THE NEW STAR THAT WASN’T ONE
When Danish astronomer Tycho Brahe spotted a new, very bright star in the sky in November 1572, it changed the course of astronomy. Since the era of the ancient Greeks, science had been certain that the world beyond the Moon’s orbit could never change. Soon the new star was as bright as Venus. What Brahe could not know at the time was that he was not witnessing the birth of a new star but rather the death of an old one: An enormous explosion called a supernova had occurred 11,000 years earlier. Thus he was equally surprised when it faded until it was no longer visible to the naked eye less than a year and a half later.
DO STARS HAVE A LIFE AFTER DEATH?
When a human being dies, the organs begin to decompose within 72 hours of death, and the body starts to emit a pungent odor. This is the result of a biological process called autolysis, the destruction of a cell through the action of its own enzymes—basically, the cells begin to digest themselves.
In a temperate climate it may take years for a body to decompose to a skeleton. In acidic soil it could take another 20 years for the skeleton to completely decompose, but in neutral soil or sand the bones might endure for hundreds of years before finally disintegrating. The process of death is very different for stars: When and how they die depends on their mass.
The largest stars—hypergiants—can be as much as 2,000 times wider than our Sun and exhaust their fuel supply in only a few million years. The Sun, on the other hand, has used up only about half of its hydrogen reserves during its 4.6 billion years of existence and thus has around 5 billion years of life left. Stars that are smaller than half the size of the Sun are cooler,
burn their fuel more slowly, generate less light, and live for trillions of years. But regardless of their size, all stars are constantly resisting the crushing forces of their own gravity, which they do by burning their fuel and releasing enough energy to prevent implosion. When they expend all of their energy, they collapse under their own weight.
Extremely large stars with more than 12 times the mass of our Sun die with the biggest bang in all the universe— the supernova—and turn into a stellar black hole. Medium-size stars (8 to 12 solar masses), on the other hand, may also explode in a supernova but will then condense into a neutron star with a diameter of just 12 miles.
Smaller stars (under 8 solar masses) get to die a relatively peaceful death: After burning out, they implode and shrink to a diameter of no more than 20,000 miles. (For comparison, the diameter of our Sun is 865,000 miles.) This shrinking star now ekes out the rest of its existence as a barely bright white dwarf—at least that had been the belief up until now. But German astrophysicists recently proved that there are uncanny exceptions…
Seven years after the discovery of the mysterious X-rays by the NUSTAR telescope, it seemed as if the search for an “undead” star would remain unsuccessful. Then researchers at the University of Erlangen-nuremberg led by the astronomer Roberto Raddi came across an actual zombie. It was proof stars can actually follow a very different, very dark path after death.
The majority of stars exist in binary or multiple systems with other stars, and their relationship is characterized by intense attraction. Some rare white dwarfs are supermassive; due to their tremendous gravitational force, they siphon off gases from a neighboring star that accidentally gets too close, effectively becoming vampires. The absorption of extra matter throws the delicate mass-gravity balance of the white dwarf off kilter, causing it to first implode and then explode in a kind of mini supernova. White dwarfs that are not entirely destroyed in the explosion haunt the galaxy as zombie stars that move at stupendous speed.
The newly discovered zombie stars have thus far stayed well hidden. One reason may be that their great speed allows them to escape the irresistible gravity of Sagittarius A* and leave the Milky Way. So far Raddi and his team have identified three of these livingdead hypervelocity fugitives. But how many zombies are skulking through the cosmos—and what if one of them comes for us? Mercifully the risk is so low that there’s no need to worry—yet.
“This new result just reminds us that the galactic center is a bizarre place.” Chuck Hailey professor of physics at Columbia University