How big is Gacrux?

Gacrux is a red giant star in the constellation Crux, the Southern Cross. It is about 100 million kilometers (62 million miles) across, roughly 74 times the width of the Sun. That is the width of its visible surface, and like every star's size it is an estimate.

Size

Stars have no solid surface, so a star's size is worked out from how wide it looks in the sky and how far away it is. Gacrux looks about 25 thousandths of an arcsecond wide, where an arcsecond is 1/3,600 of a degree. That is about the width of a human hair seen from 800 meters (half a mile) away. The star is about 89 light-years from Earth, and a light-year is the distance light travels in one year.

A 2024 study of red giants lists this width and distance, along with a radius 73.9 times the Sun's, which makes Gacrux about 103 million kilometers across. An earlier measurement of its apparent width, 24.7 thousandths of an arcsecond, gives about 100 million kilometers. Its surface temperature is only about two-thirds of the Sun's. Its brightness also changes slowly, and a 2026 study of data from the TESS satellite classed it as a long-period variable star.

Size comparisons

Gacrux is about 74 times as wide as the Sun, which is about 1.39 million kilometers across.
If Gacrux took the Sun's place, its surface would reach about nine-tenths of the way to the orbit of Mercury.
Its width is about seven-tenths of the distance from Earth to the Sun, which is about 150 million kilometers.
The blue supergiant Rigel is only a little larger, about 1.1 times as wide as Gacrux.
If the Sun were shrunk to the size of a basketball, Earth would be a 2-millimeter bead 26 meters away, and Gacrux would be a ball about 18 meters (58 feet) across.

A giant with a wind

Red giants such as Gacrux blow gas off their surfaces in a stream called a stellar wind. Using the Hubble Space Telescope, a 2024 study estimated that Gacrux loses about half a billionth of the Sun's mass each year. At that rate, it would take the star about 6,000 years to lose as much mass as Earth has.

Hubble also detected ultraviolet light from the wind as far as about 190 times the star's radius from its center. That is about 66 times the distance from Earth to the Sun. Gacrux's ultraviolet spectrum, its light spread out by wavelength, shows the wind's signs clearly. Astronomers use it as a guide to the more tangled light of the supergiant Betelgeuse.