Pinning down the Milky Way's spin

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Artist's impression of the local neighbourhood of the Sun and its setting within our galaxy, the Milky Way (see insert above). The figure shows the positions of some bright stars (in white) in the sky as well as the eight Cepheids used in the investigation (in blue). After the rotation of the Milky Way had been accounted for (red arrow), it seemed that the Cepheids were all 'falling' towards the Sun (blue arrows; these are not to scale: in reality the blue velocities are typically a factor one hundred smaller than the velocity around the Milky Way). New, very precise measurements with the HARPS instrument have shown that this apparent 'fall' is due to effects within the Cepheids themselves and is not related to the way the Milky Way rotates. The motion indicated by the blue arrows is thus an illusion. The scale of the image is given in light-years (ly).



Pulsating stars enable a new precise determination of the rotation of our Galaxy
New, very precise measurements have shown that the rotation of the Milky Way is simpler than previously thought. A remarkable result from the most successful ESO instrument HARPS, shows that a much debated, apparent 'fall' of neighbourhood Cepheid stars towards our Sun stems from an intrinsic property of the Cepheids themselves.

The result, obtained by a group of astrophysicists led by Nicolas Nardetto, will soon appear in the journal Astronomy & Astrophysics.

Since Henrietta Leavitt's discovery of their unique properties in 1912, the class of bright, pulsating stars known as Cepheids has been used as a distance indicator. Combined with velocity measurements, the properties of Cepheids are also an extremely valuable tool in investigations of how our galaxy, the Milky Way, rotates.

"The motion of Milky Way Cepheids is confusing and has led to disagreement among researchers," says Nardetto. "If the rotation of the Galaxy is taken into account, the Cepheids appear to 'fall' towards the Sun with a mean velocity of about 2 km/s."
http://www.eso.org/public/outreach/press-rel/pr-2008/pr-30-08.html