V509 Cassiopeiae is a yellow hypergiant, one of the rarest types of stars. Current estimates suggest there are no more than 15 of these stars in the entire galaxy. Yellow hypergiants are an extremely short-lived phase of late stellar evolution of massive O-type stars. These stars evolve from red hypergiants to blue hypergiants in a phase that lasts only a few thousand years. Many stars lack the mass to go through this phase, or have a much greater mass and end as a supernova.

V509 Cassiopeiae currently has 11 solar masses and a radius 600 times larger than Sol; this would encompass the Sol system past the orbit of Mars. V509 has varied significantly in temperature and size over periods as short as 1 year, indicating that this phase is highly unstable. The attached photo is from a metal-rich orbiting planet at 2300 light-seconds distance.

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