How big is Polaris?

Polaris is today's North Star, and its main star is the nearest and brightest known example of the pulsating stars called Cepheids. That star is about 64 million kilometers (40 million miles) across, about 46 times the width of the Sun, according to measurements published in 2024. The figure is the width of its visible surface, which swells and shrinks very slightly as the star pulsates.

Size

Polaris is really three stars, and the explorer shows the brightest, called Polaris Aa. Between 2016 and 2021, astronomers measured how wide it looks with the CHARA Array on Mount Wilson in California, which combines the light of several telescopes to see very fine detail. It is about 3.1 thousandths of an arcsecond across, where an arcsecond is 1/3,600 of a degree. That is the width of a human hair seen from about 6.6 kilometers (4.1 miles) away.

Turning that angle into a true size needs the distance, which was long disputed. Polaris itself is too bright for some of the instruments that measure distances, and published estimates have ranged from about 320 to 520 light-years. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The 2024 study used the distance to Polaris B, a fainter star in the same system, measured by the Gaia spacecraft: about 447 light-years.

At that distance, Polaris Aa's radius is 46.3 times the Sun's. A measurement from 2006, made with a different instrument, gives 46 times the Sun's at the same distance. At the shortest of the published distances, the same apparent width would make the star only about 33 times as wide as the Sun.

Polaris Aa is a variable star. It pulsates about every four days, and its width is expected to change by only about 0.4% over each cycle, or about 260,000 kilometers.

Size comparisons

Polaris Aa is about 46 times as wide as the Sun and about 5,000 times as wide as Earth.
About 99,000 Suns would fit inside it, by volume.
If it took the Sun's place, its surface would reach more than halfway to the orbit of Mercury.
It is about 1.8 times as wide as Arcturus, another giant star in the explorer. Aldebaran, the next smaller star in the explorer, is almost the same width, about 63 million kilometers across.
If the Sun were shrunk to the size of a basketball, Polaris Aa would be a ball about 11 meters (36 feet) across.

The North Star

Polaris is the North Star because Earth's axis of rotation points close to it. The axis slowly wobbles, like a spinning top, in a cycle of about 25,800 years called precession, so the North Star changes over time. A couple of thousand years ago, the North Stars were two other stars, Kochab and Pherkad.

A star with company

Polaris Aa has a close companion, Polaris Ab, which circles it about every 30 years in a stretched, oval orbit. At their closest, the two are about 6.2 astronomical units apart, where one astronomical unit is the average distance from Earth to the Sun. By tracking this orbit, the 2024 study found that Polaris Aa has 5.13 times the Sun's mass. The third star, Polaris B, is 18 arcseconds away in the sky.

Polaris Aa is unusual in other ways, too. Its pulsations grew weaker for much of the 20th century and began to grow stronger again around 2000. Images of its surface made from the CHARA measurements show what appear to be spots, including a large bright one, although some of the features may be flaws in how the images were made.