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.
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.
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.
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.