Short answers, pulled from the story.
The Ursa Major Moving Group, also known as Collinder 285, is the closest stellar moving group to Earth, with its core about 80 light-years away. It is significant because it is the nearest cluster-like object to Earth and contains most of the stars of the Big Dipper, meaning that familiar asterism is largely composed of physically related stars.
Richard A. Proctor discovered the Ursa Major Moving Group in 1869. He noticed that most stars of the Big Dipper share common proper motions directed toward a single point in Sagittarius, identifying them as a physical association rather than a chance alignment.
The Ursa Major Moving Group formed approximately 418 million years ago from a single protostellar nebula. A separate age estimate based on Hipparcos data places the common age of its stars at about 500 million years.
Dubhe (Alpha Ursae Majoris) and Alkaid (Eta Ursae Majoris) are not members of the group. Both stars are somewhat further away and move in very different directions from the rest of the Big Dipper stars.
Sirius was long considered a possible member, but research published in 2003 by Jeremy King et al. at Clemson University indicates it is too young to belong. Its motion in the same general direction as the group appears to be coincidence.
The stream stretches from the constellation Cepheus to Triangulum Australe. Individual stream members range in distance from about 29 light-years (Gamma Leporis A) to 350 light-years (Zeta Crateris in Crater).