Observations indicate that all Be stars are rotating rapidly, at up to 90% of the velocity at which gravitational force is balanced by centrifugal force at the star's equator (~400 km/s). In effect, material at the surface of the star is almost in orbit, so that only a slight additional force is necessary to move it into the circumstellar disk.
The rapid rotation of Be stars confuses the issue of their position in the color-magnitude diagram, making it difficult to see how they fit into the stellar evolutionary sequence. Depending on the inclination, the combination of oblateness and gravity darkening can displace a B star from the ZAMS into the subgiant and giant regions, causing it to appear more evolved than it really is. Light from the circumstellar envelope also contributes to the difficulty of observing the properties of the underlying star.
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