Polaris
Polaris (α UMi / α Ursae Minoris /
Alpha Ursae Minoris) is the brightest star in the
constellation Ursa Minor. It is very close to the north
celestial pole (42′ away
as of 2006), making it the current northern
pole star.
"Polaris" comes from
Stella Polaris, the
Latin form of its common name "Pole Star". The rarely used
Greek name
Cynosura (Κυνόσουρα) means "tail of the dog" and is the source of the English word
cynosure. Other names include the "
North Star", the "Lode Star", or sometimes "Polaris Borealis".
Polaris is a
trinary system, consisting of a large yellow
Cepheid variable (α UMi A), distantly (about 2700
AU or 380 billion km) orbited by a bright
yellow dwarf (α UMi B). Polaris B can be seen with even a modest telescope and was first noticed by
William Herschel in 1780. In 1929, it was discovered by examining the
spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab). In January 2006,
NASA released images from the
Hubble telescope, directly showing all three members of the Polaris trinary system. The nearer dwarf star is in an orbit of only 20 AU (3 billion km) from Polaris A, explaining why its light is swamped by its close and much brighter companion.
Polaris is 431
light years (132
pc) from Earth, according to
astrometric measurements of the
Hipparcos satellite. Concerning the detailed physics, α UMi A is an
F7 supergiant (
Ib) or bright giant (
II). The two smaller companions are: α UMi
B an
F3V main sequence star, orbiting in 2700 AU distance, and
C a very close dwarf on a 20 AU orbit. Recent observations show that Polaris may be part of a loose
open cluster of type
A and
F stars.
The giant star of Polaris is a classical
Population I Cepheid variable (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 suggests that Polaris is 2.5 times brighter today than when
Ptolemy observed it (now 2mag, antiquity 3mag). The astronomer Edward Guinan 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."
Because α UMi lies nearly in a direct line with the axis of the
Earth's rotation "above" the
North Pole — 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 and for
astrometry. The antiquity of its use is attested by the fact that it is found represented on the earliest known
Assyrian
tablets. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the
Moon 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; the rest of the time it is only an approximation and must be corrected using tables or a rough
rule of thumb.
|
An artist's concept of Polaris' system |
Due to the
precession of the
equinox, 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 and
Vega. In the near future, Polaris is due to become an even more accurate pole star; the distance between Polaris and the pole will reach a minimum (of just under 1/2 degree) in 2100.
In the
Northern Hemisphere, it is easy to find Polaris by following the line traced from
Merak to
Dubhe (β and α
Ursae Majoris, also known as the Pointers), the two stars at the end of the bowl of the
Big Dipper. One can also follow the central point of the "W" shape of
Cassiopeia. Polaris is not visible from the
Southern Hemisphere except from an elevated position near the
equator.
Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky. Although Polaris is a relatively bright star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 46th brightest star in the night sky. The brightest star in the sky (besides the
Sun) is
Sirius (see
Winter sky and
List of brightest stars).
There is no real
southern pole star. The only star visible to the
naked eye that is close to the south celestial pole is the dim
Sigma Octantis, sometimes called Polaris
Australis. However, the bright Southern Cross (
Crux) points fairly accurately towards the south celestial pole.
*In
astrology Polaris is one of the
Behenian fixed stars and has the symbol
.
*In
DC Comics, Polaris is the star around which
Thanagar, home planet of the
Hawks,
orbits.
*The Polarans are also a faction in
Ambrosia Software's
Escape Velocity Nova.
*In the computer game
Freespace 2, Polaris is the capital system of the
Neo-Terran Front during their rebellion against the
Galactic Terran Vasudan Alliance.
*The constellation
Ursa Major is inscribed on the
Latter-day Saint temple in
Salt Lake City,
Utah. This is symbolic of the use of the constellation to find Polaris in the sky, which in turn points north. The temple similarly provides direction for members of the church.
*
Info on Polaris*
Polaris: astrometric orbit, position, and proper motion*
Polaris Ab imaged by Hubble
