Alpha Centauri B is the smaller and fainter of the two Sun-like stars at the heart of Alpha Centauri, the nearest star system to the Sun. It is about 1.2 million kilometers (750,000 miles) across, about 86% of the width of the Sun. That figure is the width of its photosphere, the glowing layer that looks like a star's surface.

Size

Alpha Centauri B was measured with an interferometer, which combines the light from several telescopes so that together they see much finer detail than any one of them. In a study published in 2017, a team led by Pierre Kervella used the European Southern Observatory's Very Large Telescope Interferometer. They found that the star appears 6.0 thousandths of an arcsecond wide, where an arcsecond is 1/3,600 of a degree.

Combined with the star's distance, that gives a radius 0.8632 times the Sun's, and a width of about 1.20 million kilometers. The measurement is precise to about half a percent. An earlier measurement with a different instrument, published in 2003, found the same size.

Alpha Centauri B belongs to a different class of star from the Sun: K1 V, where the Sun is G2 V. Its surface is about 540 degrees Celsius cooler than the Sun's, and it has about 94% of the Sun's mass. Being both smaller and cooler, it gives off about half as much light as the Sun.

Size comparisons

• Alpha Centauri B is about 86% as wide as the Sun, the next larger star in the explorer, and has nearly two-thirds of the Sun's volume.
• It is about 94 times as wide as Earth.
• Its partner, Alpha Centauri A, is about 1.4 times as wide.
• It is about 5.6 times as wide as Proxima Centauri, the small red dwarf that is the third star of the system.
• If Alpha Centauri B were shrunk to the size of a basketball, Alpha Centauri A would be a ball 34 centimeters (13.4 inches) across. The two stars would circle each other between about 340 meters and 1.1 kilometers apart.

A pair of stars

Alpha Centauri A and B orbit each other once every 79.9 years. Their orbit is a stretched oval, so the distance between them changes from about 11 to 36 astronomical units. One astronomical unit is the average distance from Earth to the Sun, about 150 million kilometers.

The pair is about 4.37 light-years from the Sun, where a light-year is the distance light travels in one year, about 9.5 trillion kilometers. Proxima Centauri is slightly closer to the Sun, at 4.24 light-years.

A planet that probably does not exist

In October 2012, the European Southern Observatory (ESO) announced a planet with about Earth's mass orbiting Alpha Centauri B every 3.2 days. It had been found with the HARPS instrument at La Silla Observatory in Chile. HARPS looked for a tiny back-and-forth wobble in the star's motion, caused by the pull of a planet's gravity. The wobble was no more than 51 centimeters per second, about the speed of a crawling baby.

In 2015, Vinesh Rajpaul and colleagues reanalyzed the same measurements. They showed that the signal almost certainly came from the pattern of dates on which the star had been observed, not from a planet. In January 2017, ESO added a note to its announcement saying that the planet probably does not exist.

Sources

• The radii and limb darkenings of Alpha Centauri A and B: Interferometric measurements with VLTI/PIONIER. Kervella et al., Astronomy & Astrophysics, 2017.
• The diameters of Alpha Centauri A and B: A comparison of the asteroseismic and VINCI/VLTI views. Kervella et al., Astronomy & Astrophysics, 2003.
• Close stellar conjunctions of alpha Centauri A and B until 2050. Kervella et al., Astronomy & Astrophysics, 2016.
• A discontinuity in the Teff-radius relation of M-dwarfs. Rabus et al., Monthly Notices of the Royal Astronomical Society, 2019.
• Planet Found in Nearest Star System to Earth. European Southern Observatory, 2012.
• Ghost in the time series: no planet for Alpha Cen B. Rajpaul et al., Monthly Notices of the Royal Astronomical Society: Letters, 2015.
• Sun: Facts. NASA, 2026.
• Sun Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.