Astrophysics of the Diffuse Universe by Michael A. Our purpose in writing this book is to show how p… More.
Stellar core - Wikipedia
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Principles of Stellar Interferometry by Andreas Glindemann. The imaging process in stellar interferometers is… More. Shelve Principles of Stellar Interferometry. A corrected reprint of the? This book, written by the designer of the worlds… More. A stellar core is the extremely hot, dense region at the center of a star. For an ordinary main sequence star, the core region is the volume where the temperature and pressure conditions allow for energy production through thermonuclear fusion of hydrogen into helium.
This energy in turn counterbalances the mass of the star pressing inward; a process that self-maintains the conditions in thermal and hydrostatic equilibrium. The core is surrounded by the stellar envelope, which transports energy from the core to the stellar atmosphere where it is radiated away into space. Main sequence stars are distinguished by the primary energy generating mechanism in their central region, which joins four hydrogen nuclei to form a single helium atom through thermonuclear fusion.
The Sun is an example of this class of star. Above this spherical radiation zone lies a small convection zone just below the outer atmosphere. At lower stellar mass , the outer convection shell takes up an increasing proportion of the envelope, and for stars with a mass of around 0. The low mass end of the VLMS range reaches about 0. The temperature of the core region for a VLMS decreases with decreasing mass, while the density increases.
For a star with 0. Even at the low end of the temperature range, the hydrogen and helium in the core region is fully ionized. Below about 1. For stars above this mass, the energy generation comes increasingly from the CNO cycle , a hydrogen fusion process that uses intermediary atoms of carbon, nitrogen, and oxygen.
In the Sun, only 1. For stars at 1. This results in a stronger thermal gradient, which creates convective instability. Hence, the core region is convective for stars above about 1. For all masses of stars, as the core hydrogen is consumed, the temperature increases so as to maintain pressure equilibrium. This results in an increasing rate of energy production, which in turn causes the luminosity of the star to increase. The lifetime of the core hydrogen—fusing phase decreases with increasing stellar mass.
For a star with the mass of the Sun, this period is around ten billion years. Once a star has converted all the hydrogen in its core into helium, the core is no longer able to support itself and begins to collapse.
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It heats up and becomes hot enough for hydrogen in a shell outside the core to starts fusion. The core continues to collapse and the outer layers of the star expand. At this stage, the star is a subgiant. Very-low-mass stars never become subgiants because they are fully convective. Stars with masses between about 0. They spend several billion years on the subgiant branch, with the mass of the helium core slowly increasing from fusion of the hydrogen shell.
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Eventually the core becomes degenerate and the star expands onto the red giant branch.