V509 Cassiopeiae
A yellow hypergiant that has crossed the yellow evolutionary void.
V509 Cassiopeiae (V509 Cas or HR 8752) is one of two yellow hypergiant stars in Cassiopeia, the other being Rho Cassiopeiae. Located roughly 11,000 light-years from Earth, it has varied in apparent magnitude from below +6 historically to a peak of +4.6 and now around +5.3. It is classified as a semiregular variable star of type SRd and is undergoing strong mass loss as part of its rapid evolution.
Quick Facts
- Distance
- 11,000 light-years
- Apparent magnitude range
- +4.6 to +5.3
- Variable type
- semiregular variable (SRd)
- Spectral type (companion)
- B1V
- Companion discovered
- 1978
Facts from the source article.
Observations
HR 8752 is visible to the naked eye, yet it carries no Bayer or Flamsteed label and appears in no earlier star catalogues before the 1800s. When it was first listed in the Radcliffe Observatory catalogue in 1840, it shone at 6th magnitude, and astronomers assume it had been that dim or fainter beforehand. The star flickers slightly over roughly a year, but its average brightness steadily rose, hitting magnitude 5.0 by the 1950s. It brightened further to magnitude 4.75 by 1973, though the start of that change went poorly observed. It reached a peak of magnitude 4.6 in 1976, then fell quickly to magnitude 4.9 by 1979, and wobbled between 4.75 and 4.85 for the following decade. After that, its brightness generally faded, with irregular shifts of less than 0.1 magnitude, down to magnitude 5.3 in 2000. As of 2025, it has held steady at magnitude 5.3 for at least eight years. There are old records of new stars in Cassiopeia that might match earlier outbursts of HR 8752, but this link is very uncertain. Astronomers have tracked its spectral type and colour for over a hundred years. The star was seen as odd and likely very luminous, but not variable, and was even proposed as a spectral standard for type G0Ia. Its colour, measured as the difference between blue and visual magnitudes (B-V), may have dipped slightly from about 1.2 in 1900 to 0.8 in the 1960s. Then it reddened sharply to as much as 1.6 in 1973, dropped quickly to 0.02 by 2000, and has stayed roughly constant since. Detailed observations from 1960 onward also reveal rapid colour swings of about 0.2 magnitudes over 1 to 5 years, layered on top of the longer-term trends.
Properties
HR 8752 is not just flickering like most unstable stars; it appears to be making a long-term evolutionary shift from a cooler to a hotter state. Its effective temperature rose from 4,500 K in 1900 to 5,000 K by 1960. After erratic behavior, the star suddenly expanded and cooled in 1973. By 1977, spectral analysis showed a temperature low of 4,000 K and a peak luminosity in 1976, with a surface gravity of log(g) = -2, meaning the visible surface was essentially detached from the star. It then quickly warmed back to about 5,000 K. Beginning in 1985, HR 8752 underwent a dramatic change, heating to around 8,000 K and shrinking by 2000, with its surface gravity returning to a more typical supergiant value near log(g) = 1.0. In just a few decades, it crossed a region of the H–R diagram where no other stars are observed—an evolutionary shift never seen in any other star. As of 2022, it remained stable at 7,900 K. In 1998, its angular diameter was measured in milliarcseconds, giving a physical radius at its estimated distance. More recent CHARA array observations in 2023 found a slightly smaller angular diameter, yielding a slightly different radius. Elemental abundances are roughly solar, though some elements are enhanced due to the star's evolutionary stage.
Evolutionary state
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 main sequence 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, likely because the evolution of stars with such parameters is extremely rapid. The first crossing of the 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. The ultimate fate of all massive stars is a core collapse and some sort of supernova explosion. For some intermediate range of masses, stars are thought to undergo core collapse at the yellow hypergiant or LBV stage, resulting in a type IIb or perhaps IIn supernova. HR 8752 may be such a star, and may never make it beyond its current evolutionary state before exploding.
Possible binary
There is evidence that HR 8752 is not a single star. An excess in ultraviolet light matches what would be expected from a B1 main-sequence star, which is about 40 times dimmer than the primary in visible light, with an absolute magnitude around -4.5. The two stars must be relatively near each other, within 1400 AU, yet no radial velocity shifts show up in the primary’s spectral lines, and no lines from the secondary have been directly seen. What we observe may actually be a shell of material surrounding both stars. Some changes in the spectral line shapes might come from colliding stellar winds or from disturbances in previously expelled gas, possibly triggered when the companion passes closest in its orbit.
More in Stars of Cassiopeia
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced