WHAT IS A VAMPIRE STAR?

Our Sun is a loner, and that makes it unusual in the universe. A significant majority of the massive stars that populate the cosmos—upwards of 70%—have a close relationship with at least one smaller star, cohabitating in a binary or multiple star system. Some of these relationships are not reciprocal; instead, one partner gives more of itself than the other, to the point that it gets sucked dry. A vampire star starts its life smaller than its companion but inevitably becomes the larger one as it siphons mass and energy from the other. After sucking enough hydrogen, the vampire gets a new lease on life.

In the case of certain white dwarfs, which are technically already the dead cores of giant stars, this results in a kind of grace period. Thanks to the life essence that it has stolen from its companion star, the vampire can revive its fusion process and shine once more. But it pays a high price: When the vampire grows sufficiently large it collapses, generating enormous amounts of light and energy in a supernova — the biggest explosion ever seen in space. These events can be so bright that they outshine their entire galaxy for a few days or months and can be seen across the universe.

a completely new component of the center of our galaxy with NUSTAR’S images,” says MIT particle physicist Kerstin Perez. “However we cannot definitively explain the X-ray signal— it’s a mystery.” Everything that swirls around the center of the Milky Way— stars of all ages, as well as smaller black holes—orbits Sagittarius A*, but NUSTAR was the first telescope to successfully capture crisp images of the resulting high-energy X-rays. There are several theories to explain their origin. One of them involves the parasitic relationship of vampire and victim stars, which could give rise to an eruption of X-rays as part of the “feeding” process. In this scenario, a type of stellar zombie called a pulsar could be at work. Pulsars, which are the collapsed remains of stars after they explode in a supernova blast, can emit intense beams of radiation.

As a pulsar spins, the beam sweeps across the sky like the beacon of a lighthouse and can sometimes be detected from Earth. Another type of stellar corpse that could be involved is a white dwarf. These small stars lack the necessary mass to explode in a supernova but they’re so dense that they produce high-energy X-rays. (Incidentally, our Sun is expected to become a white dwarf some 5 billion years from now.) An alternate theory suggests that these X-rays might not be coming from stellar corpses and instead are cosmic rays emitted by Sagittarius A* as it devours material. The problem: None of the theories that have been advanced thus far is in line with previous research. “This new result just reminds us that the galactic center is a bizarre place,” says Chuck Hailey, an experimental astrophysicist at Columbia University. “Just as people behave differently when walking on the street instead of jammed on a crowded rush hour subway, stellar objects exhibit weird behavior when crammed into close quarters near a supermassive black hole.” But strange results come as no surprise to some astrophysicists: “Every time we build telescopes like NUSTAR to improve our view of the cosmos in a particular wavelength band, we can expect surprises such as this,” says Paul Hertz, who directs the astrophysics division at NASA.

In 2005 astronomers first began finding zombie stars—also known as hypervelocity stars (Hvs)—that had survived supernova explosions. They are called “hypervelocity” because while most of the stars in the Milky Way, including our Sun, move at only about 500,000 miles per hour, the HVS discovered in 2005 was moving three times faster than that. Despite intense searching, only two dozen or so more HVSS had been discovered by 2019— evidence that zombie stars are rare. Astronomers think about 1,000 HVSS may exist in our galaxy, though that’s a small fraction of its 100 billion stars.

“We can see a completely new component of the center of our galaxy with NUSTAR’S images. We can’t definitively explain the X-ray signal yet—it’s a mystery. More work needs to be done.” Kerstin Perez assistant professor of particle physics at MIT

THE VICTIM At first glance the star on the left looks far more massive than the one below—but appearances can be deceptive. In this binary star system, the one that looks bigger is in fact the victim and is losing mass and energy to the other—the vampire star. This interaction may eventually result in a supernova, either destroying both stars at once or in rare cases giving rise to a zombie star. THE VAMPIRE As the remains of star that had once been a giant, this vampire (or parasitic) star is sucking the very life essence from its victim much like a vampire— but in this case the crucial substance is hydrogen rather than blood.