Stellar Evolution in HR diagram Part I : protostar, pre-main sequence phase 🥏

All stars in the universe move through phases in their Stellar Evolution as stars are born, become stable & remain stable for about 90% of their life cycle & then eventually die. The subject term “Stellar Evolution” is used to describe this journey of a star from being born to eventually dying (for low & average mass stars, they turn into white dwarfs and eventually into black dwarfs after the cessation of nuclear fusion in their cores & upper shells, for high mass stars, they would either turn into a neutron star or a black hole once nuclear fuel in the star is emptied). Detailed explanation below__
 
For a star like ours, aka the Sun, it is presently in its stable phase, maintaining a precise balance between the inward gravitational force trying to collapse the star’s core and the outward pressure generated from nuclear fusion in the core. This balance is called the hydrostatic equilibrium, which is being maintained between the inward gravitational force & the outward nuclear fusion pressure. To add, nuclear fusion in the star’s core is the conversion of Hydrogen into Helium by fusing 4 Hydrogen atoms, thus giving a Helium atom & the release of energy as light.
 
Stars are classified based on temperature from hottest to coldest in the “Hertzsprung-Russell” diagram, as shown below. The x-axis denotes the spectral class, meaning the star’s surface temperature which increases to the left. So the hottest stars get lined up on the left while the coolest stars are placed on the right. Also, the x-axis spectral classes are ordered as O B A F G K & M from hottest to coldest, ie from left to right of the graph. The y-axis of the HR diagram denotes the Luminosity or Absolute Magnitude of stars relative to the Sun, meaning how much energy a star emits or the intrinsic brightness of a star as compared to our Sun. Here, the brighter stars are located higher on the y-axis, while the dimmer stars are located lower. The dark grey strip running diagonally from the bottom right to the upper left of the HR diagram is called the Main Sequence region/ phase of stars. It is this region where all small, medium & big mass stars lie while fusing hydrogen in their core. For all sizes of stars, nearly 90% of their Stellar life cycle is spent in this main sequence region. Our star, the Sun, which is an average or medium mass star is at present part of this main sequence region in the HR diagram. The same can be viewed in the above image. For our Sun will remain in this main sequence region till nuclear fusion in the core takes place. Once nuclear fusion ceases, as and when the core becomes bereft of hydrogen atoms & the core becomes a helium core, then the hydrostatic equilibrium between gravitational force & core pressure will be lost resulting in the Sun leaving the main sequence region & moving up in the y-axis in the HR diagram thus becoming a Red Giant & eventually when all of the nuclear fusions would cease, meaning hydrogen fusion in the outer star shell & helium fusion in the core stops (ultimately leaving a carbon core) then eventually our Sun would turn into a white dwarf & remain a white dwarf for billions of years. Also, the position of our Sun in the HR diagram would change from the Red Giant region on the right to the White Dwarf region on the left.

In the HR diagram, the main sequence & post-main sequence phases of a star’s stellar evolution can be easily understood & marked based on characteristics like a star’s surface temperature & luminosity. As in the diagram, stars are categorised as either main sequence stars or supergiants or white dwarfs, etc, but what about the newly born stars, meaning how would a protostar & its journey to becoming a star be marked or appear on this HR diagram?
 
Now, when a GMC (Giant Molecular Cloud) starts to collapse & forms the “Bok Globules”, then in due course these Bok Globules house the many proto stars & their circumstellar discs. The temperature of these obscure protostars inside the Bok Globules keeps rising & finally, when they are hot & bright enough, they embark on the HR diagram from the right side (as temperature rises from right to left & luminosity increases from bottom to top in the HR diagram). These newly on-boarded protostars then contract due to gravitational force acting inward, leading to an increase in the temperature, but with a temperature rise, the gas molecules in the protostars speed up & they start to move faster & faster, thus increasing the internal pressure of the individual protostars. This then results in the slowing down of the gravitational contraction of the protostars, which eventually slows down the temperature rise as well. After a while, when the gas particles slow down, there is not enough thermal energy for the particular protostar’s luminosity to continue brightening & so it starts to dim down. In simpler words, as a protostar contracts, its internal pressure rises, which then slows down the temperature rise & eventually decreases its luminosity.
 
The temperature of the protostar at this juncture remains quite constant, which elucidates the vertical drop in the HR diagram & thus shows the protostars evolving into pre-main sequence stars. With the protostars evolving into pre-main sequence stars, in due course the temperature in the core of these stars finally breaches the mark to kick start thermonuclear fusion of Hydrogen atoms into an Helium atom at which point these pre-main sequence stars can at last be considered as stars as they land on the “Zero age main sequence” line or the ZAMS line. The ZAMS line is generally situated or marked at the left most edge of the main sequence region & it represents a star’s evolution from being a protostar to pre-main sequence star to finally a star where the dominant energy production has become the thermonuclear fusion in the star’s core rather than the gravitational contraction force which also fuelled the star’s luminosity to glow more bright. Here, the inward gravitational contraction force is balanced by the outward thermonuclear fusion in the star’s core & so the star is now turning stable in the main sequence region of the HR diagram. The exact path which a star follows in its pre-main sequence evolution depends solely on its mass, meaning the mass of a protostar determines the shape of its path of evolution, evolving from being a protostar to a pre-main sequence star to a main sequence star. Different protostars with varying masses will have different evolution paths with different time scales. A detailed explanation regarding the evolution paths of stars coming into the main sequence region in the HR diagram will be discussed in the “Hayashi & Henyey Tracks” blog later. And thus with this, we have completed part I of Stellar Evolution of a star from being born inside a Bok Globule to its journey from a protostar to turning into a pre-main sequence star to finally into a main sequence star as a stable star in the HR diagram, where it will reside for 90% of its Stellar lifecycle. In part II of Stellar Evolution, we will discuss the post-main-sequence phase of stars & how they move around & plot themselves in the HR diagram.