The birth of a star is a dramatic event hidden deep within vast clouds of gas and dust. Understanding where a star is in its life cycle requires a framework that correlates its observable properties, allowing scientists to trace its history and predict its future. The Hertzsprung-Russell (HR) diagram is a fundamental tool in astrophysics that plots the relationship between a star’s intrinsic brightness and its surface temperature. The vertical axis represents the star’s luminosity, which is its total energy output. Stars with greater luminosity are found higher on the diagram. The horizontal axis plots the star’s surface temperature. Temperature increases from right to left, meaning the hottest stars are on the left side and the coolest stars are on the right. By plotting a star’s position on these two properties, astronomers can determine its evolutionary stage.
The Physical Characteristics of a Protostar
A protostar is the earliest stage in a star’s formation, representing a dense core of gas and dust that is contracting under its own gravity. This object has not yet initiated the stable hydrogen fusion that defines a mature star. Because they are actively gathering mass from a surrounding envelope, protostars are often significantly larger than the main-sequence stars they will eventually become. The immense energy emitted by a protostar comes from the conversion of gravitational potential energy into thermal energy as the material contracts. This makes them extremely luminous, even though their surface temperatures remain relatively cool. This combination of high luminosity and low surface temperature is a defining characteristic of the protostellar phase.
Locating the Pre-Main-Sequence Tracks
Protostars and their subsequent stage, pre-main-sequence (PMS) stars, are found on the right side of the HR diagram, a position indicating their relatively low surface temperatures. Their evolutionary path is traced by specific lines known as evolutionary tracks, which show the continuous changes in luminosity and temperature as the star contracts. The exact track a star follows depends heavily on its initial mass, leading to two primary paths: the Hayashi Track and the Henyey Track.
The Hayashi Track
Lower-mass stars, those with masses less than about three solar masses, initially follow the Hayashi Track, which is characterized by a nearly vertical movement downward on the HR diagram. During this phase, the star is fully convective, meaning energy is transported by the movement of gas throughout its interior. As the star shrinks due to gravitational contraction, its surface temperature remains nearly constant, but its luminosity drops significantly, causing the downward vertical motion.
The Henyey Track
Stars more massive than approximately 0.6 solar masses eventually develop a hot enough core to form a radiative zone, causing them to deviate from the vertical Hayashi Track and move onto the Henyey Track. This new path is nearly horizontal on the HR diagram, as the star’s luminosity remains relatively constant while its surface temperature increases. The star moves to the left on the diagram.
Transition to the Main Sequence
The pre-main-sequence phase concludes when the star reaches a specific line on the HR diagram called the Zero-Age Main Sequence (ZAMS). This moment marks the end of the star’s prolonged gravitational contraction and the beginning of its stable life.
The star’s movement ceases once the core temperature reaches the threshold needed to initiate stable, self-sustaining hydrogen fusion, typically around 10 million Kelvin. The energy generated by this nuclear fusion creates an outward pressure that perfectly balances the inward pull of gravity, achieving a state of hydrostatic equilibrium. This balance stabilizes the star’s size and energy output, locking its position onto the Main Sequence, where it will spend the vast majority of its existence.