How big is Pollux?

Pollux is a giant star in the constellation Gemini, about 34 light-years from Earth, where a light-year is the distance light travels in one year. It is about 12 million kilometers (7.5 million miles) across, about 9 times the width of the Sun. That figure is the width of the star's visible surface, worked out from how wide the star looks in the sky and how far away it is.

Size

A star has no solid surface, so its size is the width of the glowing layer that its light comes from. Pollux's apparent width was measured by interferometry, which combines the light from several telescopes to see detail too fine for any one of them. It is 7.96 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree. That is the width of a human hair seen from about 2.6 kilometers (1.6 miles) away.

Combined with the star's distance, measured by the Hipparcos satellite, that angle gives a radius 8.8 times the Sun's, or a width of about 12.2 million kilometers. Two other methods agree with it to within about 6%. Computer models that match the star's temperature and brightness give 8.3 times the Sun's radius. The star also vibrates in many small, regular rhythms, and those rhythms give 9.3 times.

Size comparisons

Pollux is about 9 times as wide as the Sun, and about 700 Suns would fit inside it, by volume.
About 960 Earths side by side would stretch across it.
If Pollux took the Sun's place, its surface would reach about one-tenth of the way to the orbit of Mercury.
The main star of Spica is about 10.4 million kilometers across, so Pollux is about one-sixth wider.
If the Sun were shrunk to the size of a basketball, Pollux would be a ball about 2.1 meters (7 feet) across, taller than a person 1.7 meters tall.

An aging giant

Pollux is further along in its life than the Sun and has nearly twice the Sun's mass. Giant stars like it began as hotter, faster-spinning stars. Its surface is now about 4,400–4,600 degrees Celsius, roughly 1,000 degrees cooler than the Sun's.

A probable planet

In 1993, astronomers reported that Pollux moves slightly toward and away from Earth in a regular cycle. In 2006, measurements spanning more than 25 years showed that the cycle had kept the same rhythm, about 590 days. That is the wobble a planet would cause by pulling on its star. A separate study at Lick Observatory confirmed the same period that year.

The planet would have at least 2.3 times the mass of Jupiter and would orbit about 1.6 times as far from Pollux as Earth is from the Sun. Giant stars are useful places to look for planets around stars more massive than the Sun. Before they became giants, such stars were hot and spun fast, which made a planet's small tug almost impossible to measure.

Pollux also has a weak magnetic field, and in 2021 astronomers used it to time the star's spin: about once every 660 days. If the spin and the 590-day cycle turned out to be the same length, activity on the star's surface could explain the wobble without a planet. For now, the planet is likely but not certain.