A massive 'zombie' star is rampaging away from its home cluster

Illustration shows a massive runaway white dwarf escaping from the Hyades star cluster.
Illustration shows a massive runaway white dwarf escaping from the Hyades star cluster. (Image credit: Robert Lea/NASA)

In what sounds like the plot of a cosmic horror story, which is perfect as Halloween is on the horizon, a long-dead "zombie" star has been caught rampaging its way out of a nearby stellar bundle dubbed the Hyades star cluster. But unlike the shambling zombies you might find in a George A. Romero movie, this massive cosmic ghoul is moving as fast as 22,000 miles per hour (10 kilometers per second) relative to the cluster from which it has escaped. 

Rather than terrify astronomers, however, this runaway white dwarf has proved an exciting find for the scientific community. That's because this fascinating object could help solve a few lingering mysteries surrounding the Hyades star cluster which is located around 153 light-years from Earth in the constellation of Taurus. For instance, it can aid scientists in determining just how big objects like this one can get without a helping hand.

Around 97% of stars in the Milky Way will end their existence as white dwarfs. Yet, despite the commonality of these dead stars, the Hyades open star cluster, consisting of several hundred stars that are believed to have formed around the same time  —  625 million years ago  —  and  from the same cloud of collapsing gas and dust, only possesses a few white dwarfs at its core. 

This has led astronomers to wonder what happened to the rest — though they have some ideas.

Related: Dead star at heart of cosmic graveyard predicts the sun’s fate 

The stars of Hyades are only loosely bound, meaning they can be ejected. One way that ejection can happen is through interactions with other star clusters; another way has to do with massive clouds of gas moving between those clusters. 

But the key here is that, because of this ejection probability, the absence of white dwarfs in the Hyades star cluster could very well be because some white dwarfs were launched out. 

This is the theory that led University of British Columbia researcher David Miller and colleagues to hunt for those white dwarfs that might have been ejected from the Hyades. For their analysis, the researchers used data collected by the Gaia spacecraft, which has been tracking over 1 billion Milky Way stars since 2013. Sure enough, the team found three white dwarfs with velocities that indicated they may have been ejected from Hyades. 

Two of these stars were 1.1 times more massive than the sun, which made it unlikely they originated in the Hyades. But the third ,  designated Gaia EDR3 560883558756079616 , was 1.3 times more massive than the sun, which pointed to it being an escapee.

"It is interesting that such a high-mass white dwarf was identified as having been born in the Hyades cluster. The Hyades is not exceptionally rich in stars nor in a particularly dense region of the galaxy; by most accounts, it is a typical moderately populated and evolved cluster," the team wrote in a paper about the discovery. "The sole factor that makes the cluster stand out is its proximity as the closest cluster to the sun. This enables the detection of older, cooler white dwarfs and the ability to trace back escaped stars with greater precision, allowing us to study the cluster in greater detail than any other."

Not only that, but typical white dwarfs have masses around 0.6 times that of the sun — meaning this runaway is one of the most massive examples of its kind witnessed by scientists.. It could therefore help astronomers better assess where the line that divides white dwarfs from other types of stellar corpses, like neutron stars, is drawn.

This dead star isn’t stitched-together Frankenstein’s monster

White dwarfs are created when stars roughly the size of the sun run out of hydrogen at their cores, fuel needed for nuclear reactions at their hearts. Without fuel, the reactions cannot continue. The end of those nuclear reactions also see the end of the outward energy that supports a star against the inward force of its own gravity.

Thus, the cores of stars at this stage of their lives collapse under their own gravity while the outer layers  —  where nuclear fusion is still occurring  —  "puff out" to expand the star until its diameter is between 10 and 100 times larger than normal. . The sun will undergo this transformation over around 5 billion years, its diameter eventually reaching that of Mars' entire orbit. At this point, our host star will also  swallow the inner planets  —  including Earth.

The puffed-out material of a dying star continues to expand and cool over time, eventually forming a planetary nebula.  The core, meanwhile, gradually smolders away to become a white dwarf stellar remnant consisting of matter that is prevented from collapsing further. That matter is saved by a rule of quantum physics called electron degeneracy, which prevents an entity from cramming its material too closely. This electron degeneracy pressure can be overcome, however, if a star exceeds around 1.4 times the mass of the sun  —  the so-called Chandrasekhar limit  —  with stars pushing past this limit pulling in material so closely that they become either neutron stars or black holes

White dwarfs usually approach, and sometimes exceed, the Chandrasekhar limit by pulling material from a companion star and siphoning it to their surfaces. This triggers thermonuclear explosions at the surface of the white dwarf that make it appear almost as if it is springing back to life.

So what all this means when these dead stars are discovered close to that mass limit, they are usually the product of material from two stars, not one  —  almost like a stitched-together Frankenstein’s monster made of  two stellar bodies. 

But this Hyades escapee white dwarf seems to be the product of just one star, making it  perhaps the most massive single progenitor star white dwarf ever spotted. 

"This provides a critical observational benchmark for white dwarfs created from single progenitor stars, demonstrating that single stars can produce white dwarfs with masses close to the Chandrasekhar limit," the team behind this discovery wrote. "The combination of the unremarkable nature of the Hyades cluster and the benefits of its proximity suggest that open star clusters may be producing ultramassive white dwarfs, including white dwarfs which push the Chandrasekhar limit, more commonly than previously thought."

The team’s research has been accepted for publication in The Astrophysical Journal with a preprint available on the paper repository arXiv.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.