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Ursa Minor, the Little Bear

16,000 light-years away, nova explosion explained

Two-thirds of the way up in our northern sky is the den of Ursa Minor, the Little Bear. The brightest stars of this constellation form an asterism widely known as the Little Dipper. The difference between the two is a few fainter stars that extend the constellation to the south and west. On these August evenings, Ursa Minor is standing up on its tail, stretching its front paws higher into our sky.

The North Star, Polaris, is the star at the end of the tail of the Little Bear, Ursa Minor. Ursa Minor is visible every night of the year that it is clear, since it never sets. It just circles around the North Celestial Pole, which is right next to Polaris. On August evenings, Ursa Minor is standing on its tail. (Image courtesy Bert Stevens)

The Little Bear and the Big Bear (Ursa Major) are joined in the same myth. The nymph Callisto promised to be faithful to Artemis, but Zeus, the king of the gods, overcame her objections and the two had a son, Arcas. When Zeus’ wife Hera found out, she turned Callisto into a bear, and Callisto roamed the woods for the next 15 years, avoiding hunters. Arcas was out hunting in those same woods and came face to face with Callisto. Arcas was afraid and about to shoot her with an arrow, but something in the bears eyes made him hesitate long enough for Zeus to intervene at prevent the tragedy. Zeus changed Arcas into a bear as well and sent a whirlwind to lift them into the sky. Callisto became Ursa Major and Arcas became Ursa Minor.

The most well-known star in Ursa Minor is Polaris, the North Star. This second magnitude star is just three-quarters of a degree from the North Celestial Pole. The Celestial Pole is the extension of Earth’s pole into the sky. As the Earth turns, the stars all appear to circle around the Celestial Pole each day. However, the Pole is not fixed in the sky. The Earth wobbles the same way a top does when it is slowing down. This wobble takes the Pole in a 47-degree wide circle that takes 26,000 years to complete. Polaris will be the North Star for a long time, but the Pole will move away from it by one degree every 72 years.

Polaris is actually composed of five stars. The primary star is the bright supergiant that we see in our sky. The rest of the stars orbit the primary, but are much smaller. The primary supergiant is also a Cepheid variable star. This type of variable becomes brighter and fainter (pulse) with a regular period and is one of the “standard candles” for determining the distance to similar stars throughout the universe.

Sixteen thousand light-years away in east central Ursa Minor is the star RW Ursae Minoris. In the early part of the last century, it was an extremely faint 21st magnitude. In 1956, it suddenly brightened to sixth magnitude, right up to the edge of naked eye visibility. This nova explosion made the star a million times brighter than it had been. Even now, it is still more than six times brighter than it was before the explosion.

Novae only occur in close binary star systems. When they are young, one of the two stars in the binary is larger, so it ages more quickly. The larger star reaches the end of its life more quickly, becoming a giant star. It then loses its atmosphere, becoming a white dwarf. The other star now reaches the end of its life and balloons up as a giant star. The white dwarf’s gravity starts pulling the giant’s hydrogen atmosphere onto its own surface. The hydrogen accumulates on the surface until there is so much of it that it gets hot enough for the hydrogen to fuse into helium.

The fusion process converts about five percent of the accumulated hydrogen into helium, releasing a tremendous amount of light and heat in a violent explosion that blows remaining hydrogen into space. The hot, glowing gas expands outward from the star, making it appear much brighter. The gas slowly cools down causing the nova to fade. Unlike the much more violent Type I supernova, neither star is destroyed so the process can start all over again.

The Planets for August 2015

Mercury, Venus and Jupiter are all together in southwestern Leo near the star Regulus. They are very low on our western horizon as the month begins. Venus and Jupiter are heading westward toward the Sun and will disappear into the Sun’s glare after the first week of the month, while Mercury is heading eastward away from the Sun. It will be visible all month. They are all within six degrees of the horizon as it gets dark during that first week.


Mercury is rather far south, so this will not be a great apparition. At the middle of the month, Mercury shines at magnitude -0.2 with a disc that is 5.6 seconds-of-arc across and will be 76 per-cent illuminated. The Messenger of the Gods starts the month in Leo, drifting southeast into Virgo as the month ends. Mercury is low on the west-northwestern horizon as the month begins and slides southward along horizon being just south of west as the month ends. Mercury stays within six degree of the horizon as it gets dark during the entire month and sets around 8:45 p.m.

On August 1, Venus’s disc is just 6 percent illuminated and is 52.8 seconds-of-arc across shining at magnitude -4.3. Moving slightly south and westward, the Goddess of Love nicks the north-west corner of Sextants and the northeast corner of Hydra before settling into Cancer as the month ends.

Jupiter is moving slowly eastward in Leo, passing just a half degree north of Regulus on August 10. At the beginning of the month, Jupiter’s disc is 31.1 sec-onds-of-arc across, shining at magnitude -1.7.

Saturn sets around 12:30 a.m. It can be found almost 40 degrees up in the south-southwest, as it gets dark. The Ringed Planet stands still on Aug. 2 and then begins to move slowly eastward in eastern Libra. At midmonth, Saturn’s disc is 16.8 seconds-of-arc across while its Rings are 38.1 seconds-of-arc across and they tilt down 24.1 degrees with the northern face showing. It glows at magnitude +0.5.

Mars is just eleven degrees above the east-northeastern hori-zon as it starts to get light, having risen around 5 a.m. Mars’ disc is just 3.7 seconds-ofarc across at midmonth. The God of War starts the month in eastern Gemini moving eastward into eastern Cancer.

The annual Perseid meteor shower occurs this month just before New Moon, so this is a good year to view these meteors with no Moon in the sky. These meteors are tiny particles from Comet P/109 Swift-Tuttle, “burning up” as they hit our upper atmosphere at a speed of 40 miles per second. The shower peaks around midnight, so the entire early morning hours of Aug. 13 are the best time for viewing these meteors. Set up a folding lounge chair facing northeast, put on some warm blankets or a sleeping bag, and “keep watching the sky!”


An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.

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