How big is Luyten's Star?

Luyten's Star is a red dwarf, a kind of star smaller and cooler than the Sun, about 12.3 light-years away. It is about 445,000 kilometers (277,000 miles) across, about one-third the width of the Sun. That figure is the width of its photosphere, the glowing layer that looks like a star's surface.

Size

Seen from Earth, Luyten's Star appears less than a thousandth of an arcsecond wide, where an arcsecond is 1/3,600 of a degree. In a study published in 2019, a team led by Markus Rabus measured that width with the European Southern Observatory's Very Large Telescope Interferometer. An interferometer combines the light from several telescopes so that together they see much finer detail than any one of them.

Combined with the star's distance, the measurement gives a radius 0.320 times the Sun's. That is a width of about 445,000 kilometers, uncertain by less than 2%. A 2015 study by Andrew Mann and colleagues estimated the radius a different way, from the star's temperature and total light output. It found 0.315 times the Sun's radius, about 438,000 kilometers, which agrees within the uncertainties.

Size comparisons

Luyten's Star is about one-third as wide as the Sun, and has about 3% of the Sun's volume.
It is about 35 times as wide as Earth.
It is a little wider than the distance from Earth to the Moon, 384,400 kilometers on average.
It is about twice as wide as Proxima Centauri, another red dwarf.
If the Sun were shrunk to the size of a basketball, Luyten's Star would be a ball about 7.7 centimeters (3 inches) across. It would be about 20,000 kilometers from that basketball, more than one and a half times the width of the real Earth.

A small, cool star

Red dwarfs like Luyten's Star are the most numerous stars in the universe. Luyten's Star has about 0.29 times the Sun's mass, and its surface is about 3,000 degrees Celsius, roughly 2,500 degrees cooler than the Sun's. Being both smaller and cooler, it gives off only about 1% as much light as the Sun.

Two planets

In 2017, astronomers reported two planets orbiting Luyten's Star, which is also known by its catalog name, GJ 273. They found them with the HARPS spectrograph, which measures how fast a star moves toward and away from Earth. An orbiting planet's pull makes that speed change back and forth. The method gives only a planet's smallest possible mass.

The outer planet, GJ 273b, has at least 2.9 times Earth's mass and orbits the star every 18.65 days. It lies at the inner edge of the star's habitable zone, the range of distances where liquid water could exist on a planet's surface. It receives about as much energy from its star as Earth gets from the Sun.

The inner planet, GJ 273c, has at least 1.2 times Earth's mass and orbits every 4.7 days. It receives almost seven times as much energy as Earth, too much to be in the habitable zone. When the planets were announced, Luyten's Star became the second-nearest known planetary system with a planet in its habitable zone, after Proxima Centauri.