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.

‘The surface of the white dwarf completely changes from one side to the other,’ said lead author Dr Ilaria Caiazzo, of California Institute of Technology.

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

The strange phenomena may be caused by the burnt out sun undergoing a rare phase of evolution, say the international team.

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

‘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 hydrogen side 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 use up their fuel, they collapse in on themselves. However, the pressure from such massive contraction causes the core temperature to soar, and the star then swells up to 400 times its original size – the ‘red giant’ stage.

Eventually, their outer material is blown away and their cores contract into dense, fiery-hot white dwarfs. Our Sun will begin evolving into a white dwarf in about five billion years.

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

Dr Caiazzo had been searching for highly magnetised 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 hydrogen when one side of the object was in view – and only helium when the other side swung into 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, so much so 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.

‘ZTF is very good at finding strange objects,’ said Dr Caiazzo.

Janus is published in the journal Nature.

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