WASHINGTON, July 21 – Some people are two-faced, figuratively speaking of course. Ancient Roman god Janus had two faces, literally. One face was looking ahead and the other backwards, symbolizing transitions and dualities. But a two-faced star? Yes, indeed.
Scientists have observed a white dwarf star – a hot stellar remnant that is among the densest objects in the cosmos – that they have nicknamed Janus owing to the fact it has the peculiar distinction of being composed of hydrogen on one side and helium on the other.
“Janus, the Roman God with two faces was a good choice. Moreover, Janus is the god of transition, and the white dwarf might be currently transitioning from having an atmosphere made of hydrogen to one made of helium,” said Ilaria Caiazzo, a Caltech postdoctoral fellow in astrophysics and lead author of the study published this week in the journal Nature.
The star is located in our Milky Way galaxy about 1,300 light years from Earth in the direction of the Cygnus constellation. A light year is the distance light travels in a year, 5. 9 trillion miles (9. 5 trillion km).
Janus is fairly massive for a white dwarf, with a mass 20% larger than that of our sun compressed into an object with a diameter half that of Earth. It rotates on its axis every 15 minutes – very fast considering these stars usually rotate every few hours to a few days.
“White dwarfs form at the very end of a star’s life. About 97% of all stars are destined to become white dwarfs when they die,” Caiazzo said.
“Our Sun, for instance, is burning hydrogen to helium at its core. The sun’s core will burn helium to produce carbon and oxygen when the hydrogen is gone. When the helium also is gone from the center, the sun will eject its outer layers into space in an event called a planetary nebula and the core will slowly contract and become a white dwarf,” Caiazzo added.
The good news for Earthlings is that it should be 5 billion years before any of that happens to our sun.
Janus was spotted using the Zwicky Transient Facility at Caltech’s Palomar Observatory near San Diego, with subsequent observations made by other ground-based telescopes.
After a white dwarf forms, its heavier elements are thought to sink to the star’s core while its lighter elements – hydrogen being the lightest, followed by helium – float to the top. This layered structure is believed to be destroyed at a certain stage in the evolution of some white dwarfs when a strong mixing blends the hydrogen and helium together.
Janus could be a white dwarf that is in the middle of this transitional blend process. However, the development of the hydrogen side and the helium side of Janus are puzzling.
The researchers suspect that its magnetic field may be responsible for this asymmetry. The researchers suspect that the stronger magnetic field on one side of the object could be responsible for the asymmetry.
“Many white dwarfs are expected to go through this transition, and we might have caught one in the act because of its magnetic field configuration,” Caiazzo said.
Janus is not the only exotic white star known. Caiazzo was part of a research team that in 2021 reported on one with a petite diameter slightly larger than Earth’s moon that boasted the greatest mass and littlest size of any known white dwarf.
“Every time we look at stars in a different ways, we are bound to be surprised and even baffled sometimes,” Caiazzo said. “Stellar phenomenology is extremely rich, and no two stars are the same if looked at closely enough.”
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