You're right, here are some numbers: - photosphere temperature: 5870 K - sunspot umbra temperature: 3000-4500 K Per Stefan-Boltzmann law, this can make the p

You're right, here are some numbers: - photosphere temperature: 5870 K - sunspot umbra temperature: 3000-4500 K Per Stefan-Boltzmann law, this can make the photosphere up to (5870/3000)⁴ ≈ 15 times brighter.