How big is Wolf 359?

Wolf 359 is a small, dim red star about 7.9 light-years from Earth, one of the Sun's nearest neighbors. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Wolf 359 is about 200,000 kilometers (124,000 miles) across, about one-seventh the width of the Sun. That is the width of the star's visible surface, and some estimates put it about 10% larger.

Size

Wolf 359 is a red dwarf, the most common kind of star in the Milky Way. Red dwarfs are smaller, cooler and dimmer than the Sun. Wolf 359 has about 11% of the Sun's mass, and its surface is a little less than half as hot as the Sun's.

In 2019 astronomers measured how wide the star looks from Earth, using the Very Large Telescope Interferometer in Chile. Interferometry combines the light of several telescopes to see finer detail than any one of them could. Together with the star's distance, the measurement gave a width of about 221,000 kilometers.

A 2021 study combined the same measurement with the star's brightness and with relationships built from other red dwarfs. It found a radius of 0.144 times the Sun's, which makes Wolf 359 about 200,000 kilometers across, give or take about 3%. The figure shown above matches that study.

Size comparisons

The Sun is about seven times as wide as Wolf 359 and about nine times as massive.
Wolf 359 is about 1.4 times as wide as Jupiter, but about 115 times as massive.
Proxima Centauri, the closest star to the Sun and the next larger object in the explorer, is about the same width as Wolf 359. Studies that measured both stars the same way found them within a few percent of each other.
Wolf 359's width is about half the average distance from Earth to the Moon.
If the Sun were shrunk to the size of a basketball, Wolf 359 would be a ball about 3.5 centimeters (1.4 inches) across, about 12,800 kilometers (8,000 miles) away. That distance is about the width of Earth.

A flaring star

Wolf 359 is a flare star: it often brightens suddenly for a short time as it releases a burst of energy. Its flares are strong even for a red dwarf. A 2022 study that watched it for 27 hours caught 13 flares, a rate of about one every two hours.

Activity this strong fits with the star being young. Wolf 359 also spins quickly, once every 2.7 days. A 2006 study estimated its age at about 100–350 million years, though other estimates run as high as 1.5 billion years.

The search for planets

In 2019 astronomers reported two possible planets around Wolf 359, found from tiny back-and-forth wobbles in the star's motion. A 2021 study showed that the signal of the inner one matched the star's own rotation, so it was not a planet. The outer candidate would take about eight years to circle the star.

Because Wolf 359 is so close and so young, it is a good target for photographing planets directly in infrared light. A 2023 search, which described Wolf 359's system as the fifth closest to the Sun, combined such images from the Keck Observatory with the wobble measurements. It found no brown dwarf or companion star within 10 astronomical units, where one astronomical unit is the average distance from Earth to the Sun. It could neither confirm nor rule out the outer candidate.