The Solar System takes 210 million years to complete one orbit around the center of the Milky Way. The last time we were on this same side of the Galaxy, early dinosaurs were roaming the Earth. That number, 243.72 km/s, has now been refined thanks to rare stars that streak through the Galaxy like cannonballs.
High-velocity stars are probes that sense the Galaxy's gravitational field. Their trajectories extend tens of kiloparsecs, and they feel the slightest irregularities in the potential, which are inaccessible to the slow stars of the disk. In this new study, three samples of such objects are combined: 1197 stars from the LAMOST DR7, Gaia DR2, and DR3 programs. For each star, the space velocities U, V, W in Galactic coordinates are computed, and then a velocity ellipsoid is constructed: a three-dimensional distribution of velocities showing how fast stars move in different directions.
The Oort constants are two numbers that describe how the Galaxy rotates non-uniformly: the inner regions overtake the outer ones, and this shear can be measured from the radial velocities of stars. For the three samples, noticeably different values are obtained. Program I (591 stars, distances 0.10–15.40 kpc) gives A = 15.427 ± 0.25 km/s/kpc and B = -21.095 ± 0.22 km/s/kpc. Program II (87 stars, up to 108.64 kpc) yields anomalously low A = 3.883 ± 0.51 and B = -11.029 ± 0.30. Program III (519 stars, 0.29–16.44 kpc) gives A = 16.510 ± 0.25 and B = -21.227 ± 0.22. The average values over all three programs are A = 11.94 ± 0.29, B = -17.78 ± 0.24, and the angular speed |A-B| = 25.07 ± 5.01 km/s/kpc.
Program II, which includes the most distant stars, stands out from the overall picture. These stars are likely ejected from the Galactic center, and their large distances introduce systematic errors in parallaxes, the angular shifts used to determine distances. Like a translator who is distracted by every rustle in the hall and turns a one-hour meeting into two, inaccurate parallaxes distort the result. But Programs I and III, despite the different origins of the stars, give consistent values of A and B, close to the literature. This means that high-velocity stars can be used to study the local dynamics of the Galaxy, provided that objects with reliable parallaxes are carefully selected.
Looking ahead, new data from Gaia DR4 and surveys like WEAVE and 4MOST will allow us to expand the sample and test how the origin of stars affects the derived parameters. These probes are already sensing the dark halo, the invisible mass that keeps the Galaxy from flying apart. Each fast star is a small experiment in gravity set up by nature itself. And the more such probes we include in our analysis, the more accurate the map of the Milky Way's gravitational field will become.
The Galactic rotation speed at the Sun's distance determined in this work (243.72 km/s) means that the Solar System completes a full orbit around the Galactic center in about 210 million years. The last time we were on this same side of the Galaxy, early dinosaurs were roaming the Earth.