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For B or not B
Until now, almost all known hypervelocity stars are B – type suns in the main sequence, the period of their lives where they produce energy by melting the ice. hydrogen in helium in their nucleus. Because these stars do not live more than several hundred million years, you would not expect to find them on the sidelines of the galaxy.
"These B stars should not exist there," says Brown. "There is no star formation in the outer halo of the Milky Way. It's a dead region – the halo of the galaxy contains the globular clusters of the galaxy and old, low-metal, low-mass stars. Unless you're ejected there, you would never expect to see a star B traveling at these speeds in the outside halo. "
But here they are. The best explanation for their existence is a binary star that travels too close to a huge black hole, says Hagai Perets, an astrophysicist at Technion – Israel Institute of Technology in Haifa. The black hole captures a star in a very eccentric orbit and ejects the other star in hypervelocity.
A decade after Brown and his colleagues discovered the first star of this type, there was still not enough data on authentic hypervelocity stars all over the sky. Astronomers can measure the radial velocity of a star by examining its spectrum – the light distributed in the wavelengths that compose it. If an object moves towards us, its spectral lines shift to shorter wavelengths; if it moves away, the lines oscillate at longer wavelengths. The higher the speed, the greater the shift.
Although this type of spectral analysis is simple for nearby stars, it becomes much more difficult for distant suns in the outer halo of the Milky Way. Even large telescopes can not capture enough light. That's why astronomer Ulrich Heber of the University of Erlangen-Nuremberg, Germany, thinks there are probably several low-mass hypervelocity stars waiting to be discovered. Although these tiny objects live longer than their B-type cousins, they emit much less light and therefore can not be seen so far. Nevertheless, they would be easier to detect than the even weaker remains of white dwarf of any dead star of type B.
Los Alamos theorist Jack Hills predicted super-speed stars in 1988, but astronomers did not find any until 2005.
Moving
Once astronomers are aware of the radial velocity of a star seen from Earth, they can calculate its velocity relative to the center of the galaxy. But even that tells only half of the story. To directly connect a stray star in the outer fringes of the galaxy to its theoretical point of origin at the supermassive black hole in the heart of the Milky Way, observers must also determine the movement of the star in our field of vision. This so-called correct movement is even harder to measure accurately than the radial velocity.
Astronomers determine the correct movement by observing the shift in the position of an object relative to more distant objects. For a hypervelocity star, it means measuring its movement against galaxies or background quasars, a process that takes years.
Despite their vertiginous speeds, the hypervelocity stars have own motions less than 1 milliarcsecond per year. (A milliarcsecond is equal to 0.000000005 °, or the angular size of a penny seen from around 2,300 km [3,700 kilometers] a way.)
Since ground surveys have a precision of only about 5 milliarcseconds a year, studies of the correct movement of hyper-speed stars must be made from space. This is where the Gaia mission of the European Space Agency (ESA) comes into play. This astrometric observatory, designed to measure precise positions and radial velocities of about one billion stars, produces accurate movements with an accuracy of 0.1 milliarcsecond per year. Over the next two or three years, Gaia should offer great moves to stars and new known hypervelocity candidates.
In theory, these observations will help astronomers to determine much more about the points of origin of these stars. Although researchers believe that most originate from interactions with Sagittarius A *, they still wonder if some might be intruders outside the galaxy. Perhaps they found themselves in the outer halo of the Milky Way in a stream of stars of a dwarf galaxy disturbed by the tides. Or maybe the satellite galaxy of the Milky Way, the Great Magellanic Cloud (LMC), ejected some of it into the halo of our galaxy.
"There is an unbound star B, HE 0437-5439, located near the great magellanic cloud, which could come from the LMC or the Milky Way," says Brown. "HE 0437-5439 is moving away from us and we do not know if it is oriented on our side or towards LMC." If this star originated in CML, he might be acting of a smoking gun for an intermediate mass black not yet detected. hole that ejected the star at hypervelocity.
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