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. The star is a luminous blue variable, and one of the most luminous stars known, having a luminosity varying widely above and below 1,000,000 times the luminosity of the Sun, although it is too far away to be seen with the naked eye.
History S Doradus was noted in 1897 as an unusual and variable star, of Secchi type I with bright lines of Hα, Hβ, and Hγ. The formal recognition as a variable star came the assignment of the name S Doradus in 1904 in the second supplement to Catalogue of Variable Stars. S Dor was observed many times over the following decades. In 1924, it was described as "P Cygni class" and recorded at photographic magnitude 9.5 In 1925, its absolute magnitude was estimated at −8.9. In 1933 it was listed as a 9th-magnitude Beq star with bright hydrogen lines. It was the most luminous star known at that time. In 1943, the variability was interpreted as being due to eclipses of a binary companion, orbiting with a period of 40 years. This was refuted in 1956, when the variability was described as irregular and the spectrum as A0 with P Cygni profiles and emission for many spectral lines. The brightness was observed to decline by 0.8 magnitude from 1954 into 1955. At the same time, S Doradus was noted as being similar to the Hubble–Sandage variables, the LBVs discovered in M31 and M33. The brief 1955 minimum was followed by a deep minimum in 1964, when the spectrum was compared to Eta Carinae in strong contrast to the mid-A spectrum at normal brightness. By 1969 the nature of S Doradus was still uncertain, considered possibly to be a pre-main-sequence star, but during the next decade the consensus settled on the S Doradus type variables and Hubble-Sandage variables being evolved massive supergiants. They were eventually given the name "luminous blue variables" in 1984, coined in part because of the similarity of the acronym LBV to the well-defined LPV class of variable stars. The classification system defined for the General Catalogue of Variable Stars pre-dated this and so the acronym SDOR is used for LBVs.
Surroundings
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. This is within the N119 emission nebula, which has a distinctive spiral shape. It is one of the visually brightest individual stars in the LMC, at some times the brightest. There are only a handful of other 9th-magnitude stars in the LMC, such as the yellow hypergiant HD 33579. There are several compact clusters near S Doradus, within the general NGC 1910/LH41 association. The closest is less than four arc-minutes away, contains two out of the three WO stars in the entire LMC, and the entire cluster is about the same brightness as S Doradus. A little further away is NGC 1916. Another LBV, R85, is just two arc-minutes away. This rich star-forming region also hosts a third Wolf–Rayet star, at least ten other supergiants, and at least ten class O stars. S Doradus has a number of close companion stars. The Washington Double Star Catalog lists two 11th-magnitude stars 5″ away, which at the distance of the LMC is about four light years. A much closer companion has been found using the Hubble Space Telescope Fine Guidance Sensor, 1.7″ away and four magnitudes fainter. There are other nearby stars, most notably a 12th-magnitude OB supergiant at 13″.
Variability
This star belongs to its own eponymous S Doradus class of variable stars, also designated as luminous blue variables or LBVs. LBVs exhibit long slow changes in brightness, punctuated by occasional outbursts. S Doradus is typically a magnitude 9 star, varying by a few tenths of a magnitude on timescales of a few months, superimposed on variations of about a magnitude taking several years. The extreme range of these variations is from about visual magnitude 8.6–10.4. Every few decades it shows a more dramatic decrease in brightness, to as low as magnitude 11.5. The nature of the variation is somewhat unusual for an LBV; S Doradus is typically in an outburst state, with only occasional fades to the quiescent state that is typical of most stars in the class.
The colour of S Doradus changes as its brightness varies, being bluest when the star is faintest. At the same time, the spectrum shows dramatic changes. It is typically an extreme mid-A supergiant with P Cygni profiles on many lines (e.g. A5eq or A2/3Ia+e). At maximum brightness, the spectrum can become as cool as an F supergiant, with strong ionised metal lines and almost no emission components. At minimum brightness, the spectrum is dominated by emission, particularly forbidden lines of Feii but also helium and other metals. At the deep minima these features are even more pronounced, and Feiii emission also appears. Attempts to identify regularity in the unpredictable changes of brightness suggest a period of around 100 days for the small amplitude variations near maximum brightness. At minimum brightness, these microvariations are considered to occur with periods as long as 195 days. The slower variations have been characterised with a period of 6.8 years, with an interval of 35–40 years between deep minima. The microvariations are similar to the brightness changes shown by α Cygni variables, which are less luminous hot supergiants.
The instability strip
… excerpt ends here. Continue reading the full article.
