"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.