Astronomy
How Protostars Evolve Through the T Tauri Phase
Quick fact
T Tauri stars are so named after their prototype, T Tauri, located in the constellation Taurus. They can vary in brightness by several magnitudes over days or weeks, and some eject narrow jets of material at hundreds of kilometers per second.
Why this is interesting
Every star you see in the night sky was once a chaotic, violently flickering toddler. What makes a newborn star so restless?
Read the full explanation
Understanding How Protostars Evolve Through the T Tauri Phase
Think of a protostar as a stellar embryo. It begins as a dense clump inside a giant cloud of gas and dust. Gravity pulls material inward, causing the clump to shrink and spin faster. As it contracts, the core heats up, but it is still not hot enough for nuclear fusion. This is the protostar stage. Over time, the surrounding material forms a rotating disk, which feeds the protostar. Eventually, when the central temperature reaches about 10 million Kelvin, fusion begins. But just before that stable state, the star goes through a turbulent period called the T Tauri phase. During this phase, the star is still not fully hydrostatic — it is contracting slowly, while intense magnetic activity and strong stellar winds blow off excess material. This phase can last from a few million to tens of millions of years, depending on the star's mass. The T Tauri phase marks the transition from a hidden, embedded object to a visible pre-main-sequence star that is just about to join the main sequence.
A deeper explanation
The T Tauri phase is driven by the interplay between gravitational contraction, magnetic fields, and angular momentum. As the protostar contracts, its core temperature rises, but not yet enough for sustained hydrogen fusion. Instead, the energy comes from gravitational potential energy being converted to heat. Meanwhile, the star is not yet in hydrostatic equilibrium — the pressure from the hot interior is not fully balancing gravity, so the star continues to shrink. This contraction spins up the star due to conservation of angular momentum, and the surrounding disk feeds material onto the star through accretion. However, the star's magnetic field interacts with the disk, channeling some material into funnel streams and also driving powerful outflows and jets along the rotation axis. These outflows remove excess angular momentum, allowing the star to continue contracting without spinning apart. The strong magnetic activity also causes intense X-ray and UV emission, and the star's brightness varies irregularly as spots and hot regions rotate in and out of view. Eventually, when the core reaches the critical temperature and pressure for hydrogen fusion, the star's contraction stops, the outflows cease, and the star settles into a stable main-sequence phase. The T Tauri phase is therefore a crucial stage where the star loses its initial mass and angular momentum, and the remains of the disk become the raw material for planets.