The
Ursa Major Moving Group, also known as
Collinder 285, is a nearby stellar
moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80
light years away. It is rich in bright stars including most of the stars of the
Big Dipper.
Discovery and constituents
All stars in the Ursa Major Moving Group are moving in roughly the same direction at roughly the same speed, contain roughly the same mix of
metals, and, based on stellar theory, appear to be roughly the same age. This evidence suggests to astronomers that the stars in the group share a common origin.
Based on the numbers of its constituent stars, the Ursa Major Moving Group is believed to have once been an
open cluster, having formed from a protostellar
nebula approximately 500 million years ago. Since then, the sparse group has scattered over a region about 30 by 18 light-years, whose center is currently some 80 light-years away, making it the closest
cluster-like object to
Earth.
The Ursa Major Moving Group was discovered in 1869 by
Richard A. Proctor, who noticed that, except for
Dubhe and
Alkaid, the stars of the
Big Dipper asterism all have
proper motions heading towards a common point in
Sagittarius. Thus, the Big Dipper, unlike most constellations or asterisms, is largely composed of related stars.
Some of the brighter stream members include
Alpha Coronae Borealis (α CrB or Alphecca or Gemma),
Beta Aurigae (β Aur),
Delta Aquarii (δ Aqr),
Gamma Leporis (γ Lep) and
Beta Serpentis (β Ser). More bright and moderately bright stars which are currently believed to be members of the group are listed in two sections below:
Core stars and
Stream stars.
Group members
Current criteria for membership in the moving group is based on the stars' motion in space. This motion can be determined from the
proper motions and
parallax (or distance) to the stars and
radial velocities. The
Hipparcos satellite has recently greatly improved both the proper motion and parallax estimates of nearby bright stars, refining the study of this and other moving groups .
Based on their distances (measured with Hipparcos) and
apparent magnitude, the
absolute magnitude can be used to estimate the age of the stars. The stars in the moving group appear to have a common age of about 500 million years.
Core stars
The core of the moving group consists of 14 stars, of which 13 are in the
Ursa Major constellation and the other is in the neighboring constellation of
Canes Venatici.
The following are members of the moving group closest to its center. These stars are all in
Ursa Major except where indicated.
Stream stars
There is also a "stream" of stars which are likely members of the Ursa Major Moving Group, scattered more widely across the sky (from
Cepheus to
Triangulum Australe). Only stars with
Bayer designations or
Flamsteed designations are listed here.
Non-members
The bright, nearby star
Sirius was long believed to be a member of the group, but may not be, according to research in 2003 by
Jeremy King et al. at
Clemson University. This research seems to indicate that it is too young to be a member.
Our
Solar System is in the outskirts of this stream, but is not a member, being about 10 times older. Our
Sun merely drifted in along its 250-million-year galactic orbit, and 40 million years ago was nowhere near the Ursa Major group.
