W40 IRS 1A South

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W40 IRS 1A South is a young

Not much is known about IRS 1A South, given how reddened and extinguished it is (by 10.6 magnitudes, which is why it is so dim despite being such a massive star so close to us, i.e. only 0.0058% of its light reaches us).[2] So far attempts to derive its parameters have used averages from its spectral type of O9.5 (V), and different models have given different parameters. The most recent temperature value for IRS 1A South is 31,500 Kelvin,^[4] and the most recent radius value is 7.71 solar radii.^[2] Considering these parameters and the Stefan–Boltzmann law derives a luminosity of about 52,500 solar luminosities. Assuming the distance of 436 parsecs,^[3] the apparent magnitude of about 15 and the extinction of 10.6 magnitudes,^[2] IRS 1A South has an absolute magnitude of -4.25, i.e. it's about 4,300 times brighter than the Sun in the visible wavelength.

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