Determining Our Motion Through the Galaxy
http://aasnova.org/2016/12/14/determining-our-motion-through-the-galaxy/
We know that the Sun zips rapidly around the center of the Milky Way — our orbital speed is somewhere around 250 km/s, or ~560,000 mph!
Getting a precise measurement of this velocity is useful because we can combine it with the observed proper motion of Sgr A*, the black
hole at the center of our galaxy, to determine the distance from us to the center of the Milky Way. This is an important baseline for
lots of other measurements.
But how can we measure the Sun’s revolution speed accurately? A team of scientists led by Jason Hunt (Dunlap Institute at University
of Toronto, Canada) have suggested a unique approach to pin down this value: look for missing stars in the solar neighborhood.
The stars around us should exhibit a distribution of velocities describing their orbits about the galactic center — but those stars with
zero angular momentum should have plunged directly into the galactic center long ago. These stars would have been scattered onto chaotic
halo orbits after their plunge, resulting in a dearth of stars with zero angular momentum around us today.
By looking at the relative speeds of the stars moving around us, then, we should see a dip in the velocity distribution corresponding to
the missing zero-angular-momentum stars. By noting the relative velocity at which that dip occurs, we cleverly reveal the negative of our
motion around the galactic center.
Where Are We and How Fast Are We Going?
Hunt and collaborators use a combination of the first data release from ESA’s Gaia mission and a star catalog from the Radial Velocity
Experiment to examine the motions of a total of over 200,000 stars in the solar neighborhood. They find that there is indeed a lack of disk
stars with velocities close to zero angular momentum. They then compare modeled stellar orbits to the data to estimate the relative speed
at which the dip in the velocity distribution occurs.
From this information, the authors obtain a measurement of 239±9 km/s for the Sun’s revolution velocity around the galactic center. They
combine this value with a proper motion measurement of Sgr A* to calculate the distance to the galactic center: 7.9±0.3 kpc (or about
26,000 light-years). Both of these measurements can be improved with future Gaia data releases, which will contain many orders of magnitude
more stars. This clever technique, therefore, proves a useful way of better constraining our position and motion through the Milky Way.
