intermediate-mass stars play an important role in the evolution of galaxies and in the chemical enrichment of the Universe. Understanding their internal physics is therefore part of understanding how the Universe acquired the chemical elements from which planets — and ultimately life — are made.

Magnetic fields are an essential part of stellar evolution, but their role in intermediate-mass stars remains poorly understood.

The Vega observations provide astronomers with new constraints for models explaining how these stars evolve.

The research team used sophisticated techniques including Zeeman-Doppler imaging to reconstruct magnetic maps of Vega’s surface, together with an innovative Doppler-imaging method to map variations in intensity.

A new field of stellar research

There are already theoretical ideas that could explain how relatively weak dynamos might operate inside stars whose outer envelopes were traditionally considered almost entirely radiative.

But Vega suggests that the physics may be more complicated than existing models assumed.

The next question is whether Vega is exceptional or whether other intermediate-mass stars possess similarly complex magnetic systems.

If comparable observations reveal the same phenomenon elsewhere, astronomers may have to reconsider how an entire class of stars generates and maintains magnetism.

After more than 150 years as one of astronomy’s standards of stability, Vega may now become a laboratory for discovering just how dynamic apparently stable stars can really be.

Source: Based on the French scientific report “Détection d’un champ magnétique complexe sur la surface de l’étoile de masse intermédiaire Vega”, published by CNRS Terre & Univers on 1 October 2025. The underlying study is T. Böhm et al., “Stable magnetic fields and changing starspots on Vega: An ultra-deep decadal survey at Pic du Midi and OHP”, published in Astronomy & Astrophysics (2025).