SAN DIEGO – Astronomers have discovered a rare star at Palomar Observatory that could be the first documented instance of a long-standing theory about what happens to the core of a star as it burns out.

The newfound celestial object is a rare kind of dwarf star, which are the scalding “corpses” of stars after they die, that appears to have two faces on its surface – one side is entirely made up of hydrogen, while the other is composed of helium.

Named Janus after the two-faced Roman god of transition, researchers believe that the star could be the first discovery within a class of rare white dwarfs that could shed light on a theory about the evolution of the objects at large.

“It’s something that has been predicted for a long time, but we’ve never actually seen a white dwarf going through this,” said Ilaria Caiazzo, a postdoctoral researcher at Caltech who studied the star for the last two years. “We might be seeing one (at) this moment.”

As Caiazzo explained, white dwarfs normally are formed from the remnants of stars that were once similar to the sun.

When it starts to die, the star puffs up into a red giant, or planetary nebulae, before the outer fluffy material is blown away. What remains of the star’s core contracts into dense, fiery-hot white dwarfs. Scientists believe our sun will evolve into a white dwarf in about five billion years.

Janus was discovered as Caiazzo and her team were scanning the sky for a different kind of rare white dwarf, one that is formed as two different cores merge together to become a single star. 

Using Palomar Observatory’s Zwicky Transient Facility (ZTF), an instrument that captures images of the nighttime sky that’s used to chart changes over time, they found an object that could have been a candidate for this kind of white dwarf, given quick changes in luminosity.

“I found this object and I thought it was just a normal white dwarf with features that could have some magnetic fields on its surface that was maybe brighter on one side, maybe hotter on one side or another,” Caiazzo recalled. 

Further investigation using other equipment at Palomar, as well as a camera on the Gran Telescopio Canarias in Spain’s Canary Islands, confirmed that it was rotating at a fast rate that was comparable to other merger dwarf stars.

“I was not looking for a double-faced object,” she added. “We were looking for white dwarf merger remnants and these white dwarfs looked like one of those, because it was rotating very fast – every 15 minutes, it rotates on its axis.”

Subsequent observations, however, revealed the one-of-a-kind nature of the dwarf. Using the spectrometer at the W.M. Keck Observatory atop Mauna Kea in Hawai’i, the team was able to figure out the chemical fingerprints of the star by spreading the light it emanated into wavelengths, revealing the dramatic split between hydrogen and helium on its surface.

The hydrogen appears to be brighter. An animation of the star’s composition can be found below.

“Maybe the reason the white dwarf looks like this, it might be because it’s actually transitioning between two different phases in its life,” Caiazzo said. “That’s what makes it a very interesting object, because people have had theories about the spectral evolution of white dwarfs – that (they) change the way they look, the surface composition with time.”

As she explained, heavier elements sink to the core of the white dwarf after its formation while the lighter elements – hydrogen being the lightest of all – remain on top. Helium sits right below.

At certain temperatures, this composition begins to change, either through a mixing of these two elements as the dwarf cools or heats creating convection that destroys the hydrogen, leaving helium on the top of the surface. 

But in either of these cases, scientists found Janus at the approximate transitioning temperature in a disjointed stage of evolution, given that the hydrogen and helium were clearly separated.

“It’s very surprising, because these are gaseous objects,” Caiazzo said. “They’re made of gas on their surface, so how can you keep separated two elements so cleanly?”

Why might we be seeing the very strange separation on the surface? The answer, according to the Caltech team, is likely a magnetic field on the surface of Janus that’s fostering a rare transition.

Magnetic fields tend to be asymmetric, Caiazzo explained, or stronger on one side. If that is the case with Janus, it is likely suppressing convection on one side that prevents mixing and leaves more hydrogen in place.

However, it’s hard to determine exactly why the star’s composition is the way it is, considering its novelty. To help solve the mystery, the team plans on looking for more Janus-like white dwarfs using the ZTF sky survey at Palomar Observatory.

“What’s really interesting here is that we just kind of recently started looking at the dynamic sky,” Caiazzo said. With facilities like ZTF, she explained that scientists have had a greater ability to understand space through “staring at the sky (to) look at how it changes on very short timescales.”

“This has opened a new window on stars,” she added. “Every time you open a new window on the stars, you realize that no two stars are the same and you get completely baffled.”

Janus is located in a part of the night sky that is often observed by amateur astronomers – the Cygnus Loop within the Cygnus Constellation. However, Caiazzo said that skygazers might have a difficult time finding the white dwarf, as it is an incredibly faint object.