Polaris, officially designated Alpha Ursae Minoris, is a star approximately 433 light-years from Earth. Known universally as the North Star, or Pole Star, its fame stems from its seemingly fixed location in the night sky, making it an invaluable beacon for navigation for centuries. Polaris’s significance extends beyond navigation; it is a complex physical object, classified as a yellow supergiant and the brightest member of a multiple-star system. Its relationship to Earth’s rotation, stellar variability, and composition makes it an object of continuous scientific interest.
Its Role as the North Star
Polaris is celebrated because its position aligns almost perfectly with the projection of Earth’s rotational axis into space, known as the north celestial pole. This precise alignment causes the star to appear stationary in the sky to observers in the Northern Hemisphere, while all other stars seem to circle around it. Currently, Polaris is less than one degree away from this true celestial pole.
This fixed position made Polaris a foundational tool for celestial navigation throughout history, providing a constant reference point for direction. The star’s angle above the horizon also directly corresponds to the observer’s latitude, offering a simple method for determining geographic location.
The star will not hold this privileged position forever because of a slow wobble in Earth’s axis called precession. This cycle takes roughly 26,000 years to complete, causing the celestial pole to trace a large circle across the sky. Polaris will reach its closest alignment, approximately 0.45 degrees away, around the year 2100 before slowly drifting away.
Polaris is a Variable Star
Beyond its navigational role, Polaris is a classical Cepheid variable star, meaning its brightness changes predictably over time. These stars are yellow supergiant pulsators whose outer layers expand and contract in a regular cycle, causing a periodic variation in luminosity. Polaris’s primary component, Polaris A, has a pulsation period of approximately four days.
Cepheid variables are useful in astronomy because their pulsation period is directly related to their intrinsic brightness, making them “standard candles” for measuring vast cosmic distances. Polaris is the closest and brightest Cepheid, making it an anchor for calibrating the cosmic distance ladder used to measure the size of the universe.
Unusually, Polaris has exhibited strange behavior compared to other Cepheids, with its pulsation amplitude decreasing significantly for much of the 20th century. For example, the star’s brightness variation dropped from about 0.15 magnitudes before 1960 to as low as 0.05 magnitudes a few years later. Although its pulsation period has been steadily increasing, recent data suggests this trend may be changing, making Polaris unique for studying the evolution and mechanics of these variable stars.
Structure of the Polaris System
What appears as a single point of light is actually a hierarchical triple-star system. The system is dominated by Polaris A, the yellow supergiant and Cepheid variable. Closely orbiting Polaris A is a smaller companion star, designated Polaris Ab, which is an F6 main-sequence star.
The third component is Polaris B, a more distant F3 main-sequence star that orbits the inner pair at a separation of roughly 2,400 astronomical units. Polaris B is bright enough to be resolved from Polaris A using a modest amateur telescope. This arrangement provides astronomers with a natural laboratory to study the complex interactions of a three-body system.