Stellar Evolution

For celestial bodies, ‘evolution’ doesn’t mean what it means for a living being.

The word “stellar evolution” describes the life of a star; the series of twelve stages in the life of a star.

Stage 1: Due to the dominance of Gravitational force over the heat pressure of an interstellar cloud it starts collapsing and becomes fragmented due to gravitational and thermodynamically instabilities. The dense cloud fragments start trapping produced ration and the temperature starts rising.

Stage 2 & 3: At the start of stage 2 the star starts takin spherical shape, with a 100 K temperature and density of 1012 particles/m3.

After some tens of thousands of years and more contraction of the fragmented cloud takes a diameter nearly like of our entire solar system. It is in stage 3 of the star, where it has a temperature of 10000 K, and density of 1018 particles/m3. The dense opaque centre is known as the Protostar.

Stage 4: Some 1 lakh years after the protostar reaches stage 4, with a core temperature of 106 K. At this stage release of huge gravitational energy makes the star visible. This type of star is detected by infrared rays.

Stage 5: At this stage, the star has a surface temperature of 4000 K and a core temperature of 5*106 K.

Sometimes the protostar undergoes violent surface activities, resulting in protostellar wind. In the evolutionary track, this stage of the star is known as the T-Tauri phase.

The interaction of this strong wind, with the matter around the star (at the nebular disk), results in a bipolar flow; seems two jets of matter perpendicular to the disk of the star.

Stage 6 & 7: Some 10 million years after stage 4, the ideal star is formed. Core temperature reaches 107 K, enough to ignite the nuclear fusion reaction. The surface temperature is about 4500 K; a little less than an ideal star.

Over the next 30 million years the star contracts a little more and core temperature reaches 1.5*106 K, and density reaches to 1032 particles/m3.

The star now radiates the same energy from the surface that is generated at the core.

For the next 10 billion years, the star remains in the main sequence arm of the H-R diagram.

Brown dwarf: Some fragments are too small to become a star. For these stars, the star comes in equilibrium before the nuclear fusion starts in its core. These are brown dwarf. These are scattered throughout the universe in a huge amount.

Stage 8 & 9: As the nuclear fusion turns Hydrogen into Helium, a non-burning pure helium core starts to grow.

Due to no nuclear reaction at the He core the outward pressure due to heat decreases, the gravity becomes dominant once again. The core contracts and releases gravitational energy which enhance the fusion in the shell much more. For this, the radius and luminosity of the star grows and it turns out to a red giant.

At the start of stage 8, the star remains in the Sub giant branch, following the Red giant branch in the next step.

Stage 10: After a few million years the core temperature becomes 108 K, enough to fuse He to C. The nuclear fire reignites. But the core can’t respond to these quick changes and temperature rises in a runway explosion, known as Helium flash. But it doesn’t increase the luminosity of the star.

After the helium flash, the star remains in the Horizontal branch in the evolutionary track.

Stage 11: Like the helium core, now the growing carbon core starts to shrink and the released gravitational energy once again increases the radius and luminosity of the star. The star becomes a red giant once again. This stage in the H-R diagram is known as Asymptotic red giant branch.

Stage 12: Due to intense radiation from the inside of the star, the outer layers start drifting away in space. The produced UV ray from the hot core ionizes the inner part of the cloud, producing a spectacular display, known as a planetary nebula.

Dense Matter: The core is a vast sea of electrons. At this stage, Pauli’s exclusion principle comes to play which stops electrons to collide on themselves anymore which gives a compact structure to it. In this stage, the core is the densest object of our Universe.

Stage 13: The carbon core starts to be visible, based on the energy stored in it.

Some facts about white dwarf: Most white dwarfs are composed of Carbon and Oxygen. But for some low mass stars, it is impossible to reach the point of Helium fusion, the last stage for them is Helium white dwarf.

For some high mass stars, the core produces some Oxygen which further interacts with Helium to form Neon. The last stage for them is Neon-Oxygen dwarf.

The white dwarfs continue to cool and dim, resulting in a cold, dense black dwarf.

Theoretically black dwarf is the last phase o a low mass star, but some times it undergoes different activities, such as Nova. In this case, a white dwarf takes H and He from its companion star, which swirls around the star, and fuses in the orbit, to give rise of Nova.

But Nova is a rare event, and that’s why we can say that Black dwarf is the death-phase of a low mass star.

Question :

What do you think, how can the mass difference of stars affect the death phase of a star? Why does it can vary for stars of different masses?

( You can take help of Google to answer it)