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Folk tales tell us that a person bitten by a vampire becomes a vampire himself, but researchers affiliated with the European Southern Observatory have found that's not how it works in space. In a cluster of stars called 47 Tucanae, they have found that a "vampire" star can suck matter away from a bigger star, in a process called binary mass transfer, to become a newer, hotter star.

Seeing stars as vampires is a new idea of star birth, giving us insight on the life cycles of stars and another clue as to how the universe is evolving. Researchers are also looking at the related question of how stars can collide and fuse into a new star.

The new stars resulting from collisions and “stellar vampirism” are called blue stragglers. For years, scientists weren't sure where blue stragglers came from—these young, blue stars popped up in areas populated only by older stars, noteworthy because they appeared to be lagging behind the development of their peers. Stars are traditionally formed in nebulas, clouds of matter that act as incubators for young stars, and so stars are usually grouped with others of the same age.

But blue stragglers were a mystery. "It's like taking a look in a retirement community and seeing some middle-aged people there," said Robert Rood, an astronomer and professor at the University of Virginia. "It just makes you wonder why they're there."

A team led by Franceso Ferraro, and including Rood, found that binary mass transfer can result in blue stragglers. Two stars can live side by side in a binary system, meaning the pair is bound together by gravity. If one of the stars has more mass, it will age more quickly, growing even bigger as the force holding it together is expanded beyond its means. The gravitational field of the second star then starts sucking material off the outside of the bigger star, and adds the matter to its own surface. Eventually, all of the matter from the more massive star has been added to the other. Once the two stars have been completely sucked together into a new star, it’s called a blue straggler.

Blue stragglers have surfaces that are low in oxygen and carbon. Usually, low levels of oxygen and carbon are found in the middle of older stars, not on the surface of young stars. Rood explained it like this: Imagine pouring a bucket of water into another bucket. The water at the bottom of the first bucket is what gets poured lastly into the second bucket. What was in the center of the larger, donor star now coats the exterior of the new, merged star.

Vampirism isn't the only way to give birth to a blue straggler. They can also occur when two small stars get caught up in a binary system and collide as the orbit between the stars breaks down. "It's not really a collision in the 'splat' sense," Rood said. "They just merge together."

When Ferraro and his team examined the 43 blue stragglers in the Tucanae cluster, they found 37 of the stragglers may have been caused by collisions. The other six had the low oxygen and carbon expected of binary mass transfer. Regardless of how the blue stragglers were formed, it’s clear to Ferraro that these stars, like humans, are affected by whether they evolve with companions or in isolation. “Stars that we are now observing as [blue straggler stars] would never reach this stage without the effects of the companion star,” Ferraro said in an e-mail. “Life can be quite different with a companion.”

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