How did the "zombie star" survive an explosion? The "undead" core of the star is even brighter now

Credit image: Profimedia Images
Credit image: Profimedia Images

Article by: Andacs Robert Eugen, on 02 July 2022, at 01:39 am Los Angeles time

American astronomers have observed in a relatively close galaxy a star that not only survived an incident that should have usually brought its certain death, namely a stellar explosion, ie a supernova, but became even more bright after that event.

The "zombie star", observed with the Hubble Space Telescope, belongs to the category of "white dwarfs", incredibly dense celestial objects, which have a mass close to that of the Sun, compressed into a celestial body the size of Earth. A white dwarf is the remaining nucleus of a star that ejected much of its matter into space at the end of its life cycle, as is estimated to happen to our Sun in about 5 billion years.

The recently detected white dwarf is gravitationally locked in orbit next to another star, forming a stellar pair called the "binary system", which, due to its gravitational force, has attracted and incorporated into its structure much of the matter previously held by that star. Less fortunate "companion".

This is when the problems arose. By behaving in this way, the white dwarf reached a mass about 1.4 times that of the Sun, which caused thermonuclear reactions in its core, which caused it to explode into a supernova, an event that should have destroyed it.

"We were quite surprised that the star itself was not destroyed, but even survived, and is now brighter than it was before it exploded," said Curtis McCully, an astronomer at the Las Cumbres Observatory in California and coordinator of the study published this month in the Astrophysical Journal.

"During the explosion, radioactive material was produced. It is the one who fuels the radiance of the supernova. However, some of that material remained on the core of the surviving star and acts as a fuel to heat the core," added the American researcher.

That white dwarf is in the spiral galaxy NGC 1309, which is about three-quarters the size of the Milky Way. The white dwarf is located 108 million light-years from Earth.

There are several types of supernovae, categorized according to the size and composition of the original star and the power of its explosion.

The newly discovered star will help researchers better understand "Iax-type" supernovae. In this category, a white dwarf experiences a fugitive nuclear fusion of carbon and oxygen after gaining matter relatively quickly, as happened to the star that "stole" matter from its companion. But the explosion does not destroy the white dwarf, leaving behind an "undead" core.

"We called these objects 'zombie stars' for this very reason. They're dead, but not really dead. At first, many of the scientists' computer simulations of supernovae ended before they blew up the entire white dwarf. It's interesting to see that the new discovery tells us something about the real physics that happens during these supernovae," said Curtis McCully.

Scientists have so far detected about 50 such supernovae, but so far have not been able to identify a surviving white dwarf ("zombie star").

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