Transcript: Distance is a fundamental stellar property. Without knowing distance it’s impossible to measure the luminosity or absolute brightness of a star, and so without measuring distance it’s impossible to know the true nature of a star seen in the sky whose flux is measured whether it’s a giant star, a main sequence star, or a dwarf. Parallax is difficult to measure from the ground. Typical image sizes from ground-based observatories are about an arcsecond or fraction of an arcsecond. Image positions can be measured to about a tenth of that, and so that only allows the possibility of measuring parallaxes of a few tenths of an arcsecond which limits us to distances of a few parsecs, the nearest dozen or so stars. From space the image sizes go down by a factor of ten or twenty to 0.1 arcseconds or 0.05 arcseconds. The position accuracies can be measured ten times better than that to a hundredth of an arcsecond or less which opens up a distance range of a hundred parsecs. There are twenty-five thousand stars within a hundred parsecs. In 1989 ESA launched the Hipparcos satellite which used a highly elliptical orbit and several years of observations to directly measure the parallax of a hundred thousand stars. Thus we have the distances of a large stellar population sample within a few hundred lightyears of the Sun.