How big is Arcturus?

Arcturus is a red giant star and one of the brightest stars in the night sky. It is about 35 million kilometers (22 million miles) across, about 25 times the width of the Sun. That is the width of its visible surface, whose apparent size astronomers have measured directly.

Size

Arcturus is bright and large, so its width has been measured many times with interferometers, instruments that combine the light of several telescopes to see very fine detail. Seen from Earth, it is about 21 thousandths of an arcsecond across, where an arcsecond is 1/3,600 of a degree. That apparent width is the width of a human hair seen from about 1 kilometer (0.6 miles) away.

Arcturus is about 36.8 light-years away, where a light-year is the distance light travels in one year, about 9.46 trillion kilometers. The distance comes from parallax, the tiny shift in a star's position as Earth travels around the Sun. A 2011 study by Ramírez and Allende Prieto combined the apparent width and the distance, averaged the result with an estimate from models of how stars age, and found a radius of 25.4 times the Sun's, precise to about 1%.

Some ways of modeling the star's edge suggest that its apparent width is slightly larger. That would give a radius of 25.6–26.1 times the Sun's, or about 36 million kilometers across.

Size comparisons

Arcturus is about 25 times as wide as the Sun and about 2,800 times as wide as Earth.
About 16,000 Suns would fit inside it, by volume.
If Arcturus took the Sun's place, its surface would reach almost a third of the way to the orbit of Mercury.
It is about 1.3 times as wide as the largest star of Alnitak, the next smaller star in the explorer. Aldebaran, another giant star, is about 1.8 times as wide as Arcturus.
If the Sun were shrunk to the size of a basketball, Arcturus would be a ball about 6.1 meters (20 feet) across.

An old star

Arcturus is estimated to be about 7 billion years old, older than the Sun, which formed about 4.6 billion years ago. Compared with its hydrogen, it has only about a third as much iron as the Sun. Its chemistry and its motion through the galaxy link it to a population of stars that astronomers call the Milky Way's thick disk.

Because it is bright, nearby and well measured, Arcturus is a standard reference for astronomers who study other giant stars. Many interferometers also observe it as a test object.

The Sun's future

Arcturus has about the same mass as the Sun, 1.08 times as much, but it is already a red giant. Its surface is about 4,000 degrees Celsius (7,300 degrees Fahrenheit), cooler than the Sun's 5,500 degrees Celsius (10,000 degrees Fahrenheit). The Sun is still a yellow dwarf star, but NASA expects it to expand into a red giant late in its life, large enough to swallow Mercury and Venus, and possibly Earth.