![]() Spatial scanning was initially developed as a way to increase the signal-to-noise of exoplanet transit observations, but it has also greatly improved the prospects of astrometry - precisely determining the separations between astronomical objects. Brown and coauthors used a technique called spatial scanning to greatly broaden the reach of the parallax method. Thomas Brown (Space Telescope Science Institute) and collaborators used the Hubble Space Telescope to determine the distance to NGC 6397, one of the nearest metal-poor globular clusters and anchor for one stellar population model. The blue ellipse represents the parallax motion of a star in the cluster, exaggerated by a factor of ten thousand. Top: An image of NGC 6397 overlaid with the area scanned by Hubble (dashed green) and the footprint of the camera (solid green). But stellar evolution models that anchor on the more-distant, ancient, metal-poor globular clusters have been hampered by the less-precise indirect methods used to measure distance to these faraway clusters - until now. These precise properties can be readily determined for young, nearby open clusters using parallax measurements. Stellar evolution models are often anchored to a reference star cluster, the properties of which must be known precisely. Precise distance measurements aren’t only important for setting the scale of the universe, however they can also help us better understand stellar evolution over the course of cosmic history. This technique has come a long way since it was first used in the 1800s to measure the distance to stars a few tens of light-years away with the advent of space observatories like Hipparcos and Gaia, parallax can now be used to map the positions of stars out to thousands of light-years. Distances to nearby stars are often measured using the parallax technique - tracing the tiny apparent motion of a target star against the background of more distant stars as Earth orbits the Sun. Gaia is on track to map the positions and motions of a billion stars. An artist’s impression of the European Space Agency’s Gaia spacecraft.
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