D  e  s  e  r  t     E  x  p  o  s  u  r  e     May 2005

Constellation of the Month: Ursa Major, The Great Bear

Ursa Major, the Great Bear, is our May constellation of the Month. This is the third largest constellation, and during May swings high in our northern sky. Not only did the ancients in Greece and Rome think of this grouping as a great bear, but a number of North American tribes (Algonquin, Iroquois, Illinois and Narragansett, among possibly others) did likewise. This constellation contains the famous asterism known as the Big Dipper, though in England it is known as the Plough. The French and Irish both consider the asterism to be a chariot.

Click on the image for a bigger map. Facing north and looking two-thirds of the way up in the sky you will find Ursa Major, our Constellation of the Month. This view is how the sky will look at 8:45 p.m. Saturn is far to the left in Gemini. From this vantage point, the Great Bear and the Big Dipper are upside-down. You can follow the arrow from the Pointer Stars down to Polaris, the Pole Star.

The story behind the Great Bear begins with King Lycaon. His daughter, Callisto, was chosen to be one of the band of beautiful nymphs who followed the goddess Artemis, sister of Apollo and patroness of childbirth and babies. Despite the nymphs' vow of chastity, Zeus was not above taking advantage of them. One night, Zeus came to Callisto in the disguise of Apollo. He overcame her scruples and took advantage of her. Eventually, Callisto delivered a son, named Arcas (from the Greek arktos or "bear"). Zeus knew that Artemis would be angry with Callisto for breaking her vow, and if his wife, Hera, found out, Callisto's life would be in jeopardy. To protect her, Zeus turned Callisto into a great bear. She retained her human mind, so she was unable to make friends with other animals, yet at the same time, people might kill her, thinking that she was a bear. It was a lonely existence.

One day, Callisto's son Arcas spied her in the woods and aimed his arrow at her. Seeing this, Zeus turned Arcas into a bear and he immediately recognized his mother. Zeus grabbed both of them by the tail and swung them around and around, stretching their tails far longer than those of earthly bears. Zeus let go of them, flinging them up into the sky, where they became Ursa Major and Ursa Minor.

As always, Hera eventually found out about Zeus' dalliance and the honored place he gave Callisto and her son in the sky. Outraged, Hera went to visit her friend Oceanus, god of the ocean, to whom she complained that the two bears had displaced her from her place in the sky. She pleaded with Oceanus to keep the bears penned up. Oceanus promised her that the two would never be allowed to travel far and would never be allowed to enter Greek waters.

Oceanus' stricture keeps the two bears from ever entering the water. All the stars in our sky turn daily around the north pole in Ursa Minor's tail, keeping the two bears above the northern horizon as seen from Athens or Rome. From our latitude, all the stars of the Big Dipper asterism do go below the horizon, except for Dubhe, the northwest star of the Big Dipper. This and all the stars in Ursa Minor are called circumpolar, because they circle the pole but never set.

The seven stars of the Big Dipper are not related to each other and are each moving in a different direction. Indeed, the Dipper has only appeared as such for the last 50,000 years, and in a similar time it will become so distorted as to be unrecognizable. This quick motion is because all the stars are mostly within 600 trillion miles of our Sun, very near when you consider that our galaxy is almost 2 million trillion miles across.

The star in the middle of the Big Dipper's handle forms an interesting pairing with a fainter star. Alcor and Mizar are only a fifth of a degree apart, but Alcor is over six times fainter than Mizar. This made the pair a test for keenness of vision among Native Americans. These two stars are moving through space together, but are over 17 trillion light years apart, so astronomers do not think they are gravitationally bound together into a double-star system. Mizar itself is actually part of a triple star with Mizar A and Mizar B only 14.4 seconds of arc apart, while the third member of the family, Mizar C, is over 700 seconds away. In turn, Mizar A and B are each a double star, but the stars are so close together that they cannot be separated with even a powerful telescope. In 1650, Mizar was the first binary star system to be discovered. The whole group of six stars is 458 trillion miles away from us.

Speaking of multiple stars, the first double star system to have its orbit computed is Alula Australis (Xi Ursae Majoris), located at one of the bear's feet. The two stars that make up this double are magnitude 4.3 and 4.4, but they are only 1.8 seconds of arc apart, making them easy to see in most telescopes. The computation of their orbit in 1828 by French mathematician Felix Savary marked the first time an orbit was computed for objects outside our solar system. More recent observations have shown that there are actually four or five stars in this system and that it is 160 trillion miles away.

The Planets for May 2005

Still in Gemini, Saturn continues to fade as the Earth pulls away in their race around the Sun. Starting at magnitude -0.2, Saturn ends the month at -0.1. Moving lower in the western sky after sunset, Saturn's ball is now only 17.3 seconds of arc across at midmonth. The rings are tilted at 23.3 degrees, with the southern face showing, and are 39.2 seconds of arc across.

Getting higher in the southeast as evening begins, Jupiter is near the 2.9-magnitude star Porrima. Virgo will be home to Jupiter all month. At magnitude -2.4, Jupiter joins Saturn in the "fading and shrinking" club after having reached opposition last month. In May Jupiter is 42 seconds of arc across. The Moon will be near it on Thursday evening, May 19.

In Aquarius this month, Mars is in the morning sky, rising about 3 a.m. Slowly getting larger, Mars is 7.2 seconds of arc across and glows at magnitude +0.5.

Mercury is low in the east shortly before sun-up. Having been at its best last month, Mercury will spend the first part of May sinking back toward the Sun and invisibility. At mid-month it will rise around 5:30 a.m., at magnitude -0.4.

If you happen to see a bright star in the west just after sunset at the end of May, you will know that Venus is about to make an entrance, so "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.

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