How big is Gliese 229A?

Gliese 229A is a red dwarf, a kind of star smaller and cooler than the Sun, about 18.8 light-years away. It is estimated to be about 730,000 kilometers (450,000 miles) across, a little over half the width of the Sun. That figure is the width of its photosphere, the glowing layer that looks like a star's surface. The star is best known for its companion, Gliese 229B, the first brown dwarf found to have methane in its atmosphere.

Size

The width of Gliese 229A given here is worked out from other measurements. A star's total light output depends on its surface temperature and its surface area, so knowing both the temperature and the light output gives its size. In 2019 a team led by Andreas Schweitzer used this method and found a radius 0.523 times the Sun's, a width of about 730,000 kilometers.

They took the temperature from the star's spectrum, the way its light spreads into colors, and the light output from its brightness and its distance. A 2015 study by Maldonado and colleagues, working from features in the spectrum, found a radius of 0.54 times the Sun's, about 750,000 kilometers. The two estimates agree within their stated uncertainties.

Gliese 229A is classed M1 V, where the Sun is G2 V. Its surface is about 3,600 degrees Celsius, roughly 1,900 degrees cooler than the Sun's. It has about 0.58 times the Sun's mass and gives off only about 5% as much light.

Size comparisons

Gliese 229A is a little over half as wide as the Sun, and has about one-seventh of the Sun's volume.
It is about 57 times as wide as Earth.
It is almost twice as wide as the distance from Earth to the Moon, which is 384,400 kilometers on average.
Alpha Centauri B, the next larger star in the explorer, is about 1.65 times as wide.
If Gliese 229A were shrunk to the size of a basketball, the two brown dwarfs that make up Gliese 229B would circle each other about 2 meters apart, on average about 1.6 kilometers from the ball.

The brown dwarf companion

Gliese 229B was discovered in 1995. It is a brown dwarf, an object with too little mass to fuse hydrogen in its core as stars do, so it cools and fades over time. Gliese 229B was the first object clearly identified as a T dwarf, a cool kind of brown dwarf with methane in its atmosphere. It orbits Gliese 229A at an average distance of about 33 astronomical units, where one astronomical unit is the average distance from Earth to the Sun.

For years Gliese 229B was a puzzle. Its orbit showed that it has about 71 times the mass of Jupiter, yet it was several times fainter than models predicted for that mass. In 2024 two teams solved the puzzle: Gliese 229B is two brown dwarfs, Gliese 229Ba and Gliese 229Bb. They have about 38 and 34 times Jupiter's mass, and circle each other every 12.1 days, only about 16 times the Earth–Moon distance apart.