Vampires are ancient undead entities that are able to maintain a youthful appearance by feeding off the blood of the living. This is, of course, what folklore and teen fiction novels would have us believe. It appears, however, that the cosmos plays host to its own "blood-sucking" stars that feed off other stars, rejuvenating their appearance. But these vampires are far from fictional; they are known as "blue stragglers" and their true parasitic nature has finally been revealed.
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Before we understand the mystery, we have to first appreciate that astronomers have triumphed in recent decades in the understanding of how stars evolve.
In its most basic form, stellar evolution begins in a dense interstellar cloud of gas that collapses under gravity. Often these clouds have enough hydrogen to seed many stars inside their stellar nurseries. When fusion is sparked in their cores, the stars inside the nursery evolve at similar rates and form a cluster of stars. Over time, the stars age and deplete their supply of hydrogen. Eventually, these stars die as supernovae (if they are massive enough) or, if they are of a similar mass as our sun, swell into red giants, eventually shedding the vast majority of their plasma, leaving a "dead" stellar husk of a white dwarf behind.
Astronomers are able to "see" a Main Sequence star's maturity by measuring its luminosity against its color and, according to the famous Hertzsprung Russell Diagram, they are able to learn something about its mass, temperature and age. Generally speaking, young stars are bright, blue and hot whereas older stars along the Main Sequence are dim, red and cool.
Stellar Vampires This evolutionary model has been complicated in observations of some clusters that are known to be pretty old, but seem to contain oddly young stars. These stars shouldn't be there - they appear blue in an otherwise red population. There are no clouds of gas hanging around that could spawn these blue stragglers but there they are, often outshining their cooler siblings.
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It has been suspected that blue stragglers aren't young per se, rather they have been rejuvenated; basically they are older stars that have found a way to interrupt their natural evolution, and turn back the cosmic clock. But how?
The majority of stars in our galaxy aren't loners like our sun - around half of the stars in our galaxy have formed close gravitational bonds since birth, creating binary star systems where two stars evolve in a gravitational dance around one another. Half of these stellar binaries orbit so close that material may pass from one star to its partner; the gravitational heft of a larger star steals superheated plasma from its weaker sibling. Sometimes the orbits of binaries will shrink, causing two stars to merge as one.
In the merger scenario, it's not hard to imagine that colliding two stars together to form a new, more massive star could generate a bright and apparently young blue straggler. But in new research, astronomers have focused on one blue straggler's emissions to reveal that it is, in fact, a stellar vampire.
A Parasitic Past Using observations by the Hubble Space Telescope, researchers led by Robert Mathieu, of the University of Wisconsin-Madison, found emissions from a blue straggler contained sinister information of its parasitic past. The star in question is located around 5,500 light-years away and, at first, appears to be a single star. But on closer inspection of its emission spectrum, it was found that the star is in fact one of a binary pair of stars. Its stellar partner is a small, dense, ancient white dwarf.
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From our understanding of closely-orbiting binary stars, it's apparent that the blue straggler was the more massive of the two and started "sucking" its sibling's plasma away, bulking up its own mass and fueling its rejuvenation into a hotter, younger-looking star. Eventually, all that was left of its stellar "victim" was a tiny white dwarf that remains in orbit to this day, like the petrified remains of a stellar skeleton.
The physics behind this binary star interplay and the presence of blue stragglers are key components of our understanding of stellar evolution for an estimated 25 percent of stars in our galaxy, so it's good to know that some of the stars out there aren't quite as young as they look.
Source: University of Wisconsin-Madison press release.