Hyades hī´ədēz [key], in astronomy, open star cluster in the constellation Taurus, located immediately to the right of the bright star Aldebaran. The cluster is about 130 light-years from the earth. It consists of about 100 stars all moving in the same direction. Its shape is that of an oblate spheroid, with most of the stars lying within 20 light-years of the center. Most of the stars in this cluster are of spectral class G and K and are average in size, with temperatures that are comparable to that of the sun. The nebular variable star T Tauri is part of this cluster. The Hyades is important as a fundamental calibration point in the distance scale of the universe. Because it is one of the closest of the open clusters, measurements of parallaxes of its stars as well as the relationship between the apparent and absolute magnitudes of some of its member stars yield similar distance results. In 1908 L. Boss discovered that the proper motions of all the Hyades stars were converging as an effect of perspective, and that their motions were really parallel. Once the convergent point for the cluster was established, the distance could be calculated. With the distance to this cluster known accurately, absolute magnitudes for its members could be determined. Thus the Hyades stars are used as calibration for similar stars in far more distant clusters.
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