How big is Rigel?

Rigel is a blue supergiant star in the constellation Orion, the Hunter. It is about 110 million kilometers (68 million miles) across, roughly 79 times the width of the Sun. That figure is an estimate of the width of the star's visible surface.

Size

Stars have no solid surface, and Rigel's size comes from two measurements: how wide it looks in the sky and how far away it is. Interferometers, which combine the light from several telescopes, show that it looks about 2.6–2.75 thousandths of an arcsecond wide. An arcsecond is 1/3,600 of a degree. The Hipparcos space mission measured its distance as about 860 light-years, where a light-year is the distance light travels in one year.

A 2012 study combined these to put Rigel's radius at 78.9 times the Sun's, give or take 7.4. That makes the star about 110 million kilometers across. A 2018 study using the Navy Precision Optical Interferometer found a radius 74 times the Sun's, or about 103 million kilometers across.

The distance is the larger source of doubt, since it is uncertain by about 9 percent. In 2022 a team estimated it another way, by combining measurements of the glowing hydrogen gas around the star with computer models. Their answer agreed with Hipparcos.

Size comparisons

Rigel is about 79 times as wide as the Sun, which is about 1.39 million kilometers across.
If Rigel took the Sun's place, its surface would reach almost to the orbit of Mercury.
Its width is about three-quarters of the distance from Earth to the Sun, which is about 150 million kilometers.
Rigel is about 1.3 times as wide as the giant star in Albireo, one of its smaller neighbors in the explorer.
If the Sun were shrunk to the size of a basketball, Earth would be a 2-millimeter bead 26 meters away, and Rigel would be a ball about 19 meters (62 feet) across.

A hot, massive star

Rigel's surface is roughly twice as hot as the Sun's. The star gives off about 120,000 times as much energy as the Sun. It has about 21–23 times the Sun's mass.

Stars like Rigel end their lives as Type II supernovae, the explosions of massive stars when their cores collapse. A 2020 study listed 31 stars within about 3,300 light-years whose cores are expected to collapse, and Rigel is the 10th nearest of them.

A star that pulses

Rigel's brightness and the motion of its surface change slightly, because the star pulsates in many rhythms at once. Twenty-eight days of watching with the MOST satellite, together with more than six years of measurements from the ground, found 19 separate pulsations. They repeat over periods from about 1.2 to 75 days. Stars that vary in this way are called Alpha Cygni variables, after Deneb, whose other name is Alpha Cygni.

Rigel also has a companion, Rigel B, which appears about 9.5 arcseconds away in the sky. Rigel B is itself a pair of stars, too close together to see separately.