In a world first, astronomers observed a star with a solid outer surface

Stars are essentially large fiery balls of plasma, which is often referred to as the "fourth state of matter" as it forms when gas is heated into a soup of positively charged ions and negatively charged electrons.

Now, though, an international group of astronomers has observed a star with a solid outer layer for the first time, casting doubt on widely held preconceptions about the cosmic giants lighting the night sky.

The star's magnetic field is so strong it appears to have "frozen" its surface into a solid outer crust, a press statement reveals.

Magnetar observation was "completely unexpected"

Astronomers made the discovery while investigating data from the Imaging X-ray Polarimetry Explorer (IXPE), a NASA satellite designed to measure the polarization of X-ray light from stars and other space objects.

The team studied IXPE's data on a type of star called a magnetar. The magnetar in question, 4U 0142+61, is located some 13,000 light-years from Earth in the Cassiopeia constellation. Magnetars are a type of neutron star — a collapsed supergiant star —named because they have extremely powerful magnetic fields. The new observation, outlined in a new paper in the journal Science, is the first time scientists have analyzed one of these star types in polarized X-ray light — which essentially shows the direction electromagnetic waves are vibrating.

Surprisingly, the polarized X-ray data revealed that magnetar 4U 0142+61 doesn't have an atmosphere, as the scientists on the project had expected. Most strikingly, the polarization angle at higher energies was flipped 90 degrees when compared with light from lower energies. This indicates that the magnetar has a solid surface surrounded by a magnetic field. According to scientists, the "solid crust of the star is thought to be composed of a lattice of ions, held together by the magnetic field. The atoms would not be spherical but elongated in the direction of the magnetic field."

"This was completely unexpected," said Professor Silvia Zane, co-lead author of the study. "I was convinced there would be an atmosphere. The star's gas has reached a tipping point and become solid in a similar way that water might turn to ice. This is a result of the star’s incredibly strong magnetic field. But, like with water, temperature is also a factor — a hotter gas will require a stronger magnetic field to become solid."

Scientists set out to find more solid surface stars

The new paper marks the first time a magnetar observation indicated the star type has a solid crust. Though the finding is surprising, it's in line with the widely believed theory that magnetars have powerful magnetic fields. More research is needed as the international group of scientists said other potential explanations must be ruled out.

Next, they plan to study hotter magnetars to investigate the roles of temperature and magnetic field strength on the outer surface of the stars further. More observations will also help to rule out any other explanations and confirm that the scientific community has made a historic first-ever observation of a star whose surface is not characterized by the "fourth state of matter".