How big is KY Cygni?

KY Cygni is a red supergiant star in the constellation Cygnus, about 4,800 light-years from Earth. A light-year is the distance light travels in a year, about 9.46 trillion kilometers. Published estimates put the star at between about 1.2 and 2 billion kilometers across, roughly 880 to 1,420 times the width of the Sun. Those figures are the width of its visible surface, and they differ because thick dust hides the star and makes its true brightness hard to measure.

Size

A star has no solid surface, and KY Cygni's width is worked out from two things: the total light it gives off, and the temperature of its surface. A cool star that gives off a great deal of light must be very large. KY Cygni's surface is about 3,500 kelvins, on a temperature scale that starts at absolute zero, against about 5,800 for the Sun.

The total light is the hard part. A 2005 study estimated that dust in space between KY Cygni and Earth dims its visible light about 1,300-fold. Infrared light gets through the dust far more easily. Using infrared light, the study found a radius of 1,420 times the Sun's, which makes the star about 2 billion kilometers across.

The 2005 study got twice that radius from visible light but suspected that figure was too high, and it put the uncertainty of its estimates at about a fifth. A 2024 study used a distance based on measurements by the Gaia space telescope, about 4,800 light-years, and found 883 times the Sun's radius. That makes KY Cygni about 1.2 billion kilometers across. The star's brightness also changes irregularly.

Size comparisons

KY Cygni is between about 880 and 1,420 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 4.1–6.6 astronomical units out, where an astronomical unit is Earth's average distance from the Sun. That is past the orbit of Mars in every estimate, and past the orbit of Jupiter in the largest.
It is between 1.2 and 1.9 times as wide as Betelgeuse, a nearer and better-studied red supergiant.
At the largest estimate it is as wide as VY Canis Majoris, one of the widest red supergiants known in the Milky Way as of 2024. At the smallest, it is about three-fifths as wide.
If the Sun were shrunk to the size of a basketball, KY Cygni would be a ball 210–340 meters (700–1,100 feet) across, the length of two to three American football fields.

How big a red supergiant can grow

The 2005 study measured 74 red supergiants in the Milky Way. It found that the largest, including KY Cygni and Mu Cephei, had radii of roughly 1,500 times the Sun's, and called them the largest ordinary stars then known. That size matched the largest that computer models of how stars change allowed for red supergiants in the Milky Way. Only two unusual stars had been reported as larger.

A future supernova

Red supergiants are massive stars in one of the last stages of their lives, and many end as supernovae, the explosions of massive stars. KY Cygni is in a 2024 catalog of the Milky Way's red supergiants, compiled for astronomers watching for the next supernova in the galaxy. A collapsing star gives off a burst of tiny particles called neutrinos before its explosion becomes visible. Detectors that catch them could give an early warning and a rough direction, and the catalog would help astronomers work out which star is exploding.