Main Sequence
Pre-Main Sequence Stars
Now, we will elaborate on a star's entrance to the main sequence. On the HR diagram, the entrance of a protostar onto the main sequence is marked by the Hayashi and Henyey tracks (shown in blue).
The Hayashi tracks are the blue lines on the right side of the graph that are almost vertical, while the Henyey tracks are the blue lines on the left side of the graph that are almost horizontal (leftwards).
The red dashed lines labelled in years are isochrones at the given ages. In other words, stars \(10^5\) years old lie along the curve labeled \(10^5\), and similarly for the other 3 isochrones.
The Hayashi track is a nearly vertical curve that a protostar takes on the H-R diagram. After a protostar ends its contraction and becomes a T Tauri star, it is extremely luminous. The star then follows the Hayashi track downwards, becoming several times less luminous but staying at roughly the same surface temperature, until it enters the Henyey track.
The Henyey track by contrast is nearly horizontal in nature. It happens just after nuclear fusion starts, where the star slowly collapses to reach hydrostatic equilibrium. During this time, the star becomes hotter but stays at the same luminosity. Different stars have different pre-main sequence tracks they take. Note that the more massive stars enter the main sequence on the left side, becoming bluish stars, and vice versa for less massive stars. The Hayashi track is dominant for less massive stars, which start out as blazing T-Tauri stars but then settle down into yellow or red dwarves before starting nuclear fusion. The Henyey track starts almost immediately for more massive stars, as they start nuclear fusion earlier owing to their huge size, and then slowly settle into hydrostatic equilibrium.
Leaving the Main Sequence
The sun will spend 90% of its lifetime as part of the main sequence. This number is highly dependent on the star's mass, with heavier stars spending less time as main sequence stars. Main sequence stars fuse hydrogen in their cores.
Main sequence stars will get hotter and more luminous over time. As the number of hydrogen atoms per unit mass decrease, so too does their energy output provided through nuclear fusion. This results in a decrease in pressure support (radiation), which causes the core to contract in size until the increased density and temperature bring the core pressure in to equilibrium with the layers above. The higher temperature causes the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium, and hence becoming more luminous over time. This explains why the main sequence is a broad band rather than a narrow line as stars move up and to the right on this band as they age.