How big is Antares?

Antares is a red supergiant, a huge and comparatively cool star in the late stages of its life. It shines red at the heart of the constellation Scorpius. Antares is about 970 million kilometers across, roughly 700 times the width of the Sun. That is the width of its visible surface, and it could be off by up to a fifth because the star's distance is uncertain.

Size

A star's width is worked out from two measurements: how wide it looks in the sky and how far away it is. The apparent width of Antares has been measured with the Very Large Telescope Interferometer in Chile. It combines the light of several telescopes to see finer detail than any one of them can. A team led by Keiichi Ohnaka measured it at a little over 37 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree.

The distance comes from the star's parallax, the tiny shift in its position as Earth travels around the Sun. It places Antares about 550 light-years away, where a light-year is the distance light travels in a year. From these two measurements, Ohnaka's team found in 2013 that Antares is 680 times as wide as the Sun, or about 950 million kilometers. In 2017 the European Southern Observatory, which runs the interferometer, gave about 700 times.

The parallax is small and hard to measure, so the distance could be anywhere from about 470 to 670 light-years. Across that range, the same apparent width means a star between about 820 million and 1.15 billion kilometers across. The star's brightness changes slowly and irregularly, but two measurements of its width made a year apart, in 2009 and 2010, agreed to within 0.2%.

Size comparisons

Antares is about 700 times as wide as the Sun, which is about 1.39 million kilometers across.
If Antares took the Sun's place, its surface would reach past the orbit of Mars, about 60% of the way to the orbit of Jupiter.
More than 300 million Suns could fit inside it, by volume.
Betelgeuse, one of the next larger stars in the explorer, is about 760 times as wide as the Sun, which makes it about one-tenth wider than Antares.
If the Sun were shrunk to the size of a basketball, Antares would be a ball about 170 meters (560 feet) across, about one and a half times the length of an American football field.

A star near the end of its life

Red supergiants form from stars born with 9–40 times the mass of the Sun. Antares probably began life with about 15 times the Sun's mass and has since shed about three Suns' worth of material, leaving about 12. Ohnaka's 2013 study put its age at 11–15 million years. Antares is expected to end its life as a supernova, the explosion of a massive star.

Antares has a companion, a much hotter star that appears about 2.7 arcseconds away in the sky. The two circle each other roughly once every 2,700 years. The companion allows astronomers to measure fairly precisely how fast Antares is losing mass.

A turbulent atmosphere

In 2017, Ohnaka's team used the interferometer to make the first map of how gas moves in the atmosphere of a star other than the Sun. It showed huge clumps of gas rising and falling at up to about 20 kilometers per second, in an atmosphere that reaches out to about 1.7 times the star's radius. The team concluded that convection, the rising of hot gas and sinking of cooler gas, cannot explain these motions on its own.

The visible surface is churning too. In 2025, astronomers made the first images of it with spectropolarimetry, which measures the direction in which the star's light waves vibrate at each wavelength. The images show convection cells that last several months. The largest span about 30% of the star's radius, almost the distance from Earth to the Sun.