Polaris

Polaris, Alpha Ursae Minoris (α UMi), commonly known as the North Star, is the closest relatively bright star to the north celestial pole. It lies at an estimated distance between 323 and 433 light years (99 to 133 parsecs) from Earth and has an apparent magnitude that varies between 1.86 and 2.13. It is classified as a classical Population I (metal-rich) Cepheid variable and is the nearest star of this kind to Earth.

Polaris is easily visible to the unaided eye, but not exceptionally bright. It is the brightest star in the constellation Ursa Minor, but only the 48th brightest star in the sky. This, however, does not mean that it is not particularly luminous. As a yellow supergiant, the star is immense and only appears faint because it is so distant from Earth.

Star system

Polaris is a triple star system consisting of the yellow supergiant Polaris Aa and two white (spectral type F) main sequence stars, Polaris Ab and Polaris B. Polaris Aa and Ab are in close orbit with each other and Polaris B is orbiting the pair.

Polaris Aa has the spectral classification F7Ib. It is a supergiant star with 5.4 solar masses and a radius 37.5 times that of the Sun. It is 1,260 times more luminous than the Sun with a surface temperature of about 6,000 K. It was the first variable star of its type to have its mass calculated from its orbit.

Polaris (Alpha Ursae Minoris), image: Wikisky

In 1929, a study of the star’s spectrum revealed that Polaris was in fact two stars in a tight orbit, confirming an earlier theory that the main component was a binary star.

Polaris Aa is a low-amplitude Cepheid and, being the nearest such star to Earth, it has been a frequent object of study. Cepheids are important in astronomy because they are used as standard candles – objects with known luminosities – to measure distances. With classical Cepheids, astronomers use the relation between their periods and luminosity to determine distances to objects both within the Milky Way and outside our galaxy. Cepheids were a critical factor that helped American astronomer Edwin Hubble prove that the Andromeda Galaxy (Messier 31) was not a nebula within the Milky Way, as previously believed, but an external galaxy.

Classical or Population I Cepheids are typically bright giants or low-luminosity supergiant stars with 4 to 20 solar masses, 1,000 to 50,000 solar luminosities and a few tens to a few hundred solar radii. They have the spectral classification of F6 – K2. They are pulsating variable stars whose temperature, radius and spectral type change as they pulsate. Their brightness changes from a few tenths of a magnitude to 2 magnitudes. These stars have a well-defined period-luminosity relationship; the longer the pulsation period, the more luminous they are.

Astronomers had suspected the primary star in the Polaris system to be a variable since 1852, but its variability was not confirmed until 1911, when Danish astronomer Ejnar Hertzsprung demonstrated it. Hertzsprung went on to determine the distances to several such stars using parallax in 1913. Both his work and Edwin Hubble’s relied on American astronomer Henrietta Leavitt’s discovery of the period-luminosity relationship in 1908. Leavitt was investigating variable stars in the Magellanic Clouds at the time and published her findings in 1912.

Polaris varies in brightness from magnitude 1.86 to 2.13, but the amplitude is not the same as it was at the time of discovery. Before 1963, it was more than 0.1 magnitude and slowly decreasing until 1966, when it had a dramatic decrease to less than 0.05 magnitude. The amplitude has varied unpredictably since, but stayed close to those values. A paper published in 2008 reported that it was increasing again, which is unprecedented in a star of this type.

Polaris Aa has a period of about four days, but the period has not stayed the same either. Astronomers noticed a gradual increase by roughly 4-5 seconds per year, with a break between 1963 and 1965. The star’s temperature varies only slightly as the star pulsates, but the variation itself is erratic, from 50 K to 170 K or more. Astronomers have speculated that the unpredictability may be due to the star’s orbit with Polaris Ab.

Polaris Ab orbits the primary star at a distance of 18.8 astronomical units, roughly equal to the distance between the Sun and Uranus. It belongs to the spectral class F6V, indicating a white main sequence dwarf. It has 1.26 solar masses and a radius 1.04 times solar. It is three times more luminous than the Sun.

Polaris B orbits the main pair at a distance of 2,400 astronomical units. It is a main sequence star of the spectral type F3V with 1.39 solar masses and 1.38 solar radii. It shines with 3.9 solar luminosities and has an estimated surface temperature of 6,900 K.

Polaris B was discovered by the German-born British astronomer William Herschel in August 1779 using a reflector telescope. The star can be observed even in modest-sized telescopes.

At one time, astronomers believed there were two other, more distant stars in the system, designated Polaris C and Polaris D, but these stars were later discovered to not be physically related to the three components of the Polaris system.

In January 2006, images taken by the Hubble Space Telescope showed all three members of the Polaris star system. It was the first time the close companion was seen directly.

Polaris star system imaged by the Hubble Space Telescope, credit: NASA/ESA/HST, G. Bacon (STScI)

North Star

Polaris does not mark the exact location of the north celestial pole, but it is very close to it. It lies in line with the Earth’s northern axis of rotation, almost directly above the North Pole and, for hypothetical observers at the pole, the star would be directly overhead. With the rotational axis pointed almost directly at the star, Polaris does not rise or set for northern observers. Because it is so near the north celestial pole, it appears motionless in the sky and other stars appear to move in a circle around it. This makes Polaris highly useful in navigation and astrometry.

Polaris is currently moving closer to the north celestial pole. It will come to the closest approach on March 24, 2100 (at declination +89°32’50.62’’) and then slowly begin to move away. The star’s apparent movement is the result of the Earth’s axial precession (the precession of the equinoxes), a slow change in the orientation of the Earth’s rotational axis. The gradual shift occurs in a cycle of about 25,772 years and, as a consequence, the pole stars also change. Polaris is currently the closest visible star to the north celestial pole, but it will not stay the North Star forever, just as Sigma Octantis (Polaris Australis) will not be the marker of true south forever.

Polaris took over as the North Star from Kochab, Beta Ursae Minoris, around the year 500 CE. Kochab, the second brightest star in Ursa Minor and the brightest star in the bowl of the Little Dipper, held the title from 1500 BCE to 100 CE. Today, Kochab and Pherkad, Gamma Ursae Minoris, are known as the Guardians of the Pole. The two stars mark the outer edge of the Little Dipper’s pan and appear to rotate around Polaris and the north celestial pole.

In late antiquity, the pole was at the same angular distance from Polaris as it was from Kochab, and this was the time when Polaris, the brighter of the two, started to become more important as a navigational star. In antiquity, before Polaris had become the nearest visible star to the pole, the entire Ursa Minor constellation was used for navigation. Polaris moved to within several degrees of the pole in the early Middle Ages, which is when it started being called the “polar star.”

Polaris will be succeeded by Errai, Gamma Cephei. Around the year 3000, the pole will be halfway between Polaris and Errai and, around 4200, Errai will reach its closest point to the pole.

Of all the north stars, Polaris is the second closest to the pole. The only star that marks true north more accurately as the pole star is Thuban, Alpha Draconis. Thuban comes within 0.2° of the pole, while Polaris comes within 0.5°. However, Thuban is considerably dimmer at magnitude 3.65, which made it slightly less useful as a marker when it was the North Star from the 4th to the 2nd millennium BCE.  The brightest of the north stars, Vega, has an apparent magnitude of 0.026, but only comes within 5° of the pole.

Other than Polaris, Errai (Alrai), Vega, Thuban and Kochab, the stars that act as indicators of true north or near-north are Iota and Beta Cephei, which share the title of the North Star, Alderamin (Alpha Cephei), Deneb (Alpha Cygni), Fawaris (Delta Cygni), Iota Herculis, Tau Herculis, Edasich (Iota Draconis), and Kappa Draconis, which shares timing with Kochab.

North Stars (marked red), image: Roberto Mura

Polaris will be the North Star again around the year 27800, but it will not be as close to the pole as it is now or when it had the title around 23600 BCE and came even closer to the pole than it is now.

Facts

The proximity of Polaris to the north celestial pole in the sky means that its distance from the horizon matches the observer’s latitude. For observers at the North Pole, the star is directly overhead, and the further south the observer is, the closer the star is to the northern horizon.  For example, observers in New York can see it 41 degrees above the horizon because the city is located at latitude 41°N.

Polaris is surrounded by a small semicircle of dim stars known as the Engagement Ring. Polaris is the diamond in the ring, formed by about 10 brighter and several fainter stars that can be seen in a small telescope. The stars that form the asterism belong either to Ursa Minor or to the neighbouring Cepheus constellation.

Polaris, image: DSS / Giuseppe Donatiello (CC0 1.0)

Polaris’ distance is still uncertain. The revised parallax obtained from the Hipparcos satellite data gives a distance of 433 light years, but older estimates are slightly closer. Recent studies based on spectral analysis also indicate that the star may be 323 light years (99 parsecs) away.

The accuracy of the data collected by the Hipparcos satellite in 1989 and 1993 has been questioned in the cases of Cepheid variables in binary star systems. Even though the data has been re-examined and confirmed, there is still no single value that has been widely accepted.

A more recent astrometry mission launched with the Gaia space observatory, the successor to Hipparcos, in 2013 was originally limited to stars dimmer than magnitude 5.7, but proved to work even with magnitude 3 stars. The Gaia Data Release 2 did not provide a parallax for Alpha UMi A, but the distance estimate deduced from it is 136.6 ± 0.5 parsecs (443.57 light years) for Alpha UMi B. The estimate places the star further away than previous ones, but is significantly more accurate.

Research conducted in the last two decades suggests that Polaris is 4.6 times brighter today than it was at the time Ptolemy observed it (137 CE). A team of scientists began to monitor the star in 1999 and discovered that, after reaching a minimum, the amplitude of the star’s pulsations was increasing again. As they researched historical records, they discovered that the star was fainter the further back they went.

Cepheids have been very important for a very long time because their consistency allows astronomers to use them as standard candles. However, Polaris is not the only star of this type found to exhibit unexpected changes in its brightness in the long term. Most of the 15 other Cepheids included in the study showed similar changes, which indicates that these stars are more complex than previously believed.

Name

The name Polaris is short for stella polaris, Latin for “polar star.” The name dates back to the Renaissance era, when Polaris came within a few degrees of the north celestial pole. The Dutch physician, geographer and mathematician Gemma Frisius mentioned the star as “stella illa quae polaris dicitur,” or “the star which is called ‘polar’” in 1547. He determined the star’s distance from the pole to be 3°7’.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) officially approved the name Polaris for Alpha Ursae Minoris Aa on June 30, 2016.

Polaris has been known by many other names throughout history. The star’s name in Old English was scip-steorra, or “ship-star,” and an even older name was lodestar, meaning “guiding star.” The Old Norse name leiðarstjarna and Middle High German leitsterne share the same linguistic root. In the late 13th century, the star became known as Stella Maris, or “the star of the sea.”

By the early modern period, the name Cynosura was also used for the star. It is the old name for the constellation Ursa Minor, derived from the Greek κυνόσουρα, meaning “the dog’s tail.”  The constellation was associated with a dog, not a bear, in ancient times.

In Hindu Puranic literature, Polaris was known as Dhruva, which means “immovable” or “fixed.”

The old Arabic name for the star was Al-Judeyy. The name dates back to pre-Islamic astronomy, before Polaris was as close to the pole as it is today.

Medieval Islamic astronomers knew the star by several different names: Mismar (“needle” or “nail”), al-kutb al-shamaliyy (“the northern axle”), and al-kaukab al-shamaliyy (“the north star”).

In the Berber language of the Tuareg people in North Africa, Polaris is known as Tatrit tan Tamasna, or “star of the plains.” The name reflects the star’s prominent role in navigating the great deserts.

The Inuit name for Polaris is Niqirtsuituq. The star is depicted both on the flag of the Inuit territory of Nunavut, the northernmost territory of Canada, and on the flag of Alaska, the northernmost state of the United States.

Location

Polaris is quite easy to find because it is relatively bright and part of the Little Dipper, a familiar northern asterism. However, since the other stars that form the Little Dipper are fainter than Polaris and cannot be seen from urban locations, it is easier to use the stars of the larger and brighter Big Dipper to find Polaris and true north. Dubhe (Alpha Ursae Majoris, mag. 1.79) and Merak (Beta Ursae Majoris, mag. 2.37), the two stars at the end of the Big Dipper’s bowl, are commonly used to find the star. They are known as the Pointer Stars because an imaginary line extended from Merak through Dubhe leads directly to the North Star.

Polaris (Alpha Ursae Minoris) as seen by the Hubble Space Telescope, image: NASA/HST

The distance from Dubhe to Polaris is about five times the distance from Merak to Dubhe. Polaris is the brightest star in that direction. The two other relatively bright stars in this area of the sky are Kochab and Pherkad, the Guardians of the Pole. The Pointer Stars always point toward Polaris as the Big Dipper keeps going around it. The Dipper stars take 23 hours and 56 minutes to complete a circle around the star.

Constellation

Polaris is the luminary of Ursa Minor and marks the tip of Smaller Bear’s tail. In the Little Dipper asterism, it is the star at the end of the handle.

Ursa Minor is one of the Greek constellations, easily recognizable for the Little Dipper asterism on a clear, dark night. It is one of the smaller constellations, only the 56th in size. Like its neighbours Cepheus and Draco and the larger Ursa Major, Ursa Minor is circumpolar to northern observers, which means that it can be seen year-round from locations north of the equator.

Ursa Minor constellation map by IAU and Sky&Telescope magazine

Ursa Minor does not contain any particularly bright deep sky objects. Notable galaxies in the constellation include the Ursa Minor Dwarf (mag. 11.9), a dwarf spheroidal galaxy discovered in 1955, the barred spiral galaxy NGC 6217 (mag. 11.2), and the supergiant elliptical radio galaxy NGC 6251 (mag. 14.3).

The best time of year to see the stars and deep sky objects of Ursa Minor is during the month of June.

The 10 brightest stars in Ursa Minor are Polaris (Alpha UMi, mag. 1.86 – 2.13), Kochab (Beta UMi, mag. 2.08), Pherkad (Gamma UMi, mag. 3.05), Epsilon Ursae Minoris (mag. 4.19), 5 Ursae Minoris (mag. 4.253), Zeta Ursae Minoris (mag. 4.32), Yildun (Delta UMi, mag. 4.36), RR Ursae Minoris (mag. 4.44 – 4.85), 4 Ursae Minoris (mag. 4.80), and Eta Ursae Minoris (mag. 4.95).

Polaris – Alpha Ursae Minoris

Alpha Ursae Minoris Aa

Alpha Ursae Minoris Ab

Alpha Ursae Minoris B