Rutgers researchers discover ‘zombie star’


A team of Rutgers researchers last August announced the discovery a type of star that lives after its death.

This class of star known as a white dwarf is a non-reacting mass left behind after a star’s death. If it is paired with much larger stars, the white dwarf may absorb enough material to cause a supernova, said Saurabh Jha, an associate professor in the Department of Physics and Astronomy.

This may leave behind a reacting mass, which University researchers have dubbed a “zombie star.”

“The ‘zombie star’ is just a cute name we came up with, but the idea is that mass can be transferred from one star to another, and this can ‘resurrect’ a white dwarf,” Jha said. “If enough material is absorbed, the star can explode.”

Curtis McCully, a post-doctoral Rutgers researcher, found this supernova in 2012 at the Lick Observatory in California, but it was not the first time that region of space had been documented.

The Hubble Space Telescope took pictures of the region in 2005 and 2006, before the star had exploded. This allowed McCully to compare the before-and-after pictures and study the star before its death. 

This type of explosion is called a type Ia supernova, said Jha, who was on the team of researchers that discovered the zombie star. 

Normally, a type Ia supernova leaves nothing behind, said Alyson Brooks, an assistant professor in the Department of Physics and Astronomy. The explosion discovered in 2012 appears to have left a remnant behind that behaves like the core of a star.

This newly-discovered explosion, termed Iax, Jha said, received the nickname “zombie star” because nuclear fusion still occurs even after the star has died.

The discovery of this zombie star is exciting because it provides evidence for the standard model for type Ia supernovae, Brooks said.

“The problem is by the time you see a supernova, one of those stars is missing,” she said. “But in this case, they had data from before the supernova went off, so they could look for that companion star, and they found it.”

This is one of the first confirmations of this model to be discovered, she said.

McCully said the explosion’s brightness was a factor in determining whether or not the supernova they observed was something new.

“These type Iax’s are weaker explosions than normal, so the ejecta, the exploded material, is moving slower,” he said. “We find the densities of these objects are very high even two years after the explosion.” 

Type Ia supernovae are responsible for the discovery that the universe is expanding, said Carlton Pryor, undergraduate program director in the Department of Physics and Astronomy. They have led to the hypothesis that dark energy exists.

“Most of these dwarfs explode with the same amount of brightness,” Jha said. “We use that to study how distant they are. We see a dimmer one — we know it’s far away. A brighter one is nearby.”

In 1998, these explosions were used to prove the universe is expanding, and this rate of expansion is accelerating, Jha said. 

Type Ia supernovae are expected every few centuries, he said, and are used as cosmic yardsticks. Now, type Iax supernovae can be used to study progenitor systems, the binary star systems that exist before the supernova occurs.

Type Iax supernovae are not nearly as common as type Ia supernovae, he said, but they expect to see more of them in the future.

McCully said the team will be returning to the Hubble Space Telescope in September 2015 to continue studying the supernova.


Melanie Groves

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