Polaris, known universally as the North Star, is often mislabeled in popular media as a supergiant star. The primary component of the system, Polaris A, is not a true supergiant but is more accurately categorized as a bright giant star. This complex classification places it in an intermediate stage of stellar evolution. It is highly luminous and massive, but it does not reach the size and brightness extremes of a full supergiant.
The Actual Stellar Classification of Polaris
The most precise astronomical classification for Polaris A is an F7 Ib-II star. This designation is derived from the Morgan-Keenan system, which places stars on the Hertzsprung-Russell (H-R) diagram based on their surface temperature and luminosity. The letter “F” indicates its yellow-white color and surface temperature of about 7,200 Kelvin.
The key to its classification lies in the Roman numerals Ib-II, which denote its luminosity class. Class II stars are termed bright giants, indicating Polaris A is a massive star that has evolved off the main sequence and expanded significantly. Polaris A is estimated to have a mass of approximately 5.4 to 6.5 times that of the Sun and shines with the luminosity of about 2,500 Suns. Although it has ceased core hydrogen fusion, it has not yet undergone the full expansion necessary to be categorized as a definitive, high-luminosity class I supergiant.
Distinguishing Supergiants from Bright Giants
Stellar classification uses the luminosity class to separate stars of the same temperature but different sizes and brightness. Luminosity class I is reserved for the most expansive and luminous stars, the true supergiants, which are further divided into Ia (most luminous) and Ib (less luminous). These stars, such as Betelgeuse or Rigel, are at the extreme end of stellar size, often hundreds of times the Sun’s diameter, representing a later stage of evolution than Polaris.
Bright giants, designated as luminosity class II, represent a distinct, slightly less extreme category. These stars are substantially larger and brighter than normal giants (Class III) but fall short of the absolute magnitude and radius thresholds of supergiants. Polaris A, with a radius about 46 times that of the Sun, fits perfectly into this intermediate class. Its Ib-II classification reflects the fact that its properties straddle the boundary between a lower-luminosity supergiant and a bright giant.
The Unique Physical Characteristics of Polaris
Beyond its steady classification, Polaris A is scientifically significant because it is the closest classical Cepheid variable star to Earth. Cepheids are massive, young, pulsating stars whose brightness fluctuates in a predictable cycle due to internal expansion and contraction. This pulsation is caused by helium ionization in the star’s outer layers, which acts like a valve to periodically trap and release energy.
Polaris A’s brightness varies with a period of approximately four days, a cycle that has been precisely tracked by astronomers. For decades, the amplitude of its brightness variation was steadily decreasing, leading to speculation that it might stop pulsating entirely. However, recent observations suggest its variability may be increasing again, which makes Polaris a unique laboratory for studying the evolution of Cepheid stars.
The star system itself is not a single entity but a triple-star system. The main star, Polaris A, is orbited by two companions. The most distant companion, Polaris B, is an F3V main-sequence star that orbits Polaris A at a distance of about 2,400 astronomical units. A much closer companion, Polaris Ab, is a low-mass F6 main-sequence star that orbits the primary in a very tight path. The presence of these companions allows astronomers to calculate the mass of the primary star with greater accuracy, which is essential for understanding the physics of Cepheid variables.
Polaris’s Astronomical Role as the North Star
The fame of Polaris stems not from its stellar classification but from its alignment with Earth’s axis of rotation. Polaris is currently positioned less than one degree away from the North Celestial Pole, which is the point in the sky directly above Earth’s North Pole. This proximity makes it appear nearly motionless in the sky, while all other stars in the Northern Hemisphere appear to circle around it.
This role as the North Star, or Pole Star, is a temporary coincidence due to a phenomenon called axial precession. Precession is a slow, wobble-like motion of the Earth’s axis, similar to a spinning top that is slowing down. This wobble causes the North Celestial Pole to trace a large circle in the sky over a period of roughly 26,000 years.
The alignment of Polaris with the pole is continually changing. Around 2,500 BCE, the star Thuban in the constellation Draco served as the pole star. Polaris will reach its closest alignment to the pole in the 21st century before the pole begins to drift away toward other stars. In approximately 12,000 years, the bright star Vega will take over the role of the northern pole star.