VY Canis Majoris

In the southern winter constellation Canis Major, near brilliant Sirius, is a faint red star that the French astronomer Jérôme Lalande recorded in 1801. It looks unremarkable. But VY Canis Majoris is one of the largest stars known: if it replaced the Sun, its surface would reach beyond the orbit of Jupiter.

How big?

Measuring a star this distant and this fuzzy is hard, and older estimates of its size ran as high as 3 000 times the Sun’s radius. In 2012 Markus Wittkowski and colleagues measured its angular size with the Very Large Telescope Interferometer and found a radius of about 1 420 Suns, with a temperature of about 3 490 K (3 220 °C) and a brightness about 270 000 times the Sun’s. It lies about 3 820 light-years away.

For all its size, VY Canis Majoris is not especially heavy: about 17 times the Sun’s mass, perhaps 25 at birth. Its outer layers are extremely thin, spread through a volume billions of times larger than the Sun’s.

A star losing itself

VY Canis Majoris is throwing off its outer layers at one of the highest rates of any star of its kind, around 0.0006 solar masses a year. That material has built up a large, lumpy nebula of dust and gas around it, first noticed in 1917. In 2021 Roberta Humphreys and colleagues used the Hubble Space Telescope to trace arcs and knots in the nebula back to separate outbursts, the most recent in the last few centuries. Each outburst produced dust that dimmed the star. That explains why it has faded from about magnitude 6.5 in the early 1800s to about 8.5 today.

Knots in the nebula once fooled observers in the 19th century into reporting companion stars. They were clouds of dust and gas lit by the star.

Its end

VY Canis Majoris is near the end of its life. According to ESA, it may explode as a supernova, or it may simply collapse and form a black hole. Some recent work suggests it could first shed enough mass to become hotter and bluer before it dies.