V509 Cassiopeiae

Star in the constellation Cassiopeia

| b v=+1.0 +1.7
| variable=SRd
| prop mo ra=−2.787
| dist ly=
| dist pc=4,810

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. 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. 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 de reddened to account for interstellar extinction, but the small change corresponds to records of the spectrum and are considered to be real. The colour then reddened dramatically to a B−V value of as much as 1.6 magnitudes in 1973, dropped rapidly to 0.02 by 2000, and has remained about constant since then. The detailed observations available since 1960 also show rapid colour variations of about 0.2 magnitudes on scales of 1–5 years super imposed on the overall trends.

It appears that HR 8752 is not just varying in brightness and fluctuating in temperature and size like most unstable stars, but is actually undergoing a secular evolutionary shift from cooler to hotter temperatures. The temperature can be estimated with some accuracy from the spectral and colour observations. The calculated effective temperature increased from 4,500K in 1900 to 5,000K in 1960. At that stage the luminosity was around and the radius
The star then varied erratically until 1973 when it rapidly expanded and cooled. A detailed spectral analysis in 1977 reported a temperature low of 4,000K, with a peak luminosity in 1976 of with a radius of The surface gravity at this time was calculated to be log(g) = 2, indicating that the visible surface was effectively detached from the star. The star then rapidly returned to around its previous temperature of 5,000K, a luminosity of , and radius of .

Prior to 1973, HR 8752 was a cool yellow hypergiant with an early G spectral type. Following a dramatic shedding of its outer layers, it has now jumped to mid A hypergiant and is not expected to return to its cool state. Models of a ZAMS star show it crossing the "yellow evolutionary void" instability region first towards cooler temperatures, then later back towards hotter temperatures. The yellow evolutionary void is named because very few stars are found in that part of the H–R diagram. This is likely to be because the evolution of stars with such parameters is extremely rapid, perhaps even almost instant in astronomical terms. The first crossing of the yellow evolutionary void is very rapid but the star does not experience major instability. The second crossing, returning to hotter temperatures after a time as a yellow hypergiant, involves crossing a region, or possibly two regions, where the star experiences major instability, expected to show as episodes of strong mass loss. HR 8752 has crossed the first of the two major zones of instability and is expected to migrate to even hotter temperatures over a timescale on the order of a thousand years. Based on its current observed state, HR 8752 is estimated to now have left from an initial and is likely to become a relatively low luminosity luminous blue variable before evolving further into a Wolf–Rayet star.

HR 8752 may have a companion. Measurements of the ultraviolet spectral distribution show an excess that corresponds to the output of a B1 main sequence star. The absolute magnitude was estimated at 4.5, approximately 40 times fainter than the primary at visual wavelengths. Although the stars must be fairly close (< 1400AU), no radial velocity variations have been detected in the spectral lines of the primary, and no lines are observed which can be attributed directly to the secondary. The observed spectrum may be mostly from a shell surrounding both stars. It has been suggested that some variations in spectral line profiles are caused by variations in colliding winds or disturbances of previously ejected material, caused during a periastron passage of the companion.