Polaris
The current northern pole star and closest Cepheid variable.
Polaris, also known as Alpha Ursae Minoris and commonly called the North Star, is a yellow supergiant star located in the northern circumpolar constellation Ursa Minor. It is the brightest star in that constellation, with an apparent magnitude that varies around 1.98, and is easily visible to the naked eye at night. Because it sits less than one degree from the north celestial pole, it currently serves as the northern pole star, and its steady position in the northern sky makes it a useful navigational aid.
Though it appears as a single point of light to the naked eye, Polaris is actually a triple star system. The primary component, a yellow supergiant designated Polaris Aa, orbits a smaller companion called Polaris Ab, and this pair almost certainly orbits a third star, Polaris B, at a wider distance. Polaris B was discovered in August 1779 by William Herschel, while the inner pair was not confirmed until the early 20th century.
As the closest Cepheid variable star, Polaris Aa plays a key role in the cosmic distance ladder. The revised Hipparcos stellar parallax gives a distance to Polaris A, while the Gaia mission provides a distance for Polaris B.
Polaris Aa is an evolved yellow supergiant of spectral type F7Ib, with a mass of 5.4 solar masses. It is the first classical Cepheid to have its mass determined from its orbit. The two smaller companions are Polaris B, an F3 main-sequence star orbiting at a certain distance, and Polaris Ab (or P), a very close F6 main-sequence star with a given mass. In January 2006, NASA released Hubble Space Telescope images showing all three members of the system.
Polaris B can be resolved with a modest telescope. William Herschel discovered it in August 1779 using one of the best reflecting telescopes of his time.
The variable radial velocity of Polaris A was reported by W. W. Campbell in 1899, suggesting it is a binary system. Since Polaris A is a known Cepheid variable, J. H. Moore demonstrated in 1927 that changes in its line-of-sight velocity came from a combination of a four-day pulsation period and a much longer orbital period with a large eccentricity of about 0.6. Moore published preliminary orbital elements in 1929, giving an orbital period of about 29.7 years and an eccentricity of 0.63. This period was confirmed by proper motion studies by B. P. Gerasimovič in 1939. As part of her doctoral thesis in 1955, E.
Quick Facts
- Spectral type
- F7Ib (Polaris Aa)
- Companions
- Polaris Ab (F6 main-sequence), Polaris B (F3 main-sequence)
- Orbital period
- 29.7 years (preliminary, 1929)
- 30.46 years (1955)
- refined later
- Eccentricity
- 0.6–0.64
Facts from the source article.
Lore & Background
Polaris appears to the naked eye as a single point of light but is a triple star system. The primary, Polaris Aa, is an evolved yellow supergiant and a classical Cepheid variable with a low amplitude. Its variability was suspected since 1852 and confirmed by Ejnar Hertzsprung in 1911. The brightness range is 1.86–2.13, but the amplitude has changed over time, decreasing rapidly after 1966 and later erratically varying. The period of roughly 4 days has steadily increased by about 4.5 seconds per year except for a hiatus in 1963–1965. Authors disagree on whether Polaris is a fundamental or first-overtone pulsator and on whether it is crossing the instability strip for the first time. The outer companion Polaris B was discovered in August 1779 by William Herschel. The inner binary Aa/Ab was confirmed in the early 20th century. In 2024, researchers using the CHARA Array improved the orbit solution and confirmed the Cepheid radius and mass.
Reader's Guide
Polaris's role as the pole star stems from its proximity to the north celestial pole, making it nearly motionless in the sky while all northern stars appear to rotate around it. This provides a fixed point for celestial navigation and astrometry; the star's elevation above the horizon gives the approximate latitude of the observer. Despite popular belief, it is not the brightest star in the night sky, ranking 47th in apparent magnitude. As the closest Cepheid variable, Polaris Aa is foundational to the cosmic distance ladder. Its distance, revised by Hipparcos and later by Gaia, anchors measurements of other Cepheids. The star's brightness has reportedly increased since Ptolemy's time, a change described as 100 times larger than predicted by current stellar evolution theories. The system's peculiarities—rapid period change, amplitude variations, and possible tidal forcing from the companion—continue to challenge models. In 2024, tentative imaging of surface features on Polaris Aa showed large bright and dark patches that change over time.
More in Named Stars
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced