The stars moving rapidly along the initially thin bar are like water travelling quickly through the fire hose.”Īs the galaxy’s central bar became wider and therefore more stable, it stopped thrashing about, like a fire hose with a wider nozzle. “When the water flow from the hose is high, it will violently throw itself from side to side. “Much of the physics of buckling or bending instability is due to what is known as the ‘firehose instability,’” Wegg explains. The red and blue arrows refer to the bright and faint red clump stars, respectively, observed by Vásquez's team. This sketch shows two such banana-like trajectories, outlining the peanut-shaped bulge. Such weird orbits seem like they ought to be impossible, or at least unstable, but they're actually maintained by the gravitational field associated with the Milky Way's rectangular bulge.Ībout half the stars in the Milky Way's bulge travel along banana-shaped orbits that take them in and out of the galaxy's plane. Gravitational interaction between stars rotating in the disk created the central bulge, initially shaped like a squashed football.īut this bar-shaped bulge wasn’t gravitationally stable - it buckled, sending stars on banana-like orbits that gave the bulge its distinctive peanut shape. What they found was expected from models of our galaxy’s evolution, yet nevertheless completely bizarre: half the stars follow banana-shaped orbits that plunge them in and out of the galactic center as if they were on a giant swing.īoth of these studies suggest a peaceful history for our galaxy, an evolution on its own rather than by colliding and merging with another big galaxy. Comparing images of the galactic center taken 11 years apart from the 2.2-meter telescope at La Silla Observatory in Chile, Vásquez’s team tracked the movements of 454 red clump giant stars. Another recently released study, this one by Sergio Vásquez (Pontifical Catholic University of Chile) and colleagues, took on the challenging task of mapping the motions of bulge stars. How does the peanut-like bulge take shape? Through banana-like orbits, of course. Previous surveys had found peanut-shaped hints before, but this was the first survey to collect data without assuming a model for the bulge’s shape. Their map shows the bulge has a distinctly peanut-like shape. Wegg and Gerhard counted the number of red clump stars in each section of the bulge to map out the whole center in three dimensions. Zooming in on Milky Way's bulge shows this star-filled patch of sky, which is but a tiny part of the VISTA Variables in Via Lactea (VVV) survey.ĮSO / VVV Survey / D. By measuring how bright the stars appear to be, astronomers can calculate how far away they are. These stars should all have the same absolute brightness. Wegg and Gerhard plucked 22 million “red clump” giants from the survey, stars in the twilight of their lives that burn helium rather than hydrogen in their cores. So to make sense of our galaxy’s bustling downtown, Christopher Wegg and Ortwin Gerhard (Max Planck Institute for Extraterrestrial Physics, Germany) looked to VISTA Variables in Via Lactea ( VVV), a near-infrared survey that pierces the dusty veil to image individual, faraway stars along the galaxy’s plane. Those dust bands block much of the visible light from view. The Milky Way is a striking celestial sight as much for its dark bands of dust as for its milky river of starlight. Dust enshrouds much of the central bulge, obscuring observations. ![]() The Milky Way bulge, shown here above the 3.6-meter telescope at La Silla Observatory in Chile, lies 27,000 light-years away.
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