Polaris is only the 46th brightest star in the night sky but it's an important one that's been of immense value to navigators. What makes it special is that it's positioned almost exactly at the North Celestial Pole making it the current Northern Pole Star. The star is about three quarters of a degree from the pole and is edging closer; on March 24, 2100 it will be less than half a degree distant but after that will slowly start to move away.

Polaris (Fred Espenak)

Polaris is a multiple star system located 434 light-years from Earth that shines with a combined apparent magnitude of +1.97. The main component (α UMi Aa) usually referred to as simply Polaris is a type F7Ib yellow-white supergiant that dominates the system. It's a classical Cepheid variable with a period of slightly more than 4 days. There is evidence that the brightness variations have decreased and then increased over the past few years. Before 1960, the change was about 0.15 magnitudes but a few years later it was down to 0.05. Now it appears the range is on the increase and with modern imaging and magnitude measuring techniques, it will be interesting to see how it changes over the coming years. Since one of the nearest Cepheid's - important stars for measuring astronomical distances - Polaris is heavily studied by astronomers. It's 2,300 more luminous than the Sun and 46 times larger than our star.

The other system components are two nearby yellow-white dwarf companions, Polaris B (α UMi B) and α UMi Ab and two more distant dwarfs, α UMi C and α UMi D. Through amateur telescopes only Polaris itself and the B star are visible forming a notable double. The B star shines at magnitude +8.7 with a generous separation of 18 arc seconds from its much brighter companion. Polaris B was discovered by Sir William Herschel in 1780 and is an easy object when viewed through a scope of 80mm (3.1 inches) aperture or more. A magnification of 75x easily splits the pair. In space the two stars are a long way apart with a revolution period amounting to many thousands of years.

In 1929, α UMi Ab was discovered spectrospically and subsequently imaged by the Hubble telescope in 2006. The nearer dwarf star orbits only 18.5 AU (2.8 billion km) from the main star which is equivalent to about the distance between our Sun and Uranus, explaining why its light is swamped by its close and much brighter companion.

Finding Polaris is easy. An imaginary line extending northwards from the Big Dipper pointer stars Dubhe and Merak heads straight to the pole star. In addition to its navigational importance, Polaris is a wonderful marker for Northern Hemisphere astronomers setting up equatorial mountings. It's a pity there is no southern counterpart.

Polaris and Ursa Major

The system is also known by numerous names. One ancient name for Polaris was Cynosure from the Greek meaning "dogs tail". Also commonly used names are the "North Star", the "Steadfast Star" and the "Guiding Star". It's always interesting to take a time exposure photograph or image of the region centered on Polaris with the trails very evident. Since it's not exactly at the pole, even Polaris leaves a short trail.

Finder Chart for Polaris (Alpha Ursae Minoris)

Finder Chart for Polaris (Alpha Ursae Minoris) - pdf format

Polaris Data Table

Bayeralpha (α) UMi
Flamsteed1 UMi
ConstellationUrsa Minor
RA (J2000)02h 31m 49s
DEC (J2000)+89d 15m 51s
Distance (ly)434
Apparent Mag.1.97
Absolute Mag.-3.6
Spectral TypeF7Ib
Radius (Sol)46
Surface Temp (K).6,015
Luminosity (Sol)2,300
Age (million years)70
Other designationsHR 424, HD 8890, HIP 11767, North Star
Notable featureClosest bright star to the North Celestial Pole

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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