The Ursa Major Moving Cluster — A New Way To See The Big Dipper

The Big Dipper rides high in the northern sky in spring. Stellarium

The best way to look at the stars is to lie flat on your back. If you do that in April and May you’ll stare straight up at the Big Dipper. Even if you live in a light-polluted location, the Dipper is relatively easy to see this time of year because it’s high in the sky. The familiar ladle-shaped arrangement of seven bright stars is probably the most familiar asterism in the night sky after Orion’s Belt. An asterism is the brightest, easiest part of a constellation to see. With the Dipper, it’s part of a larger figure called Ursa Major the great bear.

Most constellations figures are made of unrelated stars connected into patterns that tell a story. The Dipper is an exception. Instead of a chance alignment of suns, it’s the core of the nearest star cluster, named the Ursa Major Moving Cluster. That rank usually goes to the Hyades star cluster in Taurus, the one shaped like the letter “V” and containing  the bright orange star, Aldebaran. It’s located about a fist to the left of the familiar Pleiades cluster. Several hundred stars comprise the Hyades, and they form a gravitationally-bound swarm about 151 light years from Earth.

From this diagram you can see that by distance and direction of movement (green arrows), Alkaid and Dubhe are chance background stars and not cluster members. Stellarium with additions by the author

The Ursa Major Moving Cluster (UMMC) is a much looser, sparser group of only 13 or 14 stars about 78 light years distant or half again as close as the Hyades. It’s barely a cluster, the reason it’s often described as “group,” but all the stars are related as they are in more familiar clusters. They were born together as siblings in a great cloud of gas and dust roughly 500 million years ago and now occupy a volume of space some 30 light years long by 18 light years wide.

Five of the Big Dipper’s stars form the group’s core. The other two — Alkaid, located at the end of the handle, and Dubhe, the topmost star in the bowl — are unrelated. They’re fakers that look convincing but only lie along the same line of sight. While the true cluster members are all around 80 light years away and and move as a group to the southeast, both Alkaid and Dubhe move in the opposite direction and are more than 100 light years from Earth. Several fainter stars in Ursa Major and one star in the neighboring constellation, Canes Venatici the Hunting Dogs, comprise the cluster. There are also 42 additional stars broadly scattered across the sky from Cetus the Sea Monster to Lupus the Wolf that belong to a larger “stream” of stars related to and possibly members of the cluster.

Use this map to find the other members of the Ursa Major Moving Cluster. All the named stars except Dubhe and Alkaid are group members. The white lines connect bright cluster members; the wider blue ones connect to non-members. Star brightnesses, called magnitudes, are given in parentheses. The faintest member is HD 109647 at magnitude 8.5. A pair of 50mm binoculars will show them all under a reasonably dark sky.  Stellarium with additions by the author

How do we know the stars of the UMMC are related? In 1869, English astronomer Richard Proctor discovered they were all moving together through space, drifting slowly in the direction of the constellation Sagittarius at the rate of on full moon diameter every 16,000 years. Astronomers identify related stars by finding which ones share the same motion and have the same age.

Our solar system is located on the outskirts of the cluster but is much older and unconnected. As we go about our lives, this sparse collection of suns is headed in our direction moving at 7 miles a second (11.5 km/sec). In about a million years, the UMMC will pass us at a distance of about 51 light years.

The Hyades star cluster in Taurus. The bright star Aldebaran is not a member but a chance alignment. Bob King

Star clusters like the UMMC and Hyades are called open clusters because they’re only loosely bound by the mutual gravity of their members. Sometimes a star’s individual motion causes it to break away and leave its siblings behind. Star clusters are also subject to bigger forces intent on ripping them apart. As a cluster revolves about the galactic center, massive clouds of dust and gas as well as tides produced by the shear mass of stars in the Milky Way take it apart star by star. A typical open cluster survives for only a few hundred million years, a short time compared to the sun’s expected 10-billion-year-lifetime.

The five bright Dipper stars along with 78 Ursa Majoris and 37 Ursa Majoris are visible with the naked eye. For the others you’ll need a pair of binoculars. Lie back, relax and get to know a different side of the Big Dipper.

5 Responses

  1. Interesting post, Bob! Thank you! With my light polluted skies the Big Dipper’s about the only thing I am guaranteed to see. On many nights it can be hard to see Megrez here in the Philly suburbs.

  2. kevan hubbard

    Indeed a great article about one of the biggest but most overlooked open clusters.id forgotten which ones where not members now I know alkaid and dubhe.i think that the stars,or some of them, around Orion’s belt are a cluster too?I expect if you, assuming life exists other than on Earth?,where to look at the Ursa Major cluster from a star about 400 light years away,say canopus,it would look a bit like we see m39 a poor open cluster.

    1. astrobob

      Hi Kevan,
      The Orion Belt stars aren’t related in a cluster sense, but it’s possible other stars in the belt area belong to a stream.

      1. kevan hubbard

        Bob you’re quite right about Orion’s belt and I’ll be jiggered those belt stars are an incredible distance away between 1200 and 2000 light years…. Deneb distances! strange ,although they are unrelated,that all three of the main belt stars are all at such vast distances?now what was going on on Earth 1200 years ago

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