The location of B supergiants in the Hertzsprung-Russell diagram ( HRD ) represents a long-standing problem in massive star evolution .
Here we propose their nature may be revealed utilising their rotational properties , and we highlight a steep drop in massive star rotation rates at an effective temperature of 22 000 K. We discuss two potential explanations for it .
On the one hand , the feature might be due to the end of the main sequence , which could potentially constrain the core overshooting parameter .
On the other hand , the feature might be the result of enhanced mass loss at the predicted location of the bi-stability jump .
We term this effect “ bi-stability braking ” and discuss its potential consequences for the evolution of massive stars .