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Subrahmanyan Chandrasekhar: Theorist of Stellar Evolution
Chandrasekhar limit, white dwarfs, and black hole formation
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Subrahmanyan Chandrasekhar was one of the most important theorists of stellar evolution, the study of how stars change over time. As a young physicist, he calculated that white dwarf stars can remain stable only below a certain mass, now called the Chandrasekhar limit. This result helped explain why some dying stars quietly fade while others collapse into neutron stars or black holes. His work connected quantum physics, relativity, and astronomy in a way that reshaped modern astrophysics.
The key idea is that a white dwarf is supported by electron degeneracy pressure, a quantum mechanical pressure that resists gravitational collapse. Chandrasekhar showed that if the star's mass is greater than about 1.4 times the mass of the Sun, this pressure is not strong enough to support it. Above that limit, the star must continue collapsing or explode in a supernova, depending on its conditions. Chandrasekhar received the 1983 Nobel Prize in Physics, and NASA's Chandra X-ray Observatory is named in his honor because X-rays reveal hot gas, compact stars, and black holes.
Key Facts
- The Chandrasekhar limit is about 1.4 solar masses: Mlimit ≈ 1.4 Msun.
- A white dwarf is supported mainly by electron degeneracy pressure, not by nuclear fusion.
- If M < 1.4 Msun, a stellar core can become a stable white dwarf.
- If M > 1.4 Msun, electron degeneracy pressure cannot stop collapse of the core.
- Possible outcomes of massive stellar cores include neutron stars and black holes.
- Schwarzschild radius for a nonrotating black hole is Rs = 2GM/c^2.
Vocabulary
- Chandrasekhar limit
- The maximum mass, about 1.4 times the Sun's mass, that a white dwarf can have before it becomes unstable to gravitational collapse.
- White dwarf
- A small, dense stellar remnant formed when a low or medium mass star exhausts its nuclear fuel and sheds its outer layers.
- Electron degeneracy pressure
- A quantum pressure caused by electrons resisting being squeezed into the same low energy states.
- Neutron star
- An extremely dense stellar remnant made mostly of neutrons, formed after the collapse of a massive star's core.
- Black hole
- A region of space where gravity is so strong that nothing, not even light, can escape from inside the event horizon.
Common Mistakes to Avoid
- Treating the Chandrasekhar limit as exactly 1.4 solar masses in every situation is wrong because composition, rotation, and detailed physics can slightly change the value.
- Saying white dwarfs shine because of ongoing fusion is wrong because most white dwarfs no longer fuse elements and instead glow from stored thermal energy.
- Assuming every star becomes a black hole is wrong because lower mass stars usually end as white dwarfs and only sufficiently massive collapsed cores can form black holes.
- Confusing a supernova with ordinary stellar burning is wrong because a supernova is a rapid explosive event, not a steady energy producing stage like main sequence fusion.
Practice Questions
- 1 A white dwarf has a mass of 1.2 Msun. Is it below or above the Chandrasekhar limit of 1.4 Msun, and would it be expected to remain stable as a white dwarf?
- 2 A compact stellar core has a mass of 2.8 Msun. Using Rs = 2GM/c^2, G = 6.67 x 10^-11 N m^2/kg^2, c = 3.00 x 10^8 m/s, and Msun = 1.99 x 10^30 kg, estimate its Schwarzschild radius in kilometers.
- 3 Explain why Chandrasekhar's calculation required both quantum physics and gravity, and why this made his work important for understanding the final stages of stellar evolution.