How big is S Doradus?

S Doradus is a massive, unstable star in the Large Magellanic Cloud, a satellite galaxy that orbits the Milky Way about 160,000 light-years from Earth. The star swells and shrinks over years and decades. At its largest, S Doradus is about 530 million kilometers across, roughly 380 times the width of the Sun, and that is the size the explorer shows. At its smallest, it is about 140 million kilometers across.

Size

S Doradus is the prototype, the first known example, of the luminous blue variables. These rare, very bright stars switch between a hot, compact state and a cooler, swollen one. In 1995 the astronomer Henny Lamers modeled the changes and found that the star's radius varies between 100 and 380 times the Sun's. As it swells, its surface cools from about 20,000 to about 9,000 kelvins (degrees measured from absolute zero).

The change is real: the star itself expands and contracts, not only the gas that streams away from it. Its total power stays roughly the same, about a million times the Sun's. But a cooler star gives off more of its energy as visible light, so S Doradus looks brighter when it is large.

These sizes are calculated from the star's power and surface temperature, not measured directly, so they are estimates. They fit the pattern of other stars of its kind. A 2025 study put such stars at about 50–150 times the Sun's radius in the hot state and 200–500 times in the cool state.

Size comparisons

At its largest, S Doradus is about 380 times as wide as the Sun. At its smallest, it is about 100 times as wide.
If S Doradus took the Sun's place at its largest, its surface would reach past the orbit of Mars. At its smallest, it would reach past the average distance of Mercury from the Sun but not as far as Venus.
Its width changes almost fourfold, so its volume changes by about 55 times.
At its largest, S Doradus is a little over half as wide as Antares, a red supergiant.
If the Sun were shrunk to the size of a basketball, S Doradus would grow and shrink between about 24 meters (79 feet) and 91 meters (300 feet) across. At its largest, it would be more than four-fifths the length of an American football field.

The star that named a class

The astronomer E. C. Pickering first noticed that S Doradus changes in brightness, in 1897. Similar stars were later found in the Milky Way and in other galaxies. The swing from hot to cool and back is still called an S Doradus cycle, and S Doradus variables was one of several names for these stars. In 1984 the astronomer Peter Conti grouped them together as luminous blue variables.

Luminous blue variables are a stage in the lives of stars born with more than about 30 times the Sun's mass, marked by extreme loss of mass. They sit close to the modified Eddington limit, the point at which the push of a star's light on its outer layers almost balances the pull of its gravity. A small disturbance below the surface may then set off the swelling and shrinking. Stars of this kind are among the brightest blue stars in their galaxies.