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Caption: Figure 3.
Schematic diagram demonstrating the concept of proper-motion anomaly. A binary star system, comprised of an A component (solid black orbit and black points) and a lower-mass B component (dotted pink orbit and pink points) is shown at four separate times corresponding to approximate start and end dates of Hipparcos (left pair) and Gaia DR2 (right pair). The center of mass (gray squares) moves from left to right over time, and the true proper motion of the system over the Hipparcos and Gaia timeframes is represented by the two gray arrows. Assuming that the A component dominates the light of the system, neither Hipparcos nor Gaia will measure this true motion because the center of light will move with component A as the stars orbit their barycenter. The black arrow at left thus shows the proper motion that would be measured by Hipparcos, and the black arrow at right shows the motion measured by Gaia DR2. The disagreement between these two independent measurements is termed proper-motion anomaly and provides evidence that the system has an unseen component. (For simplicity, we have removed parallactic motion by showing only those points at the same parallax factor, as depicted by the time stamps at the bottom of the figure.)
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