How big is Kapteyn's Star?

Kapteyn's Star is a small, very old red star about 13 light-years from Earth. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Kapteyn's Star is about 400,000 kilometers (250,000 miles) across, a little less than a third the width of the Sun. That is the width of its visible surface, measured with an interferometer in a study published in 2003.

Size

Kapteyn's Star is a red dwarf, smaller, cooler and dimmer than the Sun, with a little more than a quarter of the Sun's mass. Astronomers measured its width with the Very Large Telescope Interferometer in Chile. It combined the light of two 8-meter telescopes 104 meters apart. Together they could see far finer detail than either one alone.

The star looked about 0.69 thousandths of an arcsecond wide, where an arcsecond is 1/3,600 of a degree. With the star's distance, that gave a radius of 0.291 times the Sun's. The star is therefore about 405,000 kilometers across, give or take about 9%. A 2022 estimate from the European Space Agency's Gaia mission, based on the star's brightness and temperature, is about 430,000 kilometers.

Size comparisons

The Sun is about 3.4 times as wide as Kapteyn's Star and about 3.6 times as massive.
Kapteyn's Star is a little wider than the average distance from Earth to the Moon, the next smaller object in the explorer.
It is about 2.9 times as wide as Jupiter, and about 1.9 times as wide as Proxima Centauri, the closest star to the Sun.
If the Sun were shrunk to the size of a basketball, Kapteyn's Star would be a ball about 7 centimeters (2.7 inches) across, a little bigger than a tennis ball, about 21,000 kilometers (13,000 miles) away.

A visitor from the halo

The star is named after Jacobus Kapteyn, an astronomer at the University of Groningen in the Netherlands. In 1897 he found that it moves unusually fast across the sky. It shifts by about 8.7 arcseconds a year, enough to cross a distance equal to the width of the full Moon in about 220 years.

Kapteyn's Star belongs to the Milky Way's halo, a population of stars about 11 billion years old. It circles the center of the Milky Way in the opposite direction to the Sun, on a stretched-out orbit. Studies in 2014 and 2016 described it as the nearest halo star to the Sun. Its gas holds only about one-seventh the Sun's proportion of iron, a sign that it formed early in the Milky Way's history.

A 2016 study estimated its age at about 11.5 billion years, about 2.5 times the age of the Sun. It and a group of stars moving with it may share an origin with Omega Centauri, an old star cluster. That cluster may be what remains of a small galaxy that collided with the Milky Way. A 2015 study questioned the link for the other stars in the group, however.

Planets that were not there

In 2014 a team reported two planets around Kapteyn's Star, found from tiny back-and-forth wobbles in the star's motion. One seemed to lie in the star's habitable zone, where a planet could be warm enough for liquid water. In 2015 another team showed that this signal followed the star's own rotation and activity instead. A 2021 study concluded that both signals came from the star, not from planets.