Astronomers have observed a star in a relatively nearby galaxy that not only survived conditions that would normally be doomed — a stellar explosion called a supernova — but emerged from it brighter than the explosion bright before.
Meet the “Zombie Star”.
The controversial star, seen with the Hubble Space Telescope, is a type of star known as a white dwarf, a surprisingly dense object about the mass of the sun that squeezes into the size of Earth. A white dwarf is the leftover core of a star that blew off a lot of material at the end of its life cycle, just as our sun is expected to be about 5 billion years from now.
The white dwarf’s pairing with another star — known as a binary system — is gravitationally locked in orbit, and its powerful gravitational pull sucks a lot of material from the unfortunate companion.
This is where the trouble begins. In doing so, the white dwarf reaches a mass threshold — about 1.4 times that of the Sun — which triggers a thermonuclear reaction at its core, causing it to explode in a supernova, an event that should have killed it.
“We were very surprised that the star itself was not destroyed, but actually survived and was brighter than before the explosion.” in The Astrophysical Journal.
“During the explosion, radioactive material is created. That’s what drives the brightness of the supernova. Some of that material is left in the surviving remnant and used as fuel to heat the remnant,” McCulley added.
The white dwarf is located in a spiral galaxy called NGC 1309, about three-quarters the size of our Milky Way. Like the Milky Way, NGC 1309 resembles a spinning pinwheel when viewed from above or below. This white dwarf star is 108 million light-years from Earth. A light-year is the distance light travels in 5.9 trillion miles (9.5 trillion kilometers).
There are various types of supernovae, depending on the size and composition of the star and the power of the explosion.
The star is helping scientists better understand so-called “type Iax” supernovae. In these cases, the white dwarf undergoes runaway carbon and oxygen nuclear fusion after acquiring material relatively quickly, as if the white dwarf stole it from its companion. But the explosion didn’t destroy the white dwarf, leaving behind an “undead” remnant.
“It’s for this reason that we call these objects ‘zombie stars.’ They’re dead, but not quite dead. Early on, many supernova simulations by scientists failed before they could blow up an entire white dwarf. This It’s exciting to think that this tells us about the actual physics of these supernovae,” McCulley said.
Scientists have discovered about 50 such supernovae so far, but until now no surviving “zombie star” white dwarf has been identified.
Our sun is destined to be a white dwarf, which is the fate of about 97% of stars.
“At the end of a star’s life — for stars like our sun or a little bigger — the star runs out of fuel in the core and begins to collapse into a white dwarf. In the process, the star’s outer layers are blown off into the nebula. The remaining core of this star is a white dwarf,” McCulley said.
However, our Sun cannot be a zombie star because it lacks the necessary companion star.
© Thomson Reuters 2022