How big is Alpha Centauri A?

Alpha Centauri A is the larger and brighter of the two Sun-like stars at the heart of Alpha Centauri, the nearest star system to the Sun. It is about 1.7 million kilometers (1.1 million miles) across, about 1.2 times the width of the Sun. That figure is the width of its photosphere, the glowing layer that looks like a star's surface.

Size

A star has no solid surface, so its width is measured to the photosphere. Astronomers measured Alpha Centauri A 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 four telescopes of the European Southern Observatory's Very Large Telescope Interferometer.

The star appears 8.5 thousandths of an arcsecond wide, where an arcsecond is 1/3,600 of a degree. Combined with its distance, that gives a radius 1.2234 times the Sun's, and a width of about 1.70 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 A is the same type of star as the Sun, classed G2 V, with almost the same surface temperature. It has about 1.1 times the Sun's mass. Because it is wider, it gives off about 1.5 times as much light.

Size comparisons

Alpha Centauri A is about 1.2 times as wide as the Sun, the next smaller star in the explorer, and about 1.8 times as large by volume.
It is about 134 times as wide as Earth.
Its partner, Alpha Centauri B, is about seven-tenths as wide.
If the Sun were shrunk to the size of a basketball, Alpha Centauri A would be a ball about 29 centimeters (11.6 inches) across, and Alpha Centauri B one about 21 centimeters (8.2 inches) across. The two would circle each other between about 300 and 900 meters 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. The system's third star, the red dwarf Proxima Centauri, is slightly closer to the Sun, at 4.24 light-years.

A possible planet

In August 2024, the James Webb Space Telescope took an infrared image that showed a faint point of light near Alpha Centauri A. A team led by Charles Beichman and Aniket Sanghi reported in 2025 that it could be a giant planet in the star's habitable zone. That is the range of distances from a star where liquid water could exist on a planet's surface.

If it is a planet, it is about as wide as Jupiter, with between 90 and 150 times Earth's mass and a temperature of about −48 degrees Celsius (−55 degrees Fahrenheit). It would circle the star every two to three years. The object did not appear in follow-up images in February and April 2025, possibly because it had moved along its orbit. The team called it a planet candidate that needs more sightings to confirm.