The Big Dipper and Ursa Major
When you look up into the night sky, an immense amount of scientific wonder waits to greet you. The colossal nebulae of gas and dust, alight with the fires of star formation; the swirling mass of planet-sized atmospheric storms on gas-giants both near and far; or even the beautiful but fearsome steps of stellar life-cycles, producing nuclear-powered gaseous orbs capable of warming an entire planet in life, and vaporizing an entire solar system in death.
There is so much there to captivate, and much of it only a personal telescope away from observing. And yet, with all we know about the world beyond those small points of light in the void, it is hard to escape natural inclinations when studying space. After all, to draw lines between points, or imagine images and stories in the place of these vacant expanses, is perhaps one of the oldest human endeavors. That being said, of all those ancient pictures in Western culture, at least—the Big Dipper and Ursa Major are probably among the most recognizable.
Difference between Big Dipper and Ursa Major
It is fairly common for people to use the terms Ursa Major and Big Dipper interchangeably, but the names refer to two different types of star groupings. Ursa Major is a constellation, meaning it is a recognized group of stars in the sky that form a pattern—typically in the shape of an object, person, or animal of cultural or mythological importance. That said, while many constellations have existed throughout history, the International Astronomical Union recognizes eighty-eight constellations that are used to denote regions in the night sky. Ursa Major is the third largest.
In contrast, the Big Dipper is not a constellation, but rather an asterism. Asterisms are informal groupings of stars, typically more open to interpretation and without firm boundaries. While constellations are officially recognized and highly-defined, asterisms are more open to interpretation and often bear less historical precedent. The definition of an asterism can vary, but it is important to note that their patterns of stars are not recognized among the eighty-eight constellations.
All that being said, The Big Dipper itself is actually a component of Ursa Major—the tail and flank.
By Till Credner - Own work: AlltheSky.com
A Shared and Separate History
The exact origins of constellations as a concept are unknown, but the practice of recognizing patterns in the sky and ascribing stories to them exists across all of human civilization. From the people of the Americas, to Europe and Africa, and all the way to East Asia, people have been finding these patterns and giving life to them in stories.
Ursa Major’s beginnings are similarly unknown, but the appearance of the Great Bear in dozens of cultures across the world gives rise to some fascinating theories on the migration patterns of early Indo-European peoples. Prehistoric questions aside, the first known reference to Ursa Major is from Ptolemy's 2nd century work Almagest, an ancient treatise on astronomy that listed forty-eight known constellations. However, this list was based on previous, lost work—so we may never know how long ago Ursa Major was first connected, but it has been with us ever since.
The Big Dipper has a rich history as well. Given its relative brightness compared to the rest of the Ursa Major constellation, many cultures have recognized it as a distinct star pattern—but not always as a ‘dipper’. In Ireland and the United Kingdom, it has often been identified as a plow—while in German, Slavic, and even Hungarian traditions, it has been associated with a large wagon.
Though Ursa Major’s identity as a bear is relatively uncontested, the Big Dipper’s status as an asterism leaves it much more open to interpretation.
Composition of Stars in the Big Dipper
Ursa Major and the Big Dipper contain a number of notable stars and celestial objects. Most importantly are the stars Merak and Dubhe, comprising the far wall of the dipper opposite the handle. Known as the pointer stars, a navigator can draw a line between them and then follow that line upwards by a distance roughly five times that of the space between the pointers. This should lead to Polaris, the brightest star in the constellation Ursa Minor and a staple of any traveler looking to head north.
An amazing aspect of constellations is how diverse they are. While the stars of a constellation appear close to us, they actually exist light-years apart, and at vastly different distances as well. This provides for plenty of variety in the stars, objects, and luminosity we see. For example, while Dubhe may appear as a single point of light, it is actually a binary star-system. Similarly, Marek might appear as a regular main-sequence star but has actually begun its own life-cycle transition—having burned most of its helium in nuclear fusion, it is now entering a phase of slowly cooling off.
The Moving Group of Ursa Major and the Big Dipper
Another surprising aspect of Ursa Major and the Big Dipper is the existence of the Ursa Major Moving Group. While it is incredibly uncommon for stars in a constellation to be related, this group of stars is an incredibly rare instance where the majority of stars in an asterism share a common origin. These stars were formed roughly three-hundred-million years ago, and share a similar velocity. It is believed these stars formed in the same nebula, before gravitational forces dispersed them into the wider galaxy.
Still, some have managed to stick together, as Marek is a member of this moving group.
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Where to Find Ursa Major and the Big Dipper in the Night Sky
The great thing about the Big Dipper and Ursa Major is how easy they are to find. Though barely visible in the southern-hemisphere, in the northern-hemisphere they are easy to recognize year-round above the horizon—though, most noticeably, in the Spring. And, while exact astronomical coordinates exist, the best way to find Ursa Major is simply to go outside on a dark night and look up towards the bright asterism that is the Big Dipper. With a clear night sky, neither will take long to find.
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