artist’s rendition of janus, the blue-tinted dead cinder of a star that is composed primarily of hydrogen on one side and helium on the other (the hydrogen side appears brighter). the peculiar double-faced nature of this white dwarf star might be due to the interplay of magnetic fields and convection, or a mixing of materials. on the helium side, which appears bubbly, convection has destroyed the thin hydrogen layer on the surface and brought up the helium underneath. Image credit: K. Miller, Caltech/IPAC

Two-Faced Star Exposed: Unusual White Dwarf Star is Made of Hydrogen on One Side and Helium on the Other

Maunakea, Hawaiʻi – In a first for white dwarfs, the burnt-out cores of dead stars, astronomers have discovered that at least one member of this cosmic family is two-faced. One side of the white dwarf is composed of hydrogen, while the other is made up of helium.

The findings, which include data from the Zwicky Transient Facility at Caltech’s Palomar Observatory in San Diego, California and W. M. Keck Observatory on Maunakea, Hawaiʻi Island, are published in today’s online edition of the journal Nature.

“The surface of the white dwarf completely changes from one side to the other,” says Ilaria Caiazzo, a postdoctoral scholar at Caltech and lead author of the study. “When I show the observations to people, they are blown away.” 

White dwarfs are the scalding remains of stars that were once like our Sun. As the stars age, they puff up into red giants, but eventually their outer fluffy material is blown away and their cores contract into dense, fiery-hot white dwarfs. Our Sun will evolve into a white dwarf in about 5 billion years.

The newfound white dwarf, nicknamed Janus after the two-faced Roman god of transition, was initially discovered by the ZTF, an instrument that scans the skies every night. Caiazzo had been searching for highly magnetized white dwarfs, such as the object known as ZTF J1901+1458, which she and her team found previously using ZTF. One candidate object stood out for its rapid changes in brightness, so Caiazzo decided to investigate further with the CHIMERA (Caltech HIgh-speed Multi-color camERA) instrument at Palomar, as well as with the camera HiPERCAM on the Gran Telescopio Canarias in Spain’s Canary Islands. Those data confirmed that the object, Janus, is rotating on its axis every 15 minutes.

Scientists think that magnetic fields may explain the unusual two-face appearance of the white dwarf nicknamed Janus. One side of the dead star’s surface is composed primarily of hydrogen, while the other side is helium, as seen in this artist’s animation. One theory states that asymmetric magnetic fields (seen as looping lines) may have influenced the mixing of materials in the white dwarf in such a way to have caused the uneven distribution. The white dwarf’s rotation has been sped up in this animation; normally, it rotates around its axis every 15 minutes. Credit: K. Miller, Caltech/IPAC

Subsequent observations made with Keck Observatory revealed the dramatic double-faced nature of the white dwarf. The team used the Low Resolution Imaging Spectrometer (LRIS) on the Keck I Telescope to view Janus in optical wavelengths (light that our eyes can see) as well as the Near-Infrared Echellette Spectrograph (NIRES) on the Keck II Telescope to observe the white dwarf in infrared wavelengths. The data revealed the white dwarf’s chemical fingerprints, which showed the presence of hydrogen when one side of the object was in view (with no signs of helium), and only helium when the other side swung into view. 

What would cause a white dwarf floating alone in space to have such drastically different faces? The team acknowledges they are baffled but have come up with some possible theories. One idea is that we may be witnessing Janus undergoing a rare phase of white dwarf evolution.

“Not all, but some white dwarfs transition from being hydrogen- to helium-dominated on their surface,” Caiazzo explains. “We might have possibly caught one such white dwarf in the act.”

After white dwarfs are formed, their heavier elements sink to their cores and their lighter elements—hydrogen being the lightest of all—float to the top. Over time, as the white dwarfs cool, the materials are thought to mix together. In some cases, the hydrogen is mixed into the interior and diluted such that helium becomes more prevalent. Janus may embody this transition phase, but one pressing question is: Why is the transition happening in such a disjointed way, with one side evolving before the other? 

The answer, according to the science team, may lie in magnetic fields. 

“Magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side,” Caiazzo explains. “Magnetic fields can prevent the mixing of materials. So, if the magnetic field is stronger on one side, then that side would have less mixing and thus more hydrogen.”

Another theory proposed by the team to explain the two faces also depends on magnetic fields. But in this scenario, the fields are thought to change the pressure and density of the atmospheric gasses. 

“The magnetic fields may lead to lower gas pressures in the atmosphere, and this may allow a hydrogen ‘ocean’ to form where the magnetic fields are strongest,” says co-author James Fuller, professor of theoretical astrophysics at Caltech. “We don’t know which of these theories are correct, but we can’t think of any other way to explain the asymmetric sides without magnetic fields.” 

To help solve the mystery, the team hopes to find more Janus-like white dwarfs with ZTF’s sky survey.  “ZTF is very good at finding strange objects,” Caiazzo says. Future surveys, such as those to be performed by the Vera C. Rubin Observatory in Chile, she says, should make finding variable white dwarfs even easier.


The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.


The Near-Infrared Echellette Spectrograph (NIRES) is a prism cross-dispersed near-infrared spectrograph built at the California Institute of Technology by a team led by Chief Instrument Scientist Keith Matthews and Prof. Tom Soifer. Commissioned in 2018, NIRES covers a large wavelength range at moderate spectral resolution for use on the Keck II telescope and observes extremely faint red objects found with the Spitzer and WISE infrared space telescopes, as well as brown dwarfs, high-redshift galaxies, and quasars. Support for this technology was generously provided by the Mt. Cuba Astronomical Foundation.

ABOUT W. M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.