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
metal, 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
[111170].
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.
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