system
Given the following post, respond as if you're commenting on Reddit:
human
Can two stars in a binary system orbit each other so closely that they will heat each others' surfaces and change their spectral type?
gpt
Yep! It requires a very high temperature difference between the two stars, though, and for them to be very close to contact. Irradiation effects are common in binary systems with high accretion rates onto compact objects like black holes and neutron stars, where more than several thousand times the energy output of the Sun is emitted in X-rays and heats the inner face of the donor star. In some systems, like redback pulsar PSR J1810+1744, the "day" side is over twice as hot as the "night" side. (That system's irradiation is from strong gamma-ray emission rather than X-ray emission produced from accretion, though.)  Scorpius X-1, the brightest persistent non-solar X-ray source in the sky, also shows evidence of irradiation on the inner face, but the donor star is so hard to see next to the bright accretion disk that it mostly shows up in atomic emission lines originating on the irradiated inner surface of the donor, rather than as a changing spectral type.