What powers the brightness of a T-Tauri star before nuclear ignition?
Answer
Heat from slow, final gravitational squeeze
The T-Tauri phase immediately precedes the star achieving true ignition. During this stage, the influx of new material from the surrounding cloud has largely ceased, meaning accretion-based luminosity is winding down. However, the central temperature has not yet reached the one million degrees Kelvin benchmark required for sustained hydrogen fusion. Therefore, the light emitted by a T-Tauri star is still generated by the gradual, residual energy released from its ongoing, slow gravitational contraction, a process that can persist for up to one hundred million years before fusion finally begins.
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