Read this summary of Polaris and list three new facts you learned about Polaris and list three questions
you have after reading this summary. We will discuss them in class on Wednesday and I will collect
your lists.
Polaris
(From Wikipedia, the free encyclopedia)
Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris), more commonly known as The North Star
or simply North Star, 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 "Polar Star". The rarely used
Cynosura from the Greek name Κυνόσουρα means "the dog-tailed one" and is the source of the English
word cynosure. Other names include the "North Star / Northern Star", the "Lode Star / Load Star", or
sometimes "Polaris Borealis".
Physical properties
Polaris is a trinary system, consisting of a large
yellow Cepheid variable (α UMi A), orbited by a
bright yellow dwarf (α UMi B) at a distance of
about 2400 AU (360 billion kilometers, or 224
billion miles). 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 18.5 AU (2.8 billion km; about the distance
from our Sun to Uranus) 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 bright giant (II) or supergiant (Ib). The two
smaller companions are: α UMi B an F3V main sequence star, orbiting in 2400 AU distance, and C a
very close dwarf on a 18.5 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."
Pole Star
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.
Due to the precession of the equinoxes, 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 ½ 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 48th brightest star in the night
sky. The brightest star in the sky (besides the Sun) is Sirius.
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.