Rho Cassiopeiae

Did You Know?

• Rho Cassiopeiae has no traditional proper name and, as of 2026, has not been assigned an official name by the IAU.
• Its Flamsteed designation is 7 Cassiopeiae.
• In Chinese astronomy, it is a member of the constellation Flying Serpent in the Encampment mansion.

Observation

Rho Cassiopeiae is the second brightest yellow hypergiant in the sky, after V382 Carinae, and is mostly visible in the northern hemisphere. Its variability was first described in 1901, and its spectrum was initially classified only as peculiar with a small range of variation. During a deep visual minimum in 1946, the spectrum developed features typical of an M star rather than its previous F8 class, and an expanding shell was detected. The star was eventually understood as a massive, luminous, unstable object that pulsates and loses mass, occasionally obscured by strong mass-loss events. It usually shines at apparent magnitude 4.5, but in 1946 it dimmed to 6th magnitude and cooled by over 3000 Kelvin before recovering. A similar eruption occurred in 1893, suggesting a roughly 50-year cycle, and another was observed in 2000–2001 with the William Herschel Telescope. In 2013, a shell ejection caused a half-magnitude drop and dramatic spectral changes, with weak metal emission lines and doubled H-α absorption lines in 2014, and tripled absorption lines in 2017. The brightness peaked at magnitude 4.3 before fading to 5th, then brightened again to 4.2 in 2018.

Distance

Distance measurements for Rho Cassiopeiae via parallax have been unreliable. The 2007 Hipparcos parallax is poorly constrained, and the 2018 Gaia DR2 parallax suggests a distance but is considered unreliable due to high astrometric noise. The 2020 Gaia DR3 parallax is negative and smaller than its margin of error, also with large noise. Indirect methods have been used instead. Assuming membership in the OB association Cassiopeia OB5 gives one distance, though later studies questioned that membership and found no evidence for any other group. A 2019 estimate based on spectroscopic and radial velocity observations during the 2000 eruption suggests a distance, while a 1991 estimate using extinction-corrected apparent magnitude and an assumed absolute magnitude gives another. Using the mean distance of stars with similar proper motion and reliable Gaia parallaxes yields a value. The star's radial velocity is consistent with a distance between 2500 pc and 3200 pc.

Properties

Rho Cassiopeiae is one of the most luminous yellow stars known and operates near the Eddington luminosity limit. It normally loses mass at a rate hundreds of millions of times that of the solar wind. Most of the time its temperature exceeds 7000 K, its radius is around a certain value, and it pulsates irregularly, causing small brightness changes. About every 50 years, a larger outburst blows off a substantial fraction of its atmosphere, dropping the temperature by about 1500 K and the brightness by up to 1.5 magnitudes. During the 2000–2001 eruption, the mass loss rate jumped, ejecting roughly 3% of a solar mass. The luminosity remains roughly constant during outbursts, but the radiation shifts toward the infrared. In 2023, interferometric imaging at the CHARA array gave an angular diameter; at an adopted distance of 2500 to 3100 pc, this yields a physical radius comparable to Betelgeuse. Large convection cells, hot spots, cold spots, and an extended circumstellar envelope were observed. The distance was refined to 2800 pc in 2026, giving a radius. Surface abundances show enhancement of most heavy elements relative to the Sun, but carbon and oxygen are depleted, consistent with hydrogen fusion via the CNO cycle. Sodium is strongly enhanced, indicating a dredge-up during a red supergiant stage, and the star is now evolving toward hotter temperatures while core helium burning through the triple alpha process. Its low mass and high luminosity as a post-red supergiant cause instability near the Eddington Limit, and opacity variations in zones of partial ionization of hydrogen and helium drive pulsations similar to Cepheid variables, which can lead to larger outbursts.

Evolution

Rho Cassiopeiae is a yellow hypergiant, a rare type of luminous supergiant with an effective temperature between those of red supergiants and luminous blue variables. These stars are unstable, undergoing pulsations and eruptions that vary temperature and brightness over months to years. Yellow hypergiants are post-red supergiant stars evolving rapidly to hotter temperatures as they shed outer layers. They occupy a region of the H-R diagram cooler than a boundary, and to hotter temperatures lies the yellow evolutionary void where few stars are observed. Calculations show that a luminous star in a certain temperature range is extremely unstable because the atmosphere becomes opaque at some wavelengths; when a yellow hypergiant reaches these temperatures, its atmosphere expands dramatically and the star cools again. Rho Cassiopeiae has undergone such bounces in 1946, 1986, 2000, and 2013. Outbursts occur every 10 to 20 years but vary in size and duration. The 2013 outburst was unusual, and it is predicted that the star is becoming stable and will resume evolution to hotter temperatures without further outbursts, though it may eventually become a luminous blue variable. The similar star V509 Cassiopeiae became stable around 1986 and is increasing its temperature; if Rho Cassiopeiae follows the same pattern, it will become stable around 2045.