S Doradus (also known as S Dor) is one of the brightest stars in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, located roughly 160,000 light-years away.

S Doradus was noted in 1897 as an unusual and variable star, of Secchi type I with bright lines of Hα, Hβ, and Hγ.

[23] The classification system defined for the General Catalogue of Variable Stars pre-dated this and so the acronym SDOR is used for LBVs.

[24] S Doradus is the brightest member of the open cluster NGC 1910, also known as the LH41 stellar association, visible in binoculars as a bright condensation within the main bar of the LMC.

[29] A much closer companion has been found using the Hubble Space Telescope Fine Guidance Sensor, 1.7″ away and four magnitudes fainter.

At maximum brightness, the spectrum can become as cool as an F supergiant, with strong ionised metal lines and almost no emission components.

The microvariations are similar to the brightness changes shown by α Cygni variables, which are less luminous hot supergiants.

During the quiescent phase, LBVs lie along a diagonal band in the H–R diagram called the S Doradus Instability Strip, with the more luminous examples having hotter temperatures.

[34] The standard theory is that LBV outbursts occur when the mass loss increases and an extremely dense stellar wind creates a pseudo-photosphere.

The temperature drops until the wind opacity starts to decrease, meaning all LBV outbursts reach a temperature around 8,000–9,000 K. The bolometric luminosity during outbursts is considered to remain largely unchanged, but the visual luminosity increases as radiation shifts from the ultraviolet into the visual range.

S Doradus has been calculated to be less luminous at maximum brightness (minimum temperature), possibly as a result of potential energy going into expansion of a substantial portion of the star.

[36] The temperature of an LBV is difficult to determine because the spectra are so peculiar and the standard colour calibrations don't apply, so the luminosity changes associated with brightness variations cannot be calculated accurately.

This is likely if the outburst consists only of an opaque stellar wind forming a pseudo-photosphere to mimic a larger cooler star.

The surface gravity changes dramatically and is difficult to measure from the peculiar spectral lines, and the radius is poorly defined.