R71 (RMC 71, HD 269006) is a star in the

Large Magellanic Cloud The Large Magellanic Cloud (LMC) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around , the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf ...

(LMC) in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Mensa. It is classified as a

luminous blue variable Luminous blue variables (LBVs) are rare, massive, evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of th ...

and is one of the most luminous stars in the LMC. It lies three

arc-minute A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

s southwest of the naked-eye star β Mensae.

History of observations

R71 has long been known as a

blue supergiant A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They are usually considered to be those with luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blu ...

and one of the brightest stars in the

Magellanic Clouds The Magellanic Clouds (''Magellanic system'' or ''Nubeculae Magellani'') are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both ...

. It was given a spectral type of B2.5 Iep. By 1974, R71 had increased in brightness from about 11th magnitude to about magnitude 9.2, and examination of historical photographic plates showed that there had been similar changes in the past, with the brightness peaking around 1914 and 1939. The increases in

visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

were accompanied by reddening, apparent cooling of the star. Such outbursts of 1–1.5 magnitudes from blue supergiants accompanied by cooling of several thousand kelvin, such that the

bolometric luminosity Luminosity is an absolute measure of radiated electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electromagnetic energy emitted per ...

remained approximately constant, were considered characteristic of a class of variables then known as

S Doradus 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 List ...

variables, now called luminous blue variables. The outburst of R71 lasted from about 1971 until 1977, and it then faded back to about 11th magnitude. Records are incomplete, but a new outburst was observed to begin in 2006 with the brightness increasing beyond any previous observations to magnitude 8.7. The temperature also cooled to record levels around 6,650 K and the luminosity was calculated to have increased significantly. During the rise of this outburst, periodic brightness variations with an amplitude of about 0.1 magnitudes began to be seen.

Spectrum

The spectrum of R71 in quiescence (minimum brightness) shows weak

emission line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...

s of

Hα Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emit ...

and Hβ and

absorption line Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption (electromagnetic radiation), absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. Th ...

s for the rest of the

Balmer series The Balmer series, or Balmer lines in atomic physics, is one of a set of hydrogen spectral series, six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empiri ...

. There are many strong forbidden emission lines, especially of ionised iron. The spectrum is readily classified as a B class supergiant, the peculiarities such as emission and forbidden lines not unusual for the most luminous stars. During the 1970s outburst, many spectral lines developed strong

P Cygni profile P Cygni (34 Cygni) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in ''Uranometria'' as a nova. Located about 5,300 light-years (1,560 parsecs) from Earth, it is a hypergiant ...

s, while the forbidden emission lines weakened and eventually disappeared. Many other metal absorption lines appeared and the spectrum was clearly of a cooler A-class star. In 2012, at the peak of the unusually bright outburst, the Hα lines developed into double-peaked

s and eventually to inverted P Cygni profiles. Hβ shows much weaker emission wings to the inverted P Cygni profiles, and other hydrogen lines show no emission. Helium spectral lines disappeared completely, suggesting much cooler temperatures. Absorption lines of metals dominate the visual spectrum, again indicating cool temperatures. A particularly unusual feature is the near-infrared forbidden emission lines of ionised calcium usually only seen in warm hypergiants, but also known from η Carinae during its great eruption. The spectral class is estimated to be as late as F9 to G1 during this outburst.

Variability

R71 show brightness variations in many ways typical of the luminous blue variable group of variable stars: long periods near a minimum brightness known as quiescence; micro-variations of less than 0.1 magnitude during the quiescent periods; and outbursts of a magnitude or more lasting several years at intervals of a few decades. At quiescence the star is around magnitude 11, with the lowest brightness recorded at about magnitude 11.2. The 1914, 1939, and 1970s outbursts reached approximately magnitudes 10.2, 9.9, and 9.8 respectively. The outbursts of R71 have apparently been getting progressively brighter, albeit from only a handful observed, with the 2012 outburst being unusually bright for this class of variables, reaching a peak of magnitude 8.7. It is also unusual in that periodic variations of about 0.2 magnitudes developed during the rise to maximum brightness. The period of these variations increased as the brightness increased until it was around 425 days.

Properties

R71 is one of the most luminous stars known, and the most luminous in the Magellanic Clouds during the 2012 outburst. During quiescence, it appears as like other very luminous early B supergiants, about 15,000 K and . The radius of this type of stars is not well-defined as it has a dense

stellar wind A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...

and is losing mass at about every three million years. The standard definition of a stellar surface at

optical depth In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material. Thus, the larger the optical depth, the smaller the amount of transmitted radiant power throu ...

of 2/3 gives a radius of , but a more realistic definition of the surface for this type of star is at optical depth of 20 which gives a radius of . During an outburst, R71 cools and expands although the bolometric luminosity does not change greatly, which is typical behaviour for a luminous blue variable. Because of the cooler temperature, more of the

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

is emitted at visual wavelengths and the visual brightness increases. During the abnormal 2012 outburst, the star cooled beyond the typical luminous blue variable minimum temperature of about 8,500 K, reaching something like 6,650 K and the radius increased about five times to around . The luminosity apparently also increased to over , something which has been seen in other LBVs but is thought to be unusual. This is likely to be beyond than the empirical Humphreys–Davidson limit at which stars become unstable because of their high luminosity. The brightness variations shown during the unusually cool outburst may be related to those shown by

Cepheid variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

s and

RV Tauri variable RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima. History and discovery German astronomer Friedrich Wilhelm Argelander monitored the distinctive variations in brightne ...

s. The particularly low temperature brings R71 close to the

instability strip The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillat ...

where temperature-related opacity changes in the atmosphere cause regular pulsations.

Evolution

Luminous blue variables have traditionally been considered to be in a short-lived unstable transition state before a massive star becomes a

Wolf–Rayet star Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectroscopy, spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very ...

. This has been challenged by the theory that LBVs are the result of binary mass transfer with the mass gaining star becoming a massive and unstable luminous blue variable. R71 appears to be a very massive star that did not form within a mass

star cluster A star cluster is a group of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open cluster ...

, making it likely that it did not initially form as a very massive star. Instead it may have become massive later by gaining mass from a companion. For either formation method, the next stage of evolution for R71 is likely to be a hydrogen-free Wolf-Rayet star. Eventually it is inevitable that its core will collapse, which may result in a

supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

. However, a proportion of stars similar to R71 appear to undergo progressively more violent outbursts until they are destroyed. Also, some or all Wolf–Rayet stars may collapse directly to a black hole without producing a bright supernova explosion.

References

{{Mensa (constellation) Stars in the Large Magellanic Cloud Large Magellanic Cloud Wolf–Rayet stars Mensa (constellation) Luminous blue variables 023428 269006 J05020738-7120131 Extragalactic stars