# Harvard Astronomy 201b

## CHAPTER: Important Properties of Local Thermodynamic Equilibrium

In Book Chapter on March 7, 2013 at 3:25 am

(updated for 2013)

For actual local thermodynamic equilbrium (not ETE), the following are important to keep in mind:

• Detailed balance: transition rate from j to k = rate from k to j (i.e. no net change in particle distribution)
• LTE is equivalent to ETE when $b_j = 1$ or $\frac{b_j}{b_k} = 1$
• LTE is only an approximation, good under specific conditions.
• Radiation intensity produced is not blackbody illumination as you’d want for true thermodynamic equilibrium.
• Radiation is usually much weaker than the Planck function, which means not all levels are populated.
• LTE assumption does not mean the Saha equation is applicable since radiative processes (not collisions) dominate in many ISM cases where LTE is applicable.