How big is VY Canis Majoris?

VY Canis Majoris is a red supergiant star in the constellation Canis Major, about 3,800 light-years from Earth. A light-year is the distance light travels in a year, about 9.46 trillion kilometers. The star is about 2 billion kilometers across, roughly 1,400 times the width of the Sun. That figure is the width of its visible surface, and it is uncertain by nearly a tenth either way.

Size

A star has no solid surface, so its width is measured to the depth where its glowing gas stops being see-through. In 2012 a team of astronomers measured how wide VY Canis Majoris looks from Earth with the Very Large Telescope Interferometer, which combines light from several telescopes. It spans 11.3 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree. That is how wide a human hair, about 0.1 millimeters thick, looks from about 1.8 kilometers away.

The star is wrapped in layers of water vapor and carbon monoxide gas. At wavelengths that these gases absorb, it looks about 20% to 50% wider, so the team measured it at a wavelength that avoids them. Its distance had recently been measured precisely from the tiny shift in its position as Earth circles the Sun. Together, the angle and the distance give a radius of 1,420 times the Sun's, give or take 120, so the star is 1.8–2.1 billion kilometers across.

Earlier estimates, made when the distance was less certain, ranged from 600 to 3,000 times the Sun's radius. A 2021 study gives the radius as about 7 astronomical units, where an astronomical unit is Earth's average distance from the Sun, about 150 million kilometers. That also makes the star about 2.1 billion kilometers across. A 2024 survey of red supergiants in the Milky Way describes it as one of the widest.

Size comparisons

VY Canis Majoris is about 1,400 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 6.6 astronomical units out, past the orbit of Jupiter and about two-thirds of the way to the orbit of Saturn.
It is about one and a half times as wide as the red supergiant Mu Cephei, and almost twice as wide as Betelgeuse.
By volume, nearly 3 billion Suns could fit inside it.
If the Sun were shrunk to the size of a basketball, VY Canis Majoris would be a ball about 340 meters (1,100 feet) across, as long as three American football fields.

A star that sheds its outer layers

Astronomers also call VY Canis Majoris a red hypergiant, one of a small group of stars that throw off material in large, uneven outbursts. It sits inside a dusty cloud of that material. Using the Hubble Space Telescope, a team led by Roberta Humphreys measured the motions of small knots of gas close to the star. The knots were thrown out at several different times, roughly 70, 120, 200 and 250 years ago.

Several of those dates match long spells when the star grew much fainter, in records of its brightness going back to 1800. Each deep fading typically lasted about three years. Betelgeuse faded in a similar way in 2019 and 2020, for about 150 days, as gas flowed off its surface. The team concluded that such outbursts may be a major way for red supergiants to lose mass.

What comes next

The 2012 team estimated that VY Canis Majoris began its life with 25 to 32 times the Sun's mass. They found it close to the coolest that a star this bright can be, and expect it to grow hotter next. They count it among the likeliest stars in the Milky Way to end as a supernova, the explosion that ends the life of a massive star.