Embarking on a Cosmic Odyssey: Understanding Pre-Main-Sequence Stars

This guest post was written by Menelaos Raptis. Menelaos is an undergraduate student at Franklin & Marshall College, majoring in astrophysics. His independent research focuses on determining the chemical abundances of distant galaxies as part of the CECILIA survey and some of his hobbies include playing badminton and solving difficult physics problems.

Paper 1 Title: Stellar Evolution In Early Phases Of Gravitational Contraction

Authors:  C. Hayashi

First Author’s Institution: Department of Nuclear Science, Kyoto University, Kyoto, Japan

Status: Published in Publications of the Astronomical Society of Japan [open access]

Paper 2 Title: The Early Phases of Stellar Evolution

Authors:  L. G. Henyey, R. Lelevier, R. D. Levée

First Author’s Institution: Berkeley Astronomical Department and Livermore Radiation Laboratory, University of California, USA

Status: Published in Publications of the Astronomical Society of the Pacific [open access]

Short Life, big track

Used to be a pre-main-sequence star – I am dying

Used to be cold and distant

Dusty and Unstable

Love’s track made me a contracting superhero – I am dying

They call it Hayashi track, it’s not love actually

I was shrinking and shrinking until my cells

Were filled with heat.

Until my mind was gravitationally compressed – I am dying

Got a job and acquired stability

They call it Main Sequence, it’s not really stability 

The little prince visited me and let one of his roses on my surface

It is burned and brown and not here and dark now

I am dying

I am a white dwarf

I am a black dwarf

I died 🙂       

Menelaos Raptis

Formation of a Protostar:

Figure 1. If the cloud can cool sufficiently, gravity will always cause a contraction and subsequent collapse. Many star-forming clouds appear to have collapsed spontaneously in this way. The process starts slowly, but can become more violent as the first stars form (Animation credit: ALMA, Tokuda & Onishi).

Todays’s bite delves into the fascinating journey of stars from their early gravitational phases to attaining the status of Zero-Age Main Sequence (ZAMS) stars. The exploration focuses on two distinct theoretical evolution paths, referred to as Henyey and Hayashi tracks, shedding light on the details of stellar evolution. The bite unravels the factors influencing the trajectory of stars on the Hertzsprung-Russell (H-R) diagram, emphasizing the transition from the pre-main sequence (PMS) phase to the Main Sequence.

Protostars are born within large, cold, and dense molecular clouds composed mainly of molecular hydrogen (simulation of Figure 1). These clouds can span tens to hundreds of light-years and contain both gas and dust. Some regions within a molecular cloud become more dense than others due to various factors like shock waves from nearby supernovae. When a region becomes gravitationally unstable, it can start to collapse under its own gravity, as shown in Figure 1.

The dense core within the collapsing region undergoes gravitational collapse. As it collapses in various phases, conservation of angular momentum causes it to start spinning, forming a rotating disk of gas and dust around the central concentration. Meanwhile, the surrounding material continues to collapse towards the center, forming a protostellar object, the precursor of a star. You can view the process of collapsing through a simulation here. The simulation considers a smoothed-particle hydrodynamics approach of modeling the properties of a fluid.

The material that falls toward the center forms a protostellar disk, which surrounds the central protostar-to-be. As material continues to accrete onto the central concentration, the temperature and pressure at the core increase. Eventually, nuclear fusion reactions are ignited in the core, marking the birth of a protostar. This marks the end of the protostellar phase and the beginning of the pre-main-sequence phase.

Pre-main-sequence:

The pre-main sequence (PMS) stage in stellar evolution marks the transition between the formation of a protostar and its entry into the main sequence. During this period, the star undergoes contraction, increasing internal temperatures. Under these conditions, the star is in the PMS stage, which lasts until hydrogen burning commences.

Figure 2. (Left) We see the H-R Diagram. An approximate portion of the pre-main sequence for relatively high color index (B-V) protostars is drawn on the plot. The plot was created using the  HIPPARCHOS data set, which contains accurate measurements of about 120,000 stars. (Right) A significantly more accurate representation of the pre-main sequence “area” on the H-R diagram with respect to the main sequence line. As we can see, a possible track of the pre-main sequence evolution for a protostar is depicted on the plot (Credit: Penn State Astronomy & Astrophysics).

To visualize and understand the pre-main sequence, astronomers utilize the Hertzsprung-Russell (H-R) diagram:

The paths protostars follow in the pre-main sequence are elucidated through theoretical evolution tracks, notably Henyey and Hayashi tracks. Henyey tracks detail the late phases of pre-main sequence stars on the H-R diagram, portraying the evolution of a protostar until it reaches the main sequence. These tracks are influenced by the star’s mass, metallicity, and hydrogen/helium abundance. Henyey’s calculations reveal that higher hydrogen content displaces the track downward, while lower metallicity shifts it upward. Note that there is a lower limit on what the mass of the protostar should be in order to follow the Henyey track, namely 0.6 solar masses. There is also an upper limit of 8 solar masses. There are no PMS stars with masses greater than that; high-mass stars don’t have a PMS evolution (they are born adults!).

The Hayashi tracks, depicted as vertical blue lines on the H-R diagram, outline the evolution of pre-main sequence stars in a fully convective phase. This phase, occurring in stars up to 3 solar masses, is marked by the star contracting towards its final size while maintaining convective equilibrium. Hayashi considered conservation of mass, hydrostatic equilibrium, energy transport, and other stellar structure equations to predict these tracks.

Prior to the development of the Henyey and Hayashi tracks, our understanding of stellar evolution, especially in terms of theoretical models representing a star’s life on the Hertzsprung-Russell (H-R) diagram, faced challenges. Existing models lacked precision, especially when it came to describing the evolution of protostars of different masses and compositions The Henyey and Hayashi tracks revolutionized our understanding of stellar evolution, providing detailed insights into the complex interplay of gravitational collapse, energy transport mechanisms, and nuclear processes during the early phases of a star’s life. Their significance lies in bridging the gaps of theoretical models, particularly in describing the evolution of protostars with different masses and compositions. Despite the fact that this work is originating in the mid-20th century, their enduring significance highlights their timeless contribution to our comprehension of stellar processes.

Astrobite edited by Sonja Panjkov

Featured image credit: NASA and the Night Sky Network 

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