It seems there are some misconceptions in your statement regarding the temperature of Stephenson 2-18 (St2-18) and its comparison to the Sun.
Stephenson 2-18 is a red supergiant star located in the constellation of Scutum, and it is known for being one of the largest stars by radius. Its surface temperature is indeed cooler than that of the Sun, which averages around 5,500 degrees Celsius (about 5,800 K). The surface temperature of St2-18 is estimated to be around 3,500 degrees Celsius (about 3,800 K), which is significantly lower than the Sun's temperature.
However, the core of St2-18, like that of other massive stars, reaches much higher temperatures due to the processes of nuclear fusion. In a typical massive star's core, temperatures can reach around 15 million degrees Celsius during hydrogen burning, and potentially even higher during later stages, but they do not approach a billion degrees Celsius.
To clarify, St2-18 is not a billion degrees Celsius; such temperatures are found in the interiors of neutron stars or during specific stellar events like supernovae, not in the cores of normal stars.
If you have any more questions about Stephenson 2-18 or any other astronomical topics, feel free to ask!