As long as the yellow hypergiant HR8752 has been observed spectroscopically it has shown erratic and significant fluctuations in its effective temperature. But an impressive and hitherto never observed rise in its temperature started around 1985. Since that time Teff has risen from 4600 K to 7900 K. Regular further observations are needed to see if and when this rise will stop, and what will happen thereafter. The instability is related to the fact that in its evolution the star has entered the Yellow Evolutionary Void, a region in the Hertzsprung-Russell diagram where blueward-evolving supergiants have unstable atmospheres and where the stars are also dynamically unstable. The rise in temperature started after a period of enhanced mass loss and at a time when the photospheric acceleration was directed outward. Recently (1998) the photospheric effective acceleration has again decreased to below zero. This star is the first in which dynamic instability can be studied observationally in great detail. We hypothesise that HR8752 will traverse the Void and thereafter, after a period of stability, will enter the "blue'' region of dynamic instability, and then may become a star like P Cygni.