How do T Tauri stars change before reaching the main sequence?

T Tauri stars are young, low-mass stars that represent a critical phase in stellar evolution before they reach the main sequence. During this period, they undergo several significant changes:

• Contraction: These stars begin with sizes several times larger than their eventual main-sequence dimensions. They contract over tens of millions of years, primarily powered by the release of gravitational energy.1
• Instability: T Tauri stars are characterized by erratic changes in brightness and are relatively unstable as they continue to condense.2
• Internal Temperature and Fusion: As the star contracts, its internal temperature rises. Initially, this heat destroys deuterium, followed by the breakdown of lithium, beryllium, and boron into helium.1
• Hertzsprung-Russell Progression: On the Hertzsprung-Russell diagram, these stars initially appear well above the main sequence. As they contract, they move downward and to the left until their core temperatures reach approximately ${10}^7$ K, at which point hydrogen fusion ignites and they settle onto the main sequence.1