The Big Dipper dominates the skies throughout the northern hemisphere summer.
Also known as the Chariot or the Dipper, this constellation, which has stimulated the imagination of all civilizations since ancient times, hides numerous astrophysical curiosities.
Cart, bucket and plow
Who does not recognize the Big Dipper? Its peculiar figure accompanies us on all clear nights because, being relatively close to the Pole Star, it is visible throughout the year from the northern hemisphere. But it looks particularly good on these summer nights, when its main stars draw that unmistakable silhouette high above the horizon.
It is one of the largest constellations in the sky, its seven brightest stars form an asterism so striking that they have awakened the imagination of all cultures throughout all time.
For some peoples it was a large bucket or a large ladle, for others a cart, a plow for others.
Her description as a great bear is Indo-European, appearing in both classical Greece and Vedic India.
But it seems to have parallels in the indigenous cultures of Siberia and North America as well.
Divine love affairs
Greek mythology tells how Zeus fell in love with the nymph Callisto and the tricks he used to attract her.
The enamored king of gods became transformed by adopting the appearance of Artemis to seduce the naive nymph and make her pregnant.
From these illicit love affairs Arcas was born, who over time became a good hunter and king of the Arcadians.
When Hera, Zeus's wife, found out about these love affairs, she punished Callisto by turning her into a bear.
The Big Dipper in Urania's MirrorJ atlas.
Aspin, London, US Library of Congress
In his hunts, Arcas would come across a splendid bear (his mother Calisto) and was about to kill her with his silver bow.
But Zeus intervened just in time to save Callisto by placing her in the sky as the constellation Ursa Major.
Stellar curiosities
On these summer days, the Big Dipper is visible high above the horizon throughout the night.
This weekend we have a full moon, but the lunar brightness does not prevent us from appreciating the seven brightest stars in this splendid constellation.
In constellations, stars are designated by Greek letters starting with alpha (the brightest).
However, the Big Dipper is an exception to this rule.
The seven brightest stars that make up the saucepan are designated by Greek letters starting from the bowl and continuing down the handle (and not in order of brightness).
Thus the brightest star in Ursa Major is epsilon, while alpha is the second brightest.
Locating the Polar from the Big Dipper
The alpha and beta stars are known as 'pointers' because they are very useful to locate the Pole star. If we draw an imaginary line that goes from beta to alpha and extend this segment 5 times, our gaze will rest on Polaris, the star that marks the position of the Earth's north pole. During the night, the Polar does not change places and, due to the rotation of the Earth, all the other stars seem to revolve around it. Thanks to this, the imaginary segment that passes through the stars alpha and beta can be considered as the hand of a clock that indicates the time. It is an effect that has been used since ancient times to orient yourself and have a natural clock at night, when the Sun cannot help.
Among the seven stars, the one that bears the name of the Greek dseta () in the center of the handle of the saucepan stands out.
Sharpening our eyes, we will see that it is a pair of two apparently very close stars: Mizar and Alcor.
The Arabs used these stars to assess people's visual acuity.
Although they are at a very similar distance, it is not entirely clear that Mizar and Alcor are gravitationally linked.
What is very surprising about these two stars is that Alcor (the least bright) is a binary system, while Mizar is a doubly double star (that is, a quadruple system).
This cannot be distinguished with the naked eye, but the truth is that Mizar and Alcor constitute a group of six stars.
The closest stellar association
The constellations are made up of stars that are not usually physically connected, each star is located at a very different distance from the others and it is the projection on the dome that forms the celestial figures in two dimensions. The Big Dipper is also exceptional in this regard. Five of the seven main stars are part of a large association of stars that are at the same distance, about 80 light years, and move with very similar speeds, towards a point in the direction of Sagittarius. Only the stars alpha and eta are farther apart, move in different directions, and are not part of the association.
The stars in this association, known as Collinder 285, are thought to have formed simultaneously as the same cluster, within the same interstellar cloud, about 500 million years ago.
The association is dominated by 14 main stars, 13 of which are in the constellation Ursa Major and the other in the neighboring Hunting Dogs.
Collinder 285 is the closest stellar association to Earth.
From Alaska to van Gogh, the sublime sky
The seven main stars of Ursa Major are so conspicuous that they have been used as a symbol in different circumstances that have little to do with astronomy.
They are featured on the Alaska flag, on a traditional Irish flag, and on some versions of the Swedish coat of arms.
I'm not sure if these stars are also the ones that appear on the flag of the Community of Madrid and on the shield of the capital, perhaps they are related to the bear (or osa) that accompanies the strawberry tree.
Where these seven stars shine magnificently is in the immortal 'Starry Night' that the great van Gogh painted in Arles, on the banks of the Rhone, in September 1888.
Starry night over the Rhonevan GoghMuseum d'Orsay
Let's enjoy the pleasant summer nights looking up at the sky and recognizing the constellations and stars that have been immutable witnesses of the history of mankind.
Observing the sky always reminds us that we are part, although very modest, even insignificant, but after all, part of this sublime universe.
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Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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