The Orbiting Dead

An artist's rendering of gas siphoning from a giant companion to a white dwarf disc. (Image: Space Telescope Science Institute, NASA)

An artist's rendering of gas siphoning from a giant companion to a white dwarf disc. (Image: Space Telescope Science Institute, NASA)

Stars have long been analogized to life, both for pedagogy and for style. Their “births” occur within giant clouds of gas and dust, so-called “stellar nurseries” where stars are fused from collapsed pockets of matter. We refer to their “evolution” or “life cycle” as they grow from one stage to another - including our Sun which, though now enjoying the stability of middle age, will eventually expand into a red giant and then become a white dwarf, the final naked core of what it once was. And indeed, we refer to their “deaths.” For stars that become white dwarves, it’s a slow death. They burn softly for trillions of years before going black. But for more massive stars, the death is instant and catastrophic, taking the form of a spectacular explosion known as a supernova. This violent death releases the star’s elemental contents across space and thus creates the same clouds of gas and dust that lead to new star births, perpetuating the grand cycle of stellar life.

Whether this analogy to life works because it’s the best way for us, as life forms ourselves, to understand the scientific processes or because it’s indicative of some deeper ontological connection between life and stars is unclear and maybe unanswerable; but it stands that if stars can be at least described as “alive” and “dead,” perhaps there are cases where they can be described as neither. Perhaps they can be undead. Such is the description accorded to a group of hypothetical stars, dubbed “zombie stars,” which might result from a certain type of supernova. Imagine a binary star system of two Sun-like stars orbiting close to each other: one is already a white dwarf (therefore in its final stage), and the other has evolved into a red giant. If the two stars are close enough, some of the red giant’s expanding gas envelope will spill over to the white dwarf, adding to the white dwarf’s mass. Though white dwarves wouldn’t normally explode (their style of death being the slow burn), once one attains some critical mass after siphoning enough gas from a companion, it will supernova and eject the companion star in the explosion. However, for less luminous and lower energy star systems, this supernova may not obliterate the white dwarf — it may leave behind a remnant about half the original size. This remnant is the zombie star.

The details of the process are still unclear, and indeed zombie stars have yet to be confirmed by observation, but a couple of supernova observations in recent years [1] are believed to be of this type. It raises an interesting question about whether we’ve taken our analogies too far — it’s not as if these stars were obliterated and then somehow reactivated, which would be the more accurate stellar analog of “rising from the dead.” Instead, zombie stars may be more logically described as just severely weakened stars, barely “alive” after suffering a fatal blow, remaining intact after what would normally be cosmic destruction. But the beauty of analogies is that they aren’t perfect; they play in the spaces between actual physics and our subjective experiences — experiences of both the physical world and language itself. That interplay seems to make analogies particularly powerful and memorable, even if not entirely scientifically accurate, and it's why the label “zombie star” beats out “severely weakened star” every time.

Sean Faulk, @sean_faulk20
Staff Writer, Signal to Noise Magazine
PhD Candidate, Earth, Planetary, and Space Sciences


References:
[1] NASA’s Hubble Finds Supernova Star System Linked to Potential “Zombie Star.” Nasa.gov (2014).