Various assumptions used in interpreting the observations of hydrogen recombination lines are critically assessed to confirm the gradient of electron temperature with distance from the galactic center. The total temperature increase from 5 to 13 kpc is about 2,500 K. Among many suggestions, we have singled out the decrease of trace dement abundances with the galactoccntric distance as the most viable cause for the temperature gradient. This will impose an important constraint on evolutionary models of the Galaxy.

ABSTRACT
Ⅰ. INTRODUCTION
Ⅱ. ELECTRON TEMPERATURE FROM LINE-TO-CONTINUUM RATIO
Ⅲ. GALACTOCENTRIC GRADIENT IN ELECTRON TEMPERATURE
Ⅳ. POSSIBLE CAUSE FOR THE TEMPERATURE GRADIENT
Ⅴ. CONCLUDING REMARKS
REFERENCES

• SEUNG SOO HONG(Department of Astronomy, Seoul National University)