Since the fate of the earth & humans are governed by the same laws, the earth will at that point in time also have evolved & become a sun
"A human being travels the same trajectory as the earth. In the same way that the earth came from the primordial fire, a human being is a spirit, a spark from the burning bosom of the Eternal. He has, in other words, had to travel a considerable way and, just like the sun, he has grown dull, cold and dark on the way. But human beings are predestined to regain the regions they once left, and one day, after many incarnations, they will return to their celestial Father. And since the fate of the earth and the fate of human beings are governed by the same laws, the earth will at that point in time also have evolved and become a sun."