September 28, 2026: Astronomers using archival observations from NASA’s Chandra X-ray Observatory have identified a previously unrecognized class of cosmic objects that emit exceptionally low-energy X-rays along with powerful ultraviolet radiation.

Researchers have named these objects “hypersoft X-ray sources” (HSSs). A total of 84 such sources were identified across six nearby galaxies, including the Andromeda Galaxy and the Pinwheel Galaxy (M101). The findings were published in Nature Astronomy.

A Previously Hidden Population

Unlike many familiar X-ray sources, these objects produce most of their detectable X-ray emission at the lowest energies accessible to Chandra, while showing little or no emission at higher X-ray energies.

Scientists believe much of their energy may actually be released as extreme-ultraviolet (EUV) radiation, a portion of the electromagnetic spectrum that is particularly difficult to observe because hydrogen and helium gas absorb it very effectively.

This combination may explain why the objects remained largely unnoticed despite Chandra having collected astronomical data for more than two decades.

84 Objects Found in Six Galaxies

The research team examined publicly available Chandra observations from six galaxies. The sample included two spiral galaxies—Andromeda (M31) and M101—as well as four elliptical galaxies.

The sources were found both in regions containing young, actively forming stars and in areas dominated by older stellar populations. This suggests that the phenomenon may occur in more than one type of stellar environment.

What Are These Objects?

Scientists do not yet have a single definitive explanation for the nature of hypersoft X-ray sources.

The researchers suggest that some could involve white dwarfs, neutron stars or black holes pulling material from companion stars. As matter falls toward these compact objects, it can become extremely hot and produce high-energy radiation.

Some of the less luminous sources in Andromeda are associated with known novae, providing evidence that at least a portion of the population may be connected with white-dwarf systems.

Possible Connection to Type Ia Supernovae

The discovery could have implications for one of astronomy’s long-standing questions: what systems eventually produce Type Ia supernovae?

Type Ia supernovae are important astronomical distance indicators and played a major role in the discovery that the expansion of the universe is accelerating.

Researchers have proposed that some hypersoft sources could represent stages in the evolution of white-dwarf systems that may eventually produce Type Ia explosions. However, the study does not establish that all HSSs are Type Ia progenitors. Further observations will be needed.

Another Cosmic Mystery

The newly identified sources may also contribute to the ionization of gas between stars.

Scientists have long studied how atoms in interstellar gas lose electrons. Hot, massive stars can provide some of the necessary radiation, but certain ionization processes require higher-energy photons than these stars readily produce.

Because HSSs may emit large quantities of EUV radiation, researchers believe they could provide part of this previously difficult-to-account-for energy.

More Observations Needed

The discovery opens a new area of research rather than providing a final explanation for the objects.

Scientists now need to determine how many hypersoft sources exist, what physical systems power them and how their properties change over time. Better observations at low X-ray and ultraviolet energies could help reveal their true nature.

For now, the 84 sources represent a previously overlooked population of powerful cosmic emitters and demonstrate that decades-old astronomical archives can still contain discoveries when researchers examine the data in new ways.