Milky Way's Fattening Behind Revealed

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Astronomers have long been in the dark about what is happening just beyond the center of the Milky Way. Stars in this region are mostly hidden by dust.

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The only clue has come from measuring neutral hydrogen, an indirect technique that fed computer models showing the Milky Way’s outer disk is flared.

Now, a new study adds meat to the theory with the discovery of five relatively young, pulsating stars, known as Cepheid variables in the region of the suspected flare. Cepheids' regular pulsations and well-measured changes in brightness make them good yardsticks to measure astronomical distances.

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The telltale stars were found 80,000 light-years from Earth beyond the galactic center, far above and below the Milky Way's plane.

“The presence of these relatively young -- less than 130 million years old -- stars so far from the galactic plane is puzzling, unless they're in the flared outer disk,” astronomer Michael Feast, with the University of Cape Town wrote in a paper published in this week’s Nature.

“We found the Cepheids at exactly the distance predicted for this increase in disk thickness,” Feast wrote.

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Scientists don’t know exactly why the galaxy’s outer disk is fattening.

The thickening may be because there is less mass there to gravitationally corral the gas and stars into a flatter shape, such as what exists near the sun, astronomer Patricia Whitelock, with the South African Astronomical Observatory, wrote in an email to Discovery News.

“It is complex because one needs to take into account both the mass we observe -- such as the stars and gas -- and the dark matter which we only detect via its influence on the stars. Much more theoretical and observational work would be required to confirm this, or look for alternatives,” Whitelock said.

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Scientists initially thought the Cephids, which were among a long list of potential variable stars in the direction of the galactic center, belonged to a neighbor galaxy, the Sagittarius Dwarf, which is in the process of merging with the Milky Way. Observations proved otherwise.

"We had to had to look for an alternative explanation,” Whitelock said.

More evidence may come from Europe’s Gaia spacecraft, which was launched in December to create three-dimensional maps of a billion stars and other objects in the Milky Way.

“It would be very interesting to identify large numbers of any well-understood stars at known distance, with their motion so that we could map out the distribution of mass in the galaxy,” Whitelock said.

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