How big is Mu Cephei?

Mu Cephei, also known as Herschel's Garnet Star, is a red supergiant star in the constellation Cepheus. A 2019 study estimated that it is about 1.4 billion kilometers across, some 970 times the width of the Sun. That is the width of the star's visible surface. It is uncertain by about a quarter either way, mainly because the star's distance is poorly known.

Size

A star has no solid surface, so its width is measured across its photosphere, the glowing layer that gives off most of its light. In a 2005 study, astronomers measured how wide Mu Cephei looks from Earth by interferometry, which combines light from several telescopes. In infrared light its photosphere spans 14.1 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree. Around it lies a layer of gas molecules about one-third wider.

Turning that angle into a width needs the star's distance. Published estimates range from about 1,300 to 5,900 light-years, where a light-year, the distance light travels in a year, is about 9.46 trillion kilometers. The Gaia space telescope's measurement was too uncertain to settle the question.

In 2019 a team worked out a distance of about 2,100 light-years by comparing Mu Cephei with Betelgeuse, a similar star whose distance had recently been measured again. At that distance, the star's radius is 972 times the Sun's, give or take 228, so it is 1.0–1.7 billion kilometers across. A 2005 study that placed the star farther away found a radius of 1,420 times the Sun's, which would make it about 2 billion kilometers across.

Size comparisons

Mu Cephei is about 970 times as wide as the Sun, which is about 1.39 million kilometers (865,000 miles) across.
If it took the Sun's place, its surface would reach about 4.5 astronomical units out, past the orbit of Mars and most of the way to the orbit of Jupiter. An astronomical unit is Earth's average distance from the Sun, about 150 million kilometers.
It is about a quarter wider than Betelgeuse, another red supergiant of the same type, and about two-thirds as wide as VY Canis Majoris, one of the widest red supergiants known in the Milky Way as of 2024.
From Earth, Mu Cephei looks as wide as a human hair, about 0.1 millimeters thick, held about 1.5 kilometers away.
If the Sun were shrunk to the size of a basketball, Mu Cephei would be a ball about 230 meters (770 feet) across, about as long as two American football fields.

The Garnet Star

Mu Cephei is sometimes called Erakis, but it has long been known as Herschel's Garnet Star, a name that suits its deep red color. The star looks red partly because it is much cooler than the Sun. Dust in space between the star and Earth reddens its light further. Through a telescope it appears near the edge of IC 1396, a glowing cloud of gas.

Mu Cephei's brightness also varies, in a rough cycle of about two years. The General Catalogue of Variable Stars names it as the example of red supergiants that vary in this partly regular way.

A star that is losing mass

Like other red supergiants, Mu Cephei is slowly losing its outer layers into space. In 2019 astronomers mapped the carbon monoxide gas around it with the NOEMA radio telescopes. The gas turned out to be clumpy rather than smooth. Together, the clumps and the rest of the star's wind, the gas it blows off into space, carry away about 2.5 millionths of the Sun's mass each year, roughly four-fifths of the mass of Earth.

A 2023 study followed hot gas rising from the star's surface in plumes, driven by convection, the churning of hot gas rising and cooler gas sinking. The gas reached heights of at least one-tenth of the star's radius above its surface. From there, it can easily escape the star's gravity. Mu Cephei is expected to end its life as a supernova, the explosion of a massive star.