Star Constellation Facts: Ursa Minor

Star Constellation Facts: Ursa Minor
Ursa Minor by John Chumack [www.galacticimages.com]

Ursa Minor (“Little Bear”) is situated opposite to Ursa Major (“Big Bear”), and like The Big Dipper also contains an asterism of seven main stars, in this case known as The Little Dipper. Unlike The Big Dipper, however, just three of The Little Dipper’s stars are bright, namely Polaris, Kochab and Pherkad, and you will need a dark night sky to see all seven stars clearly.

Location

Ursa Minor is the 56th largest constellation, occupies a surface area of 256 sq/degrees, and throughout history has been invaluable for navigation as it contains Polaris, also known as the North Star. The constellation is circumpolar, and can be seen by observers located between +90° and -10° of latitude.

Ursa Major Family of Constellations

Ursa Minor belongs to the Ursa Major family of constellations, along with Coma Berenices, Boötes, Camelopardalis, Canes Venatici, Corona Borealis, Draco, Leo Minor, Ursa Major and Lynx.

History

Exactly how Ursa Minor came about as a constellation is not clear, but it is widely thought to have been “created” by Thales of Miletus (625-545 BC), who was an astronomer, philosopher and one of the Seven Sages of Greece, a group of wise men who were collectively renowned throughout the Ancient World for their knowledge and wisdom. However, a dissenting school of thought holds that Thales only introduced the constellation to the Greeks, and that his knowledge of it derives from being of Phoenician descent. The Phoenicians were accomplished navigators, and Ursa Minor is an excellent pointer to true north due to its close proximity to the North Celestial Pole. In some Greek accounts, Ursa Minor is sometimes referred to as “The Phoenician”, which seems to lend some credence to the story.

Principal Stars and Deep-Sky Objects

Ursa Minor– Polaris (Alpha Ursae Minoris) is located at the end of the Little Dipper’s handle, with this yellow supergiant (F7:Ib-II) easily spotted as it has an apparent visual magnitude of 1.97. Polaris is useful for determining the direction of north, as it is aligned almost perfectly with the North Celestial Pole, being offset by just 0.7 degrees. It is located 434 light years from our solar system, but is actually a triple star system consisting of the supergiant Polaris A, and two smaller yellow-white dwarf companions called Polaris Ab and Polaris B.

– Kochab (Beta Ursae Minoris), the second brightest star in Ursa Minor, is an orange giant (K4 III) situated 132 light years from the Sun that shines with a magnitude of 2.08. It has 2.2 times our sun’s mass, and 130 times brighter. Together, the stars Pherkad and Kochab are known as the “guardians of the pole star”.

– Pherkad (Gamma Ursae Minoris), the constellation’s third brightest star, is a blue giant (A3 lab) situated 487 light years from our solar system of magnitude 3.05. It has 15 times the Sun’s radius, is 1,100 times more luminous, and has a rotational velocity of around 180 km/s.

Ursa Minor also contains Earth’s nearest neutron star called Calvera located just 250 light years away; as well as the hottest white dwarf yet discovered, H1504+65, which burns at a fiery 200,000 K. The constellation also has a total of four stars with confirmed planets. Deep sky objects in Ursa Minor includes the Ursa Minor Dwarf galaxy located 225,000 light-years from Earth; the barred spiral galaxy NGC 6217 found 67 million light-years away; and the supergiant elliptical radio galaxy NGC 6251 which is around 340 million light-years distant.

Mythology

The constellation is associated with several myths, but the story in widest circulation says it represents Ida (Ursa Minor) and her sister Adrasteia (Ursa Major), the nymph nurses who were charged with taking care of the infant Zeus, and preventing his father Cronus from discovering he was still alive. The story goes that Cronos was afraid of a prophesy that predicted his downfall at the hands of one of his own children, and in an attempt to avert his fate swallowed five of his children immediately after birth. His wife, Rhea, however, caught wind of Cronos’s plan, and tricked him into swallowing a stone instead of his sixth son, Zeus, thus allowing him to grow into adulthood and eventually overthrow his father. After Zeus assumed the Supreme-godship, in gratitude for the nyphs service he placed Adrasteia and her sister Ide in the heavens as the constellations of Ursa Minor and Ursa Major.

An unrelated, and much older myth holds that the seven component stars of the Little Dipper are in fact the seven daughters of Atlas, collectively known as the Hesperides. They tended Hera’s Orchard (Garden of Hesperides), where a tree grew that bore golden apples that could confer immortality on those fortunate, or brave enough to get close enough to eat one. As well as the nyphs, the apples were guared by Ladon, a huge hundred-headed dragon.

Meteor Showers

Ursa Minor contains just one meteor shower, the Ursids, which is associated with the comet 8P/Tuttle, also known as Mechain-Tuttle’s Comet. It is only visible from the northern hemisphere, and the shower is usually active from around December 17th to December 23rd, with its peak on the morning of December 21st/22nd. The best time to view the shower is from midnight to just before dawn, though, when observers can expect to see up to 10 meteors per hour.