Two-faced star has hydrogen on one side and helium on other

By Mark Waghorn via SWNS

A strange two-faced star has been discovered by astronomers - made of hydrogen on one side and helium on the other.

The newfound white dwarf has been nicknamed Janus - after the Roman god of transition.

Lead author Dr. Ilaria Caiazzo, of Caltech, said: " The surface of the white dwarf completely changes from one side to the other.

"When I show the observations to people, they are blown away."

We may be witnessing the burnt-out sun undergoing a rare phase of evolution, say the international team.

Dr. Caiazzi explained: "Not all, but some white dwarfs transition from being hydrogen to helium-dominated on their surface.

"We might have possibly caught one such white dwarf in the act."

The blue-tinted star is composed mainly of hydrogen on one side and helium on the other. The former appears brighter.

The phenomenon might be due to a mixing of materials, known as convection.

On the helium side, which appears bubbly, the thin hydrogen layer on the surface has been destroyed - bringing up the helium underneath.

White dwarfs are the scalding remains of stars that were once like our sun. As the stars age, they puff up into red giants.

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 five billion years.

Janus was detected by a scanner called ZTF (Zwicky Transient Facility) at the Palomar Observatory near San Diego.

Dr. Caiazzo had been searching for highly magnetized white dwarfs which she and her team found previously using the device.

Further investigations showed Janus is rotating on its axis every 15 minutes - and revealed the dramatic double-faced nature of the white dwarf.

The researchers used a spectrometer to spread the light of the white dwarf into a rainbow of wavelengths that contain chemical fingerprints.

Data revealed the presence of hydrogen when one side of the object was in view - and only helium when the other side swung into the frame.

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.

But 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.

One side may evolve before the other due to magnetic fields.

Dr. Caiazzo explained: "Magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side.

"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."

On the other hand the fields may change the pressure and density of the atmospheric gases.

Co-author Professor James Fuller, also from Caltech, said: "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.

"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."

The team hopes to find more Janus-like white dwarfs with ZTF's sky survey.

Dr. Caiazzo said: "ZTF is very good at finding strange objects." Future surveys should make finding variable white dwarfs even easier.

Janus is described in the journal Nature.