The description of the word you requested from the astronomical dictionary is given below.
core, plural cores
The core of the Sun is the centermost part of the Sun, where all the Sun's energy is produced by nuclear processes. It has a radius of about 86,000 miles (140,000 km). It contains about 1/120 of the Sun's volume (up to the visible surface), and about 1/3 of the Sun's total mass. At the very center of the Sun, the temperature is thought to be about 16 million K (28 million ℉, 16 million ℃), and the density about 150 times that of water. At the outer edge of the core, the temperature is thought to be 9 million K (17 million ℉, 9 million ℃), and the density 34 times that of water. The layer immediately around the core is the convection layer.