V509 Cassiopeiae (V509 Cas or HR 8752) is one of two yellow hypergiant stars found in the constellation Cassiopeia, which also contains Rho Cassiopeiae. HR 8752 is around 15,700 light-years from Earth. It has an apparent magnitude that has varied from below +6 in historical times to a peak of +4.6 and now around +5.3 and is classified as a semiregular variable star of type SRd. It is undergoing strong mass loss as part of its rapid evolution and has recently passed partway through the yellow evolutionary void by ejecting around a solar mass of material in 20 years. A hot main sequence companion (B1V) was described in 1978 on the basis of a colour excess in the ultraviolet. Observations Brightness HR 8752 is a naked eye star but it has no Bayer or Flamsteed designation, and is not recorded in other catalogues before the 19th century. When first recorded in the Radcliffe Observatory catalogue in 1840 it was 6th magnitude, and it is assumed it had been 6th magnitude or fainter before then. The star is slightly variable on a timescale of around a year, but the average brightness increased steadily, reaching magnitude 5.0 in the 1950s. The brightness climbed to magnitude 4.75 by 1973, but the exact onset of this event was not well observed. Since then the star has been studied much more closely. It peaked at magnitude 4.6 in 1976, then dropped quickly to magnitude 4.9 by 1979, then oscillated between magnitudes 4.75 and 4.85 for the next decade. Since then the brightness has generally decreased, with somewhat irregular variations of less than a tenth of a magnitude, to magnitude 5.3 in 2000 and may have stabilised at that level. There are possible historical records of new stars in Cassiopeia that could correspond to earlier outbursts of HR 8752, but the association is highly speculative. Spectrum Spectral types and colour comparisons for HR 8752 have been made regularly for over a century. The star was recognised as somewhat unusual and probably highly luminous, but not variable. It was actually proposed as a spectral standard for type G0Ia. The colour of the star as measured by the difference between blue and visual magnitudes (B−V) may have decreased slightly from about 1.2 in 1900 to 0.8 in the 1960s. Measurements in different eras are not always calibrated to the same spectral bands, and the values have to be