Rare Ultra-Blue Stars Among Hubble Andromeda Finds
January 12, 2012

Rare Ultra-Blue Stars Among Hubble Andromeda Finds

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Images taken by the Hubble Space Telescope have revealed both a cluster of rare ultra-blue stars and a "double nucleus" of old, reddish stars orbiting a 100-million-solar-mass black hole at the nearby Andromeda spiral galaxy, NASA officials announced on Wednesday.

The discoveries, detailed by the U.S. space agency in separate press releases, were also presented Wednesday at the American Astronomical Society (AAS) meeting in Austin, Texas. A paper detailing the Hubble team's findings will be published in The Astrophysical Journal in the near future.

NASA says that their astronomers were "surprised" when they discovered the blue stars, because typically these types of stars are young and hot. However, the ones located by Hubble in Andromeda (also known as M31) were said to be older stars that had "prematurely" shed their outer layers, thus "exposing their extremely blue-hot cores."

Furthermore, while these ultra-blue stars had been seen before in Andromeda, they say that the latest pictures from Hubble show them spread over a much larger area than expected. In fact, NASA reports that astronomers used the telescope's Wide Field Camera 3 to locate approximately 8,000 of these stars in "a stellar census made in ultraviolet light, which traces the glow of the hottest stars."

"We were not looking for these stars. They stood out because they were bright in ultraviolet light and very different from the stars we expected to see," the University of Washington's Julianne Dalcanton, leader of the Hubble survey, said in a statement.

The team says that Hubble observed the stars within 2,600 light-years of the core, and after studying them for almost an entire year, Dalcanton and her colleagues determined that the stars were "well past their prime," according to NASA.

They believe that as the stars evolved into red giants, they had "ejected most of their outer layers to expose their blue-hot cores," in contrast to the behavior of most sun-like stars, which lose far less material during the process and thus do not appear as bright in ultraviolet. They also discovered that the frequency of blue stars decreased with an increased distance from the core.

NASA reports that Dalcanton's team plans to try to create simulations that would help them learn under what conditions stars such as these would evolve in a different way than their normal counterparts.

Another new image captured by Hubble and revealed on Wednesday showed the 100-million-solar-mass black hole at the center of M31, which NASA has claims is "the sharpest visible-light image ever made of the nucleus of an external galaxy."

While the American aeronautics administration says that the region closest to the black hole from which light can still escape -- also known as the event horizon -- was too small to be seen in the images, but that it "lies near the middle of a compact cluster of blue stars at the center of the image."

"The compact cluster of blue stars is surrounded by the larger 'double nucleus' of M31," which was first discovered by Hubble two decades ago, NASA said. "The double nucleus is actually an elliptical ring of old reddish stars in orbit around the black hole but more distant than the blue stars."

"When the stars are at the farthest point in their orbit they move slower, like cars on a crowded freeway. This gives the illusion of a second nucleus," they added.

The image of the nuclear region was assembled by Tod R. Lauer of the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, who according to NASA took multiple blue and ultraviolet light exposures of the galaxy's nucleus using Hubble's Advanced Camera for Surveys high-resolution channel.

Lauer moved the telescope slightly before each image in order to "change how the camera sampled the region," they added, and by "combining these pictures, he was able to construct an ultra-sharp view of the galaxy´s core." He presented those images during this week's AAS meeting.


Image Caption: The image at left shows the nearby, majestic Andromeda galaxy. The rectangular box marks the region probed by NASA´s Hubble Space Telescope (a blend of visible and ultraviolet light). The photo (top right) is 7,900 light-years across and reveals the galaxy's crowded central region. The bright area near the center of the image is a grouping of stars nestled around the galaxy's black hole. The blue dots sprinkled throughout the image are ultra-blue stars whose population increases around the crowded hub. The square box shows a close-up view of an area around the core. The detailed image, shown at bottom right, reveals a richer population of blue stars huddled around the core. Dark dust clouds also are visible. The right-hand images, taken with Hubble, are part of a census of stars in M31 called the Panchromatic Hubble Andromeda Treasury survey. (Credit: NASA; ESA; B. Williams and J. Dalcanton, University of Washington, Seattle)


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