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Astrophotography by GrandpaJunkrat - ----- Star evolution
Cosmology from my armchair..............................
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bastion@protonmail.com The Netherlands
Bortle 7 - 9
Star evolution
Deep within the vast, dusty nebulae of the star-forming region IC 1396 in the constellation Cepheus, something special happened. A small, cold cloud of gas (mainly hydrogen) and dust, left behind by ancient stars or a shockwave from a distant supernova, began to collapse under its own gravity. This was the beginning of a new star.
The Protostar: A Star in the Making
The collapsing cloud grew denser and hotter at its center. Material fell inward, spun faster, and formed a flat, rotating disk. At the heart of this collapse, a dense, hot core formed: the protostar. It wasn't a true star yet, as no nuclear fusion was taking place. All its energy came from the heat generated by the contraction and the infalling matter. Strong magnetic fields channeled some of the infalling material into two tight beams, or jets, shooting away from the poles. These jets helped the protostar shed excess angular momentum. The protostar in IC 1396 was hidden within its own dense dust cloud, primarily visible as infrared heat.
The T Tauri Star: A Turbulent Teenager
Over time, as the protostar had gathered enough mass and its core was compressed and heated further, something important began. The temperature in the core became high enough (a few million degrees) to fuse deuterium (a heavy form of hydrogen). This wasn't the main hydrogen fusion, but it marked the transition to the T Tauri phase.
The star now broke through its dusty envelope and became visible in ordinary light, but it was a true adolescent star:
Its surface boiled due to strong convection (rising hot gases, sinking cool gases).
It had enormous starspots and very powerful magnetic fields.
These magnetic fields caused violent outbursts (flares) and strong stellar winds blown into space at high speed.
It was variable: its brightness changed irregularly.
The remaining accretion disk of gas and dust was still present, but the stellar wind slowly cleared the immediate surroundings. This is the phase where planets begin to form from the material in that disk. The star was still slowly contracting under its own weight.
The Herbig Ae/Be Star: The Bright Up-and-Comer (For Heavier Stars)
Our star in IC 1396 had about 2.5 times the mass of our Sun. Stars heavier than about 2 solar masses go through an extra, shorter but brighter phase after the T Tauri stage: the Herbig Ae/Be phase (Ae for slightly lighter, Be for slightly heavier stars like this one).
The core grew even hotter, nearing the temperature required for hydrogen fusion.
The star became brighter and hotter than T Tauri stars.
The strong surface convection decreased; energy was now mainly transported by radiation (photons) instead of bubbling gas.
Magnetic activity and stellar winds remained important but were often different in nature than in T Tauri stars.
The remnant disk of dust and gas glowed brightly in infrared light but contained much less gas than before. It was now primarily a dust disk where rocky planets or cores of gas giants might be forming.
This phase is fast: massive stars are in a hurry to become adults.
The Main Sequence: Adulthood and Stability
When the core of our star finally reached about 15 million degrees Celsius, the miracle happened: stable and balanced hydrogen fusion (via the proton-proton chain reaction) began. The enormous outward pressure from the fusion energy perfectly balanced the inward crush of gravity. The star stopped contracting and reached the Main Sequence.
This is the longest and most stable phase in a star's life:
It constantly burns hydrogen into helium in its core.
Its size, brightness, and temperature barely change.
For our star of 2.5 solar masses, this meant: a hot, white star (spectral type B or A), much brighter and shorter-lived (about 500 million years) than our Sun. It now shone as a bright point of light in IC 1396.
The End of the Hydrogen: Expanding into a Giant
But even in such a large core, the hydrogen runs out. After hundreds of millions of years, the hydrogen in the core was almost completely burned to helium "ash". Fusion stopped in the core but continued in a shell around the core.
Without the fusion pressure in the core, the core began to contract again under its own weight. This made the core hotter.
This extra heat from the core caused the outer layers to expand enormously and cool down.
The star left the Main Sequence and became first a Subgiant and then a Red Giant.
Its radius became hundreds of times larger! Its surface temperature dropped, turning it red. The star would now have swallowed any inner planets in its system.
Helium Fusion and Second Giant Phase
The contracting helium core became so hot (over 100 million degrees!) that a new fusion ignited: helium core fusion (three helium nuclei fuse into carbon: the triple-alpha process). This often started explosively (the helium flash) for stars of this mass, but stabilized afterward.
The star now burned helium in the core and hydrogen in a shell around it.
It contracted somewhat and became hotter but was still much larger than on the main sequence. This is the Horizontal Branch phase in the Hertzsprung-Russell diagram.
Eventually, the helium in the core also ran out. Fusion continued in a helium fusion shell around the carbon-oxygen core, while hydrogen fusion continued in a shell further out.
The star expanded enormously once more and became an Asymptotic Giant Branch (AGB) Star - a supergiant phase. It became unstable, pulsating, and experienced powerful thermal pulses.
The Final Breath: Planetary Nebula and White Dwarf
The outer layers of the giant were now so far from the hot core and so loosely bound that the intense radiation and the thermal pulses blew them into space. This didn't happen all at once, but in waves over tens of thousands of years.
The ejected gas formed a beautiful, often ring-shaped or complex nebula: a Planetary Nebula. This nebula, illuminated by the hot core within, would be visible for a while in IC 1396.
What remained was the exposed, glowing hot core: an extremely dense object the size of Earth, but with nearly the mass of the original star. This is a White Dwarf.
The white dwarf consists mainly of carbon and oxygen and initially glows bright white from residual heat. It has no energy source left for fusion.
Over billions of years, it will slowly cool and dim, first to a fainter Red Dwarf (cooling sequence, not a true star), then a theoretical Blue Dwarf, and eventually a cold, dark Black Dwarf - the quiet end for stars like this.
Conclusion
This life story, from a cold gas cloud in IC 1396 to a cool white dwarf, is called stellar evolution. The mass of the star at its birth determines everything: how quickly it moves through each phase, how bright and hot it is, and how it ultimately dies. Our star of 2.5 solar masses clearly went through the phases Protostar, T Tauri, Herbig Ae/Be, Main Sequence, Red Giant, Helium Burning (Horizontal Branch), AGB Giant, Planetary Nebula, and ended as a White Dwarf. Each step is a direct consequence of the battle between gravity and nuclear fusion.
For Low-Mass Stars (e.g., Sun-like):
Protostar (Class 0/I) → T Tauri Star (Class II) → Weak-lined T Tauri (Class III) → Main Sequence
For Intermediate-Mass Stars (e.g., Herbig Ae/Be):
Protostar (Class 0/I) → Herbig Ae/Be Star (Class II) → Main Sequence A/B Star
*(Note: High-mass stars (>8 M☉) skip the T Tauri/Herbig phase entirely due to rapid collapse.)*
The typical age of a Young Stellar Object (YSO) depends on its evolutionary stage, which is classified based on spectral energy distributions (SEDs) and physical characteristics. Here’s a breakdown of YSO ages by evolutionary phase:
Age: < 0.1 million years (Myr)
Characteristics: Deeply embedded in collapsing molecular cores, invisible at optical wavelengths, with strong submillimeter emission.
Age: 0.1–0.5 Myr
Characteristics: Still accreting material from an infalling envelope, with rising mid-infrared SEDs due to warm dust.
Age: 0.5–1 Myr
Characteristics: Transition phase between Class I and II, where the envelope dissipates, and a protoplanetary disk dominates.
Age: 1–10 Myr
Characteristics: Pre-main-sequence stars with prominent circumstellar disks (e.g., T Tauri stars). Herbig Ae/Be stars (intermediate-mass YSOs) fall in this range.
Age: 10–100 Myr
Characteristics: Nearly disk-free, approaching the main sequence. Variability and X-ray emission are common.
Mass Dependence: Higher-mass stars (e.g., Herbig Ae/Be) evolve faster, reaching the main sequence in < 10 Myr, while low-mass stars (e.g., T Tauri) take longer.
Uncertainties: Age estimates from HR diagrams can be skewed by variability, binarity, and accretion history.
The Standard YSO Evolutionary Sequence
For low- to intermediate-mass stars (like the Sun or Herbig Ae/Be stars), the stages are:
Protostar (Class 0/I)
Collapsing gas cloud, no fusion yet.
Strong accretion, deeply embedded in dust.
Example: L1527 IRS (Class 0).
Class II YSO (T Tauri or Herbig Ae/Be Star)
Fusion begins (deuterium or hydrogen).
Accretion continues via a protoplanetary disk.
Examples:
T Tauri stars (low-mass, <2 M☉).
Herbig Ae/Be stars (intermediate-mass, 2–8 M☉).
Class III YSO (Weak-lined T Tauri or Pre-Main Sequence Star)
Disk mostly dissipated.
Star approaches the main sequence.
Mass determines the path, not time:
T Tauri stars = Low-mass (<2 M☉) analogs of Herbig Ae/Be stars.
Herbig Ae/Be stars = Intermediate-mass (2–8 M☉) havy versions of T Tauri stars.
Both are Class II YSOs, but with different properties:
