According to stellar metamorphosis theory we can see stars that are very, very cold, only in the tens of Kelvin in their high atmospheres. Since they were really hot in the many tens of thousands of Kelvin when they were born, it is clear they have cooled down significantly. Fortunately there is a simple mathematical/physical relationship that can explain how this occurred. It is the 1st Law of Thermodynamics. Since stars produce heat, and have no mechanism for replacing the heat lost, they will cool. Therefore heat loss will continue indefinitely on all stars that shine and are not heated on their exteriors similar to Hot Jupiters. Heat loss will also continue on stars that do not shine strongly, as they are gravitationally collapsing, producing heat via chemical reactions, tugging on other objects gravitationally, their magmas are cooling, etc.