Polaris

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Polaris

Polaris (a UMi / a Ursae Minoris / Alpha

Alpha (letter)

Alpha is the first letter of the Greek alphabet. In the system of Greek numerals it has a value of 1. It was derived from the Phoenician alphabet Aleph ....
Ursae Minoris, commonly North(ern) Star
North Star

The North Star is the prominent pole star that lies closest in the sky to the celestial pole and which appears directly overhead to an observer at the Earth's North Pole; currently, this is Polaris....
or Pole Star, and sometimes Lodestar
Lodestar

A lodestar is a relatively bright, easily found, star that is used to find direction, particularly with reference to Polaris, which during the 20th and 21st centuries indicates the direction to the north within a fraction of a Degree ....
) is the brightest star in the constellation

Constellation

A constellation is a group of stars that appear to have a physical proximity in the sky. The stars in a constellation are often vastly distant from each other, but they appear close to each other from the perspective of Earth....
Ursa Minor

Ursa Minor

Ursa Minor is a constellation in the northern sky. Its name is Latin for 'little bear', contrasting with Ursa Major, the Great Bear. It was one of the 48 constellations listed by the 1st century astronomer Ptolemy, and remains one of the 88 List of constellations....
. It is very close to the north celestial pole

Celestial pole

The north and south celestial poles are the two imaginary points in the sky where the Earth axis of rotation, "infinitely extended", intersects the imaginary rotating sphere of stars called the celestial sphere....
(42′ away ), making it the current northern pole star
Pole star

A pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent position is close to one of the celestial poles, and which lies directly overhead when viewed from the Earth's North Pole or South Pole....
.

Polaris is about 430 light-years from Earth. Concerning the detailed physics, a UMi A is an F7 bright giant (II) or supergiant (Ib).

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Polaris (a UMi / a Ursae Minoris / Alpha

Alpha (letter)

Constellation

Ursa Minor

Celestial pole

Main sequence

The main sequence is a continuous and distinctive band of stars that appear on plots of stellar Color index versus brightness. These color-absolute magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell....
star orbiting at a distance of 2400 AU

Astronomical unit

An astronomical unit is a unit of length based on the mean distance from the Earth to the Sun. The precise value of the AU is currently accepted as 149,597,870,691 Plus-minus sign 6 metres ....
, and a UMi Ab, a very close dwarf with an 18.5 AU radius orbit. Recent observations show that Polaris may be part of a loose open cluster

Open cluster

An open cluster is a star cluster of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravity to each other....
of type A and F stars.

Polaris B can be seen with even a modest telescope and was first noticed by William Herschel

William Herschel

Sir Frederick William Herschel, Fellow of the Royal Society Royal Guelphic Order was a German-born British astronomer and composer who became famous for discovering Uranus....
in 1780. In 1929, it was discovered by examining the spectrum

Spectroscopy

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g....
of Polaris A that it had another very close dwarf companion (variously a UMi P, a UMi a or a UMi Ab), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA

NASA

The National Aeronautics and Space Administration is an agency of the Federal government of the United States, responsible for the nation's public list of space agencies....
released images from the Hubble telescope

Hubble Space Telescope

The Hubble Space Telescope is a Space observatory that was carried into Low Earth orbit STS-31 in April 1990. It is named after the American astronomer Edwin Hubble....
, directly showing all three members of the Polaris trinary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km; about the distance from our Sun

Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
to Uranus

Uranus

Uranus is the seventh planet from the Sun and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus ....
) from Polaris A, explaining why its light is swamped by its close and much brighter companion.

Polaris is a classic Population I Cepheid variable

Cepheid variable

A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of Radial pulsations and absolute luminosity....
(although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.

Recent research reported in Science
Science (journal)

Science is the academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious scientific journals....
suggests that Polaris is 2.5 times brighter today than when Ptolemy

Ptolemy

Claudius Ptolemaeus , known in English as Ptolemy , was a Roman Greek mathematics, Greek astronomy, geographer and astrologer. He lived in History of Roman Egypt, and was probably born there in a town in the Thebaid called Ptolemais Hermiou; he died in Alexandria around 168 AD....
observed it (now 2mag, antiquity 3mag). Astronomer Edward Guinan

Edward Guinan

Edward Guinan is currently a professor in Villanova University's Department of Astronomy and Astrophysics. He and two colleagues discovered Neptune's ring system in 1968....
considers this to be a remarkable rate of 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

Stellar evolution

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only few millions of years to trillions of years , considerably more than the age of the universe....
."

Pole Star

Because a UMi lies nearly in a direct line with the axis of the Earth

Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's rotation "above" the North Pole

North Pole

The North Pole, also known as the Geographic North Pole or Terrestrial North Pole is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets the Earth's surface....
— the north celestial pole — Polaris stands almost motionless on the sky, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation

Celestial navigation

Celestial navigation, also known as astronavigation, is a position fixing technique that was devised to help sailors cross the featureless oceans without having to rely on dead reckoning to enable them to strike land....
and for astrometry

Astrometry

Astrometry is the branch of astronomy that relates to precise measurements and explanations of the positions and movements of stars and other celestial bodies....
. The antiquity of its use is attested by the fact that it is found represented on the earliest known Assyria

Assyria

Assyria was a political state centered on the Upper Tigris river, in Mesopotamia , that came to rule regional empires a number of times in history....
n tablet

Clay tablet

In ancient times, small tablets made out of clay were used as a writing medium.From the 4th millennium BCE in the Sumerian, Babylonian, Assyrian and Hittites civilisations of the Mesopotamia region, Cuneiform characters were imprinted on a wet clay tablet with a stylus often made of reed....
s. In more recent history it was referenced in Nathaniel Bowditch

Nathaniel Bowditch

Nathaniel Bowditch was an early American mathematician remembered for his work on ocean navigation. He is often credited as the founder of modern maritime navigation; his book American Practical Navigator, first published in 1802, is still carried onboard every commissioned U.S....
's 1802 book, The American Practical Navigator, where it is listed as one of the navigational stars

Navigational stars

The navigational stars are used in celestial navigation because they are some of the brightest celestial objects due to their high luminosity and/or their proximity to our solar system....
. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon

Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth

Azimuth

An Azimuth is the angle from a reference vector space in a reference plane to a second vector in the same plane, pointing toward, , something of interest....
; the rest of the time it is only an approximation and must be corrected using tables or a rough rule of thumb

Rule of thumb

A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination....
.

Due to the precession of the equinoxes

Precession of the equinoxes

In astronomy, precession refers to a gravitationally-induced slow but continuous change in an astronomical body's rotational axis or orbital path....
, Polaris will not always be the pole star. Over tens of thousands of years, perturbations to the Earth's axis of rotation will cause it to point to other regions of the sky, tracing out a circle. Other stars along this circle were the pole star in the past and will be again in the future, including Thuban

Thuban

Thuban is a star in the constellation of Draco . A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star in ancient times....
and Vega

Vega

Vega is the brightest star in the constellation Lyra, the list of brightest stars in the night sky and the second brightest star in the northern Celestial sphere, after Arcturus....
.

Etymology and cultural significance

To the Bedouin

Negev Bedouins

The Negev Bedouin are traditionally pastoral semi-nomadic Arab tribes indigenous to the Negev region in Israel, who hold close ties to the Bedouin of the Sinai Peninsula....
people of the Negev

Negev

The Negev is a desert and semidesert region of southern Israel. The indigenous Negev Bedouin inhabitants of the region refer to the desert as al-Naqab ....
and Sinai, Polaris is known as ?????? al-jadiyy, "the billy goat". It and Suhail (= Canopus

Canopus

Canopus is the brightest star in the southern constellation of Carina , and the list of brightest stars in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53....
, a Car

Carina (constellation)

Carina , is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was formerly part of the larger constellation of Argo Navis until that constellation was divided in three....
) are the two principal stars used for nomadic wandering at night. Because it was circumpolar and hence always visible, it became associated with a steadfast nature, as opposed to Suhail, which disappears below the horizon and hence 'flees'.

A monkey

Monkey

A monkey is a nonhuman primate mammal with the exception usually of the lemurs and tarsiers. More specifically, the term monkey refers to a subset of monkeys: any of the smaller longer-tailed catarrhine or platyrrhine primates as contrasted with the apes....
's head is the emblem of the Mayan god of the pole star. In Hindu mythology

Hindu mythology

Hindu mythology is the large body of traditional narratives related to Hinduism, notably as contained in Sanskrit literature, such as the Sanskrit epics and the Puranas....
it's termed as Dhruva

Dhruva

Dhruva, ?????, in Hindu Mythology, was an ardent young devotee of Vishnu, a prince blessed to eternal existence and glory as the Pole Star by Lord Vishnu....
, meaning fixed.

Polaris

Pole Star

Etymology and cultural significance

See also

External links