How big is Regulus?

Regulus is a hot star in the constellation Leo, about 79 light-years from Earth, where a light-year is the distance light travels in one year. It spins so fast that it is flattened. It is about 5.8 million kilometers (3.6 million miles) across at its equator, about 4 times the width of the Sun, but only about 4.4 million kilometers from pole to pole.

Size

A star has no solid surface, so its size is the width of the glowing layer that its light comes from. In 2005, a study of Regulus was the first scientific result from the CHARA Array on Mount Wilson, California. The array combines the light from several telescopes to see detail too fine for any one of them. The study showed that Regulus looks wider in one direction than the other, from 1.25 to 1.65 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree.

Combining those measurements with the star's distance and its spectrum gave a radius at the equator of 4.16 times the Sun's, and a radius at the poles of 3.14 times. The star is about one-third wider across its equator than from pole to pole. A 2018 study using a different method found the same equatorial radius, 4.16 times the Sun's, and a polar radius of 3.08 times.

Size comparisons

Regulus is about 4 times as wide as the Sun across its equator, and about 3 times as wide from pole to pole.
About 450 Earths side by side would stretch across its equator.
The larger of Spica's two stars, the next larger star in the explorer, is about 10.4 million kilometers across, about 1.8 times the width of Regulus at its equator.
Vega, another fast-spinning star, is about 3.8 million kilometers wide at its equator, so Regulus is about 1.5 times as wide.
If the Sun were shrunk to the size of a basketball, Regulus would be a squashed ball about 1 meter (3.3 feet) across at its equator but only 75 centimeters (2.5 feet) from pole to pole.

Spinning near its breaking point

Regulus turns once every 15.9 hours. The Sun, by comparison, takes about 25 days. At its equator, the surface of Regulus moves at 86% of the speed at which the star would start to fly apart. The spin flings the equator outward, which is why the star is flattened.

The bulging equator is also cooler and dimmer than the poles, an effect called gravity darkening. The CHARA study measured about 15,000 degrees Celsius at the poles and about 10,000 at the equator. It was the first time gravity darkening had been measured this way in a fast-spinning star.

In 2017, astronomers used Regulus to detect, for the first time, the polarization that a spinning star's distorted shape gives its light. Polarized light has waves that vibrate more in one direction than others. The effect is tiny, but it confirmed that Regulus spins close to its break-up rate.

A hidden partner

In 2008, astronomers found that Regulus moves back and forth every 40 days, pulled by an unseen companion. In 2020, a team picked out the companion's faint light. It has about 0.3 times the Sun's mass and is about 85,000 kilometers across, roughly 7 times the width of Earth.

The companion is probably what is left of a star that lost its outer layers to Regulus. When gas flows from one star onto its partner, it can strip the giver down to its core and set the receiver spinning. Astronomers think that is how Regulus came to spin so fast.