How big is R Doradus?

R Doradus is a red giant star in the constellation Dorado, the Swordfish. It is about 410 million kilometers across, roughly 300 times the width of the Sun. That is the width of its visible surface as measured in infrared light. Seen at longer wavelengths, the star looks larger.

Size

Stars have no solid surface. A red giant also has an extended atmosphere above its visible surface, so its measured size depends on the light used to look at it. In infrared light, the Very Large Telescope Interferometer measured R Doradus as 51.2 thousandths of an arcsecond wide. An arcsecond is 1/3,600 of a degree, so R Doradus looks about as wide as a human hair seen from 400 meters (a quarter of a mile) away.

The Hipparcos space mission measured the star's distance as about 180 light-years, where a light-year is the distance light travels in one year. Together, the width and the distance give a radius of about 1.4 astronomical units, or 298 times the Sun's radius. An astronomical unit is the average distance from Earth to the Sun, about 150 million kilometers.

The ALMA telescope, which observes at wavelengths of around a millimeter, sees gas lying above the visible surface. At those wavelengths R Doradus is about 350 times as wide as the Sun, about 490 million kilometers across. The star also pulsates. Every 1,000 days or so, it switches between two rhythms, one 362 days long and the other 175 days long.

Size comparisons

R Doradus is about 300 times as wide as the Sun, which is about 1.39 million kilometers across.
If R Doradus took the Sun's place, its surface would reach beyond the orbit of Earth, about nine-tenths of the way to the orbit of Mars.
It is about 1.5 times as wide as the supergiant Deneb, one of its neighbors in the explorer, using Deneb's larger size estimate.
Betelgeuse is about 2.6 times as wide as R Doradus but about three times as far away, so R Doradus looks wider from Earth.
If the Sun were shrunk to the size of a basketball, Earth would be a 2-millimeter bead 26 meters away, and R Doradus would be a ball about 72 meters (235 feet) across, about two-thirds the length of a football field.

Large in the sky

In 1997, astronomers found that R Doradus looks wider in the sky than any other star except the Sun, wider even than Betelgeuse. A 2019 study described it as the closest asymptotic giant branch star, a kind of red giant in a late stage of its life. Its mass is similar to the Sun's, so it gives a preview of how the Sun may look in about five billion years.

Bubbles on its surface

Energy rises through a star like R Doradus in huge bubbles of hot gas, which cool and sink again. This churning is called convection. In July and August 2023, ALMA took a series of images of the star's surface over about a month. For the first time, astronomers could track the motion of these bubbles on a star other than the Sun.

The bubbles are about 75 times as wide as the Sun, and the gas in them moves at up to about 20 kilometers per second. They appear and sink back on a cycle of about one month, faster than astronomers expected from the way convection works in the Sun.