What is the difference between the Sun's core temperature and its surface temperature reading?

The core temperature ($15$ million K) is drastically higher than the surface, exemplified by the Sun's surface being around $5,500$ C equivalent.

There exists a profound thermal gradient stretching from the interior to the exterior of a star like the Sun. The core is the powerhouse, achieving approximately $15$ million K. In stark contrast, the visible surface temperature, which dictates the color of the light emitted, is comparatively cool. For the Sun, this surface measurement is equivalent to about $5,500$ degrees Celsius. This massive difference—millions of degrees versus thousands—is vital because this steep gradient is what drives the transport of intense nuclear heat outward through radiation and convection until it finally escapes into space as light.