mardi 23 septembre 2008

Ursa Minor 1

Ursa Minor
The Little Dipper or The Little Bear

The Brighter Stars of Ursa Minor
The Story
The Birth of ZeusThe Small Bear has to do with the birth of Zeus.
Zeus was an immortal god, but he was born, nevertheless. His mother was Rhea, whom the Romans knew as Ops or Cybele. His father was Cronus, who was Saturn to the Romans. Cronus was the youngest of the elder gods known as the Titans. Because of a prophecy that one of his children would dethrone him, Cronus disposed of his children as they were born. He swallowed them! Cronus had already disposed of several children this way by the time that Zeus was born.
Fooling the Old ManRhea fooled Cronus by wrapping a stone in the swaddling clothes of the baby Zeus. So Cronus swallowed the stone, thinking that he had disposed of the baby. Rhea had Zeus smuggled to the island of Crete, where the nymphs Adrasteia and Ida looked after him in the cave known as Dicte. The cave was protected by the Cretan warriors known as Curetes, who stood outside the cave making a racket to prevent the cries of the baby from being heard by Cronus.
Barfing Up the KidsThe baby Zeus remained in the cave for a year. Eventually he overthrew Cronus and forced him to regurgitate the children that he had swallowed. These children became the leaders of the younger gods, who in a ten year war overturned the rule of the Titans to take command of the cosmos.
The Nurses May Be the BearsThe Lesser Bear is identified in classical mythology with the nymph Ida. Some say that the Greater Bear is sometimes identified as Adrasteia. It is not explained how the nymphs got changed into bears.
Callisto, The Greater BearThe Greater Bear, Ursa Major, is more often identified as Callisto, one of the nymphs who formed the retinue of Artemis (Diana to the Romans). Callisto is one of the many conquests of Zeus.

Beta Ursae Minoris

Distance (Light Years) 126.4 ± 2.5
Visual Magnitude 2.07
Color (B-V) 1.47

Names For This Star

Other names for the star are Kocab or Kochah. The names of this star derive from the Arabic name Al Kaukab al Shamaliyy, "The Star of the North." The Arabic name of the star reflects an era when Kochab was nearer to the north celestial pole than Polaris.
With Kochab and Gamma Ursae Minoris are together known as "The Guardians of the Pole."

Description of the Star

Kochab is a cool, orange K4-III giant star about 190 times as luminous as the sun. This spectral implies an effective temperature of about 3600 K, a mass about 4.4 solar masses and a diameter about 20 times that of the sun.

Alpha Ursae Minoris

Distance (Light Years) 431 ± 27
Visual Magnitude 1.97
Color (B-V) 0.6

Names For This Star

Our current name for the star, Polaris, is Latin and abbreviation for Stella Polaris, "The Pole Star." The name reflects the position of the star near the north celestial pole. In fact, in English Polaris is sometimes simply referred to as The Pole Star.
The star is also referred to as The Lodestar. The name associates the star with a naturally occurring magnetic rock a lodestone that can be used to make a magnetic compass. The name "Loadestar" then reflects the position of the star above the north horizon point. Like the compass, the star points the direction of north. A similar idea is contained in another Latin name for the star, Navigatoria, that is "The Navigator's Star." Other names for the star are Alruccabah, Cynosura, Phoenice, Tramontana, Angel Stern, The Star of Arcady, Yilduz, or Mismar. For the origin and significance of these names, see Allen.

Description of the Star

Polaris A is a hot, blue F7:Ib-II bright giant or supergiant star having a luminosity about 2400 times that of the sun.
The Polaris System

Polaris A has at least two companions. The brighter of the two, Polaris B, is a yellowish F3V main sequence star of Visual Magnitude 8.20 having a luminosity about 8 times that of the sun. According to Burnham this star appears about 18.5 arc sec from the A star, corresponding to a projected separation of 2000 AU.
The A star has an additional close companion star orbiting with a 30.5 year period at 5 AU from the primary.

A Cepheid Variable Star

Polaris A is a pulsating variable star of a type known as a Cepheid variable, after prototypical star of this type, Delta Cephei. Cepheid variables are bright, giant stars that show periodic variations in luminosity. In general, the longer the period, the brighter the star. This correlation allows the luminosity (absolute magnitude) to be inferred from the period.
Polaris pulsates with 3.97 day period. The luminosity varies by 0.15 magnitude. Polaris is classified as a Population II Cepheid. Stars of this type are about 1.5 magnitudes smaller than Population I Cepheids.

The Distance Scale in the Universe

Cepheid variables have been very important in establishing a scale of distance in the universe, since the luminosity of a Cepheid variable star can be established by measuring the period of its brightness variation. Knowing the actual luminosity of the star, the distance to the star can be established from a photometric determination of its Visual Magnitude, that is, of its apparent brightness.

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