Polaris is the brightest star in the constellation of Ursa Minor. It is a triple star system and currently, it is our North Star or Pole Star. This title is given to stars that are close to the North Pole – something that changes through the passing of years due to Earth’s movement.
Key Facts & Summary
- Polaris is around 433 light-years / 133 parsecs away from the Sun.
- The primary star composing the Polaris triple star system is Polaris Aa – a yellow supergiant.
- It is in orbit with a smaller companion named Polaris Ab while the pair is in orbit with Polaris B.
- Polaris Aa has around 5.4 solar masses and it is an F7 yellow supergiant of spectral type lb. It is the first classical Cepheid to have a mass determined from its orbit.
- The other two stars are considerably smaller, with Polaris B having an estimated 1.39 solar masses and Polaris Ab 1.26 solar masses.
- Polaris B is an F3 main-sequence star orbiting at a distance of 2.400 AU while Polaris Ab is a close F6 main-sequence star with an 18.8 radius orbit.
- Polaris B can be seen with a modest telescope while Polaris Ab is outshined by Polaris A and impossible to spot without advanced tech.
- Polaris A has an apparent magnitude of 1.88 however the star is variable between 1.86 up to 2.13. It is a low-amplitude Population I classical Cepheid variable star.
- The name Polaris was coined in the Renaissance era. It is a shortened version from Latin “stella Polaris” – which translates to polar star.
- Polaris is the only star in the night sky that is stationary – or at least it appears as such – this is because it is currently positioned above the northern axis of Earth’s rotation.
- For more than 2.000 years Polaris has been aligned above the North Pole, but after 2100, Earth’s northern axis will begin to move away from the star.
- Since Polaris always points north, it has been used in aiding navigators for centuries, although it is only visible in the northern hemisphere.
Many people have relied on Polaris for navigation since at least from Late Antiquity. Though it was not always as such, Polaris wasn’t always the North Star.
From around the year 2.500 B.C., the celestial pole was near Thuban, located in the constellation Draco. Practically Thuban was the North Star when the Egyptians built the pyramids. Later, in the year 400 B.C., the pole was closer to Kochab, Beta Ursae Minoris, than it was to Alpha UMi. Polaris served as the North Star for a long time ago, for example, it served its purpose five centuries ago when the Europeans crossed the Atlantic Ocean.
Sometime in Antiquity, the constellation of Ursa Minor was used for navigation as a whole rather than a single star. Some sources claim that Polaris began to be used separately in the early medieval period since it wasn’t close/bright enough for us to see it.
Regardless, Polaris is a bright star now but it only ranks as the 50th brightest star in the night sky though it might have been even brighter in the past.
The Polaris system and its stars are believed to have formed around 70 million years ago from a nebular cloud of gas and dust. Gravity pulled the swirling gas and dust together and resulted in the triple star system that we now see and still use for navigation.
Distance, Size, and Mass
Polaris is around 433 light-years / 133 parsecs away from us based on the revised Hipparcos parallax. Some different means of calculations suggest that the star is actually 30% closer than estimated. But generally, the parallax remains at 433 light-years until further observations prove otherwise.
Polaris is around 50 times bigger than our sun. It has an estimated diameter of around 44 million miles / 70 million kilometers, and a radius of about 22 million miles / 35 million kilometers.
Its mass is estimated to be around 5.4 times that of our sun.
In terms of temperature, Polaris is slightly hotter than our sun. It has an estimated surface temperature of around 6.015 Kelvins. When it comes to its luminosity though, Polaris is around 2.900 times brighter than our sun.
It has a rotational period of 119 days and has about 112% solar metallicity. Its rotational velocity is speculated to be around 12 km / 6 mi per second and has a surface gravity of around 2.2 cgs. It is the 50th brightest star in the night sky.
Polaris is a triple star system with each star being gravitationally bound to the other. Polaris Aa is the prime star of the system having around 5.4 solar masses and it is an F7 yellow supergiant of spectral type lb.
The other two stars are similar in temperatures but much smaller. Polaris B has only 1.39 solar masses being a main-sequence star of spectral type F3 and orbiting at a distance of 2.400 AU or around 240 billion mi / 390 billion km.
Polaris Ab appears to have the smallest mass, at around 1.26 solar masses. It is an F6 main-sequence star with an 18.8 AU radius orbit or around 2 billion mi / 3.2 billion km away from Polaris Aa. This is almost the same distance between the Sun and Uranus.
For a period of time, two other stars were believed to be part of the Polaris system but further studies concluded that they didn’t have any association with the system.
Polaris Ab is 3 times more luminous than our sun, while Polaris B is almost 4 times though it would appear that Polaris B is the hottest star of the system with an average surface temperature of 6.900 Kelvins.
Role as Pole Star
Polaris almost appears motionless as it stands directly “above” the Earth’s rotational axis. Because of this, it is an excellent fixed point from which to draw measurements for celestial navigation and for astrometry.
The star has been used for navigation for many years and it will continue to do so in the future. Due to the precession of the equinoxes, Polaris will slowly move away from the celestial pole and another star will take its place.
Polaris is located in the constellation of Ursa Minor – the Little Bear. The seven stars from which we derive a bear are also known as the Little Dipper.
Polaris lies at the end of the handle of the Little Dipper. The other stars are rather faint in comparison. To find Polaris, one can draw a line between the stars Dubhe and Merak and extend it about 5 times. This will eventually lead to Polaris.
Polaris will be replaced by Vega – a much brighter star – as the North Pole star, though this won’t happen until the year 14.000. Therefore, we can still marvel and enjoy Polaris as the North Star for many years to come.
Did you know?
- Polaris is actually more accurate than any compass.
- Sir William Herschel was the first astronomer to sight Polaris B in 1780. One year later, he discovered the planet Uranus.
- The third companion, which was later revealed as a white dwarf was discovered much later even though it was closer than Polaris B. This happened in 1929 through spectrum analysis.
- The stars Dubhe and Merak seen at the front of the Little Dipper’s bowl appear to march around Polaris like sentries. Because of this, they have been named “Guardians of the Pole”.
- Columbus mentioned the two stars in the log of his famous journey across the ocean. Many navigators used them to measure the hour of the night and their position upon the sea – through the position of the stars relative to Polaris.