What is magnetar?
When a star’s mass far exceeds that of the Sun, its fate is sealed in the dramatic finale of stellar life. In the dying stages, a massive star explodes as a supernova. The leftover core may become a neutron star if its mass falls within a certain range, bright in a compact radius of only tens of kilometers. Despite their small size, these remnants pack in more mass than the Sun, with matter so dense that ordinary earthly matter cannot exist. Neutron stars are made of pure neutrons and are among the densest objects in the universe.
Some neutron stars take a different path and become magnetars. These are the universe’s strongest magnets, with magnetic field strengths around 10^11 tesla. By comparison, the strongest manmade magnets measure only a few tens of tesla. Since magnetars are distant, researchers know surprisingly little about them, including how they form. No star has been observed to naturally transform into a magnetar in real time, and their origins remain a topic of investigation.
False Wolf-Rayet
The binary system HD 45166 has puzzled astronomers for more than a century. It sits about four thousand light-years from Earth and consists of two stars: a typical star with about four solar masses and a peculiar companion. Its spectrum resembles that of very bright and hot Wolf‑Rayet stars, a class of massive stars in the late stages of their evolution that have burned a large portion of their hydrogen and are rich in helium.
HD 45166 stands apart from ordinary Wolf‑Rayet stars. Spectral analysis shows that light from this system can reveal its chemical makeup. By studying the spectrum, scientists conclude the system contains abundant oxygen, nitrogen, and carbon, with a combined mass comparable to several suns rather than being dozens of solar masses heavy.
Researchers from the University of Amsterdam, led by Tomer Shenar, compared observations from three optical telescopes—the CFHT 3.6‑meter in Hawaii, a Chilean observatory, and the Mercator telescope in the Canary Islands—to build a clearer picture of this unusual pair.
The most magnetic star
Analysis of the star’s light showed that certain spectral lines in the anomalous component of HD 45166 split noticeably due to the Zeeman effect. In simple terms, a magnetic field can separate light into distinct stripes rather than a continuous spectrum, much like a prism separates colors but in a way that reveals a star’s magnetic influence. This splitting confirms the presence of a significant magnetic field in the system.
Measurements indicate a magnetic field strength of about 4.3 tesla for the anomalous component of HD 45166. While this is modest by the standards of superconducting magnets used in big science experiments, it is extraordinary for a normal star that is not a neutron star. The temperature of the star is now understood to be around 56,000 kelvin, and its mass is close to two solar masses. The combination of a strong magnetic field with a relatively ordinary stellar makeup has led scientists to classify this object as a distinct type worthy of separate study.
These findings underscore why researchers call this object a new kind of astronomical phenomenon. The discovery excites scientists who want to understand how such stars form and behave, marking a notable advance in stellar magnetism research.
The mystery of the origin of magnetars
Computational models help scientists glimpse the future of magnetars. Simulations show a magnetar’s surface shrinking as the star collapses toward a neutron star, which concentrates its magnetic flux and strengthens the field. If the radius falls to about 12 kilometers, the magnetic inductance approaches values seen in magnetars, explaining their extreme magnetism.
Where magnetars come from remains a key question. The leading idea points to complex binary interactions rather than the slow aging of a single star. In some scenarios, two stars in a tight pair merge or exchange material, creating paths to a magnetar that are not typical for solitary stars. This past likely involved an initially triple system where two stars were closely bound while a third wandered farther away.
In one possible sequence, one star strips away its hydrogen envelope and transfers material to a companion. The hydrogen layer becomes depleted, and a dense cloud forms, altering the evolution of the remaining helium-dominated core. The result may be a highly compact, magnetized star that ultimately evolves into a magnetar.