Polaris

Stellar system

Polaris Aa is an evolved yellow supergiant of spectral type F7Ib with 5.4 solar masses (M☉). It is the first classical Cepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a 1.39 M☉ F3 main-sequence star orbiting at a distance of 2,400 astronomical units (AU), and Polaris Ab (or P), a very close F6 main-sequence star with a mass of 1.26 M☉. In January 2006, NASA released images, from the Hubble telescope, that showed the three members of the Polaris ternary system.

Polaris B can be resolved with a modest telescope. William Herschel discovered the star in August 1779 using a reflecting telescope of his own, one of the best telescopes of the time.

The variable radial velocity of Polaris A was reported by W. W. Campbell in 1899, which suggested this star is a binary system. Since Polaris A is a known cepheid variable, J. H. Moore in 1927 demonstrated that the changes in velocity along the line of sight were due to a combination of the four-day pulsation period combined with a much longer orbital period and a large eccentricity of around 0.6. Moore published preliminary orbital elements of the system in 1929, giving an orbital period of about 29.7 years with an eccentricity of 0.63. This period was confirmed by proper motion studies performed by B. P. Gerasimovič in 1939.

As part of her doctoral thesis, in 1955 E. Roemer used radial velocity data to derive an orbital period of 30.46 y for the Polaris A system, with an eccentricity of 0.64. K. W. Kamper in 1996 produced refined elements with a period of 29.59±0.02 years and an eccentricity of 0.608±0.005. In 2019, a study by R. I. Anderson gave a period of 29.32±0.11 years with an eccentricity of 0.620±0.008.

There were once thought to be two more widely separated components—Polaris C and Polaris D—but these have been shown not to be physically associated with the Polaris system.

Observation

Variability

Polaris Aa, the supergiant primary component, is a low-amplitude population I classical Cepheid variable, although it was once thought to be a type II Cepheid due to its high galactic latitude. Cepheids constitute an important standard candle for determining distance, so Polaris, as the closest such star, is heavily studied. The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911.

The range of brightness of Polaris is given as 1.86–2.13, but the amplitude has changed since discovery. Prior to 1963, the amplitude was over 0.1 magnitude and was very gradually decreasing. After 1966, it very rapidly decreased until it was less than 0.05 magnitude; since then, it has erratically varied near that range. It has been reported that the amplitude is now increasing again, a reversal not seen in any other Cepheid.

The period, roughly 4 days, has also changed over time. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965. This was originally thought to be due to secular redward evolution across the Cepheid instability strip, but it may be due to interference between the primary and the first-overtone pulsation modes. Authors disagree on whether Polaris is a fundamental or first-overtone pulsator and on whether it is crossing the instability strip for the first time or not.

The temperature of Polaris varies by only a small amount during its pulsations, but the amplitude of this variation is variable and unpredictable. The erratic changes of temperature and the amplitude of temperature changes during each cycle, from less than 50 K to at least 170 K, may be related to the orbit with Polaris Ab.

Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to second magnitude. Astronomer Edward Guinan considers this to be a remarkable change and is on record as saying that "if they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution".

Torres (2023) published a broad historical compilation of radial velocity and photometric data. He concludes that the change in the Cepheid period has reversed and is now decreasing since roughly 2010. Torres notes that TESS data is of limited utility: as a survey telescope, TESS is optimized for dimmer stars than Polaris, so Polaris significantly over-saturates TESS's cameras. Determining an accurate total brightness for Polaris from TESS is extremely difficult, although it remains suitable for timing the period.

Role as pole star

Because Polaris lies nearly in a direct line with the Earth's rotational axis above the North Pole, it stands almost motionless in the sky, and all the stars of the northern sky appear to rotate around it. It thus provides a nearly fixed point from which to draw measurements for celestial navigation and for astrometry. The elevation of the star above the horizon gives the approximate latitude of the observer. The star is often popularly believed to be the brightest star in the night sky, though this is not the case; the star's apparent magnitude of around 1.98 places it at only the 47th brightest star.

In 2018 Polaris was 0.66° (39.6 arcminutes) away from the pole of rotation (1.4 times the Moon disc) and so revolves around the pole in a small circle 1.3° in diameter. It will be closest to the pole (about 0.45 degree, or 27 arcminutes) soon after the year 2100. Because it is so close to the celestial north pole, its right ascension is changing rapidly due to the precession of Earth's axis, going from 2.5h in AD 2000 to 6h in AD 2100. Twice in each sidereal day Polaris's azimuth is true north; the rest of the time it is displaced eastward or westward, and the bearing must be corrected using tables or a rule of thumb. The best approximation is made using the leading edge of the "Big Dipper" asterism in the constellation Ursa Major. The leading edge (defined by the stars Dubhe and Merak) is referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.

The apparent motion of Polaris towards and, in the future, away from the celestial pole, is due to the precession of the equinoxes. The north celestial pole precesses in a circle over thousands of years; Polaris will be closest to the celestial pole by approximately 2100 AD, after which time the pole will begin to move away. It will pass by the star Errai (Gamma Cephei) in 4000 AD, Deneb in 10,000 AD, and Vega in 14,500 AD. The celestial pole will once again return to Polaris after a complete precession taking 26,000 years.The celestial pole was close to Thuban around 2750 BC, and during classical antiquity it was slightly closer to Kochab (β UMi) than to Polaris, although still about 10° from either star. It was about the same angular distance from β UMi as to α UMi by the end of late antiquity. The Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. However, as one of the brighter stars close to the celestial pole, Polaris was used for navigation at least from late antiquity, and described as ἀεί φανής (aei phanēs) "always visible" by Stobaeus (5th century), also termed Λύχνος (Lychnos) akin to a burner or lamp and would reasonably be described as stella polaris from about the High Middle Ages and onwards, both in Greek and Latin. On his first trans-Atlantic voyage in 1492, Christopher Columbus had to correct for the "circle described by the pole star about the pole". In Shakespeare's play Julius Caesar, written around 1599, Caesar describes himself as being "as constant as the northern star", although in Caesar's time there was no constant northern star.

Distance

Since Leavitt's discovery of the Cepheid variable period-luminosity relationship, and corresponding utility as a standard candle, the distance to Polaris has been highly sought-after by astronomers. It is the closest Cepheid to Earth, and thus key to calibrating the Cepheid standard candle; Cepheids form the base of the cosmic distance ladder by which to probe the cosmological nature of the universe.

Distance measurement techniques depend on whether or not components A and B are a physical pair, that is, gravitationally bound. If they are, then their estimated distance can be presumed to be equal. Gravitational binding of this pair is well supported by observations, and the presumption of common distance is widely adopted in historical and recent estimates.

For most of the 20th century, available observation technologies remained inadequate to precisely measure absolute parallax. Instead, the main technique was to use theoretical models of stellar evolution for both main sequence and giant stars, combined with spectroscopic and photometric data to estimate distances. Such modeling relies on theoretical assumptions and guesses, and contains much systematic error and statistical uncertainties in population data. Even by 2013, these techniques were still struggling to achieve even 10% precision in either main sequence or Cepheid modeling.

Further progress was thus limited until the advent of Hipparcos, the first instrument able to engage in all-sky absolute parallax astrometry. Its first data release was in 1997.

After the arrival of the Hipparcos data, the distance to Polaris and consequent analysis of its Cepheid variation was controversial. The Hipparcos distance for Polaris was broadly but not universally adopted. Immediately, the Hipparcos data for the nearest few hundred Cepheids appeared to clarify Cepheid models and to clear up then-tension in higher rungs of the distance ladder. However alternatives remained; particularly by Turner et al, who published several papers between 2004 and 2013.

In 2018, Bond et al. used the Hubble Space Telescope to provide an alternative direct measurement of Polaris's parallax; they summarize the back-and-forth:

Etymology and cultural significance

The name Polaris

The modern name Polaris is shortened from the Neo-Latin stella polaris ("polar star"), coined in the Renaissance when the star had approached the celestial pole to within a few degrees. The first appearance of the name stella polaris in print was in the Alfonsine Tables of 1492.

Gemma Frisius, writing in 1547, referred to it as stella illa quae polaris dicitur ("that star which is called 'polar'"), placing it 3° 8' from the celestial pole.

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Polaris for the star α Ursae Minoris Aa.

Europe

In antiquity, Polaris was not yet the closest naked-eye star to the celestial pole, and the entire constellation of Ursa Minor was used for navigation rather than any single star. Polaris moved close enough to the pole to be the closest naked-eye star, even though still at a distance of several degrees, in the early medieval period, and numerous names referring to this characteristic as polar star have been in use since the medieval period. In Old English, it was known as scip-steorra ("ship-star").

In the "Old English rune poem", the T-rune is apparently associated with "a circumpolar constellation", or the planet Mars.

In the later medieval period, it became associated with the Marian title of Stella Maris "Star of the Sea" (so in Bartholomaeus Anglicus, c. 1270s), due to an earlier transcription error.

An older English name, attested since the 14th century, is lodestar "guiding star", cognate with the Old Norse leiðarstjarna, Middle High German leitsterne.

The ancient name of the constellation Ursa Minor, Cynosura (from the Greek κυνόσουρα "the dog's tail"), became associated with the pole star in particular by the early modern period. An explicit identification of Mary as stella maris with the polar star (Stella Polaris), as well as the use of Cynosura as a name of the star, is evident in the title Cynosura seu Mariana Stella Polaris (i.e. "Cynosure, or the Marian Polar Star"), a collection of Marian poetry published by Nicolaus Lucensis (Niccolo Barsotti de Lucca) in 1655.

It was invoked as a symbol of steadfastness in poetry, as "steadfast star" by Spenser.

Shakespeare's sonnet 116 is an example of the symbolism of the north star as a guiding principle: "[Love] is the star to every wandering bark / Whose worth's unknown, although his height be taken."

In the ancient Finnish worldview, the North Star has also been called taivaannapa and naulatähti ("the nailstar") because it seems to be attached to the firmament or even to act as a fastener for the sky when other stars orbit it. Since the starry sky seemed to rotate around it, the firmament is thought of as a wheel, with the star as the pivot on its axis. The names derived from it were sky pin and world pin.

Arab world and Asia

Its name in traditional pre-Islamic Arab astronomy was al-Jady الجدي ("the kid", in the sense of a juvenile goat ["le Chevreau"] in Description des Etoiles fixes), and that name was used in medieval Islamic astronomy as well. In those times, it was not yet as close to the north celestial pole as it is now, and used to rotate around the pole.

Another name of Arabic origin, Alrucaba or Alruccabah (from al-rukba meaning knee), appears in some Western sources, starting with the Alfonsine Tables of 1492, but this was applied in error.

In the Hindu Puranas, it became personified under the name Dhruva ("immovable, fixed").

In traditional Chinese astronomy, Polaris is part of the asterism Gouchen (勾陈, "Curved Array"), along with Delta, Epsilon and Zeta Ursae Minoris, and two other stars in Cepheus.

North America

In Inuit astronomy, Polaris is known as Nuutuittuq (syllabics: ᓅᑐᐃᑦᑐᖅ).

In traditional Lakota star knowledge, Polaris is named "Wičháȟpi Owáŋžila". This translates to "The Star that Sits Still". This name comes from a Lakota story in which he married Tȟapȟúŋ Šá Wíŋ, "Red Cheeked Woman". However, she fell from the heavens, and in his grief Wičháȟpi Owáŋžila stared down from "waŋkátu" (the above land) forever.

The Plains Cree call the star in Nehiyawewin: acâhkos êkâ kâ-âhcît "the star that does not move" (syllabics: ᐊᒑᐦᑯᐢ ᐁᑳ ᑳ ᐋᐦᒌᐟ).

In Mi'kmawi'simk the star is named Tatapn.

Polynesia

A Hawaiian name for this star, used by the Polynesian Voyaging Society, is Hōkūpa‘a meaning "fixed star".

In popular culture

Polaris is depicted in the flag and coat of arms of the Canadian Inuit territory of Nunavut, the flag of the U.S. states of Alaska, Maine, and Minnesota, and the flag of the U.S. city of Duluth, Minnesota.

Ships

The Chinese spy ship Beijixing is named after Polaris.

USS Polaris is named after Polaris