ASTRON 1101 Lecture Notes - Lecture 21: Red-Giant Branch, Triple-Alpha Process, Horizontal Branch

• School: Ohio State University
• Department: Astronomy
• Course: ASTRON 1101
• Professor: Barbara Ryden

Document Summary

Low-mass stars have a mass of less than 4 times the mass of the sun (so the sun is a low-mass star) As the sun evolves, its position on the h-r diagram changes. Pre-main-sequence phase begins with energy provided by the kelvin-helmholtz (k-h) mechanism, ends with energy provided by hydrogen fusion. As hydrogen is converted to helium, the number of particles in the core decreases. Higher core temperature = higher energy production rate. The sun is now on the main sequence: in hydrostatic equilibrium, in thermal equilibrium, becoming only gradually more luminous. Energy is produced both by the k-h mechanism in the core and by the hydrogen fusion in the shell. Energy is produced faster than the star can radiate it away (loss of thermal equilibrium) Excess energy does work on the outer envelope, making it expand (and thus cool down) The star now becomes a red giant.