The North Star, known to astronomers as Polaris, has long served as a fixed point in the northern night sky, guiding travelers and explorers. This famous star is often misunderstood, particularly regarding its brightness and its true color. Its visual appearance is determined by its intrinsic physical properties combined with the distorting effects of Earth’s atmosphere.
Polaris: Our Current North Star
Polaris is the brightest star in the constellation Ursa Minor, or the Little Dipper, and its fame comes from its position rather than its luminosity. It currently holds the status of the northern pole star because it is aligned almost directly above Earth’s axis of rotation. This near-perfect alignment means that as our planet spins, Polaris appears to remain motionless in the sky.
Because of its fixed position, all other stars in the northern celestial hemisphere appear to revolve around Polaris over the course of a night. This stability makes it an indispensable tool in celestial navigation, as its elevation above the horizon directly corresponds to an observer’s latitude in the Northern Hemisphere. The star is not perfectly stationary, however, as it traces a small, tight circle in the sky, less than one degree away from the true North Celestial Pole.
The Earth’s axis experiences a slow, 26,000-year wobble, called axial precession, which causes the pole star to change over vast stretches of time. In about 13,000 years, the bright star Vega will take on the role, and Polaris will eventually return to its current position. For now, Polaris remains the celestial beacon for the Northern Hemisphere, providing a reliable marker that anchors the rotation of the entire night sky.
The Observed Color Versus Reality
To the unaided eye, Polaris appears as a white or very pale yellow point of light, which is consistent with its scientific classification. Though it is not the brightest star in the night sky, ranking about 48th in apparent magnitude, its isolated location makes it easy to spot. The star we observe is actually the primary component of a triple-star system, a yellow-white supergiant designated Polaris Aa.
While the star’s color is intrinsically yellow-white, atmospheric effects can sometimes create the illusion of color shifts. As starlight travels through Earth’s atmosphere, it encounters pockets of air with varying temperatures and densities that refract the light. This effect, called atmospheric scintillation, causes stars to twinkle, and sometimes the dispersion of light creates momentary color flashes.
Polaris often appears to twinkle less than stars lower on the horizon because its high altitude means its light passes through less of the turbulent atmosphere. Stars closer to the horizon must travel through a greater amount of atmospheric mass, making their twinkling and color changes much more pronounced. Therefore, the observed color of Polaris remains mostly stable, appearing consistently white with a hint of yellow.
The Physics of Stellar Color
The specific color of Polaris is determined by its surface temperature, a relationship foundational to stellar classification. Stars radiate energy across the electromagnetic spectrum, and the peak wavelength of that radiation, which dictates the star’s color, is directly linked to its temperature. The hotter a star is, the shorter the wavelength of light it emits most intensely.
Astronomers use the OBAFGKM spectral sequence to categorize stars, where O-type stars are the hottest and blue, and M-type stars are the coolest and red. Polaris is an F-type star, specifically classified as F7Ib, which places it firmly in the yellow-white range of the sequence. This classification corresponds to a surface temperature of approximately 6,000 to 7,500 Kelvin.
Stars like Polaris, which have temperatures similar to the Sun (a G-type star at about 5,778 Kelvin), emit most of their light in the green-yellow portion of the spectrum. Because they emit significant amounts of light across all visible colors, the human eye perceives the combined light as white or yellow-white. This yellow-white color is a direct physical result of its temperature.