This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity. The outer atmosphere of Sirius B is now almost pure hydrogen—the element with the lowest mass—and no other elements are seen in its spectrum. Since 1894, irregularities have been observed in the orbits of Sirius A and B with an apparent periodicity of 6–6.4 years. A 1995 study concluded that such a companion likely exists, with a mass of roughly 0.05 solar masses- a small red dwarf or large brown dwarf, with an apparent magnitude of >15, and less than 3 arcseconds from Sirius A. More recent (and accurate) astrometric observations by the Hubble Space Telescope ruled out the existence of such an object entirely. The 1995 study predicted an astrometric movement of roughly 90 mas (0.09 arcseconds), but Hubble was unable to detect any location anomaly to an accuracy of 5 mas (0.005 arcsec). This ruled out any objects orbiting Sirius A with more than 0.033 solar masses orbiting in 0.5 years, and 0.014 in 2 years. The study was also able to rule out any companions to Sirius B with more than 0.024 solar masses orbiting in 0.5 years, and 0.0095 orbiting in 1.8 years. Effectively, there are almost certainly no additional bodies in the Sirius system larger than a small brown dwarf or large exoplanet. In 1909, Ejnar Hertzsprung was the first to suggest that Sirius was a member of the Ursa Major Moving Group, based on his observations of the system's movements across the sky. The Ursa Major Group is a set of 220 stars that share a common motion through space. It was once a member of an open cluster, but has since become gravitationally unbound from the cluster. Analyses in 2003 and 2005 found Sirius's membership in the group to be questionable: the Ursa Major Group has an estimated age of 500 ± 100 million years, whereas Sirius, with metallicity similar to the Sun's, has an age that is only half this, making it too young to belong to the group. Sirius may instead be a member of the proposed Sirius Supercluster, along with other scattered stars such as Beta Aurigae, Alpha Coronae Borealis, Beta Crateris, Beta Eridani and Beta Serpentis. This would be one of three large clusters located within of the Sun. The other two are the Hyades and the Pleiades, and each of these clusters consists of hundreds of stars.