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Luminosity

Left to right: a red dwarf, the Sun, a blue dwarf, and R136a1. R136a1 is not the largest known star in terms of radius or volume, only in mass and luminosity.

At over 8,000,000 L_☉, R136a1 is the most luminous star known, radiating as much energy in 3.6 seconds as the Sun does in a year. It supplies ~7% of the ionizing flux of the entire 30 Doradus region, as much as 70 O7 dwarf stars. Along with R136a2, a3, and c, it produces 43-46% of the the Lyman continuum radiation of the whole R136 cluster.

The luminosity of massive stars lies very close to the Eddington limit, which is the luminosity at which the radiation pressure expanding the star outward equals the force of the star's gravity collapsing the star inward. The Eddington limit is not strictly applicable to stars sync as R136a1 that are not in hydrostatic equilibrium, so the empirical Humphrey-Davidson limit is more commonly used for stars. This means that the radiative flux passing through the photosphere of a massive star may be nearly strong enough to lift off the photosphere. Above the Eddington limit, the star would generate so much radiation that parts of its outer layers would be thrown off in massive outbursts. This would effectively restrict the star from shining at higher luminosities for longer periods.

Models estimate that a 300 M_☉ star would be 55% of its Eddington limit at birth so the ZAMS luminosity of R136a1 would be over half of its current one. Strong mass loss has caused it to shed >50 M_☉ over the past ~1.7 Myr, so its luminosity has slightly increased over time relative to its Eddington limit factor. R136a1 is currently around 80% of its Eddington luminosity and is approximately 8,700,000 L_☉. Its Eddington luminosity at birth was probably 13,000,000 L_☉ but since its mass has decreased by 20%, the Eddington luminosity has decreased to around 10,875,000 L_☉.

The apparent magnitude of R136a1, as viewed from Earth, is around 12.23. However, it is 157,000 ly away, and around 1.80±0.17 magnitudes are absorbed by dust. If it replaced the Sun in our Solar System, it would outshine the Sun by 94,000 times (M_V = −7.6), and would appear from Earth to be magnitude -39.16. The apparent bolometric magnitude would be around -44.08. due to the fact that 92% of the radiation occurs in the higher energy band. The star's absolute visual magnitude would be about -7.6, or 3 magnitudes brighter than Venus. Its absolute bolometric magnitude would be around -12.6, or almost as bright as the full moon.

The luminosity of R136a1 is rather uncertain due to conflicting measurements. The most recent survey derived a bolometric luminosity of 7,400,000 L_☉ using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. However, this measurement is expected to have an underestimation since observations rely on the Radial velocity measurement, and this approach may lead to substantial errors if R_V deviates from the adopted value. Observations largely relied on UV wavelengths where the uncertainties in the extinction are very high. The UV spectrum of R136a1 is also largely contaminated by R136a2, so measurements in this range are unreliable. However, in infrared wavelengths, the extinction is almost negligible, so values derived from infrared observations yield a more accurate value than UV observations. Another study used the infrared apparent magnitude of R136a1, the 157 kly distance to R136, and the modest interstellar extinction to derive a bolometric value of 8,700,000 L_☉, with a plus-minus error margin of 1,200,000. The uncertainties that apply to the bolometric luminosity also apply to the visual luminosity. An infrared observation derived a value of 94,000 L_☉ while an UV observation derived one of 59,000 L_☉.