The galaxy is warped, but there is still no consensus as to what produces the warping. It is now over 40 years since radio observations of neutral hydrogen revealed the gaseous disk of our galaxy to be warped. Subsequently the warp has been detected in the distribution of galactic dust, in molecular dust, and luminous stars. Roughly half of all spiral galaxies have similarly warped disks, which suggests that warps a re a common and long-lived phenomenon. Data from the Hipparcos satellite is used to determine small stellar motions in the plane of the sky that are undetectable in the gas.

Nature volume 392/ 2^nd April 1998, p471