Rho Cassiopeiae

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Rho Cassiopeiae
Observational Data
Designation Rho Cas
7 Cassiopeiae
Right ascension 23h 54m 23.0339s[1][2]
Declination +57° 29′ 57.7669″[1][2]
Constellation Cassiopeia
Type of object Hypergiant
Magnitude Apparent Mag: +4.51[3]
Absolute Mag: -8.25[3]
Distance from Earth 11648.69 ly[3]
Radial velocity -54.30 ± 0.4 km/s[1][4]
Proper motion RA: -5.449 mas/yr[1][2]
Dec: -2.613 mas/yr[1][2]
Parallax 0.9470 ± 0.2021 mas[1][2]

Rho Cassiopeiae (Rho Cas, 7 Cassiopeiae) is a hypergiant star in the constellation of Cassiopeia.[3] The star is one of the brightest stars in the Milky Way and is thought to be a type of variable star.[5] It is also a particularly rare type of star as it is a yellow hypergiant, and only six other such stars have been observed in our galaxy.[6] Its name "Rho Cassiopiae" is the Bayer designation for the star, where the Greek letter Rho indicates the star's relative brightness to the other stars in the constellation it is in. Higher letters indicate dimmer stars, so Rho Cassiopeiae's designation as rho indicates it is a dim star in Cassiopeiae. Though one of the dimmer stars in Cassiopia, it can still be observed with the unaided eye relatively easily.

Properties and structure

Rho Cassiopeia lies 11,648.69 light years from Earth.[3] It is an enormous star, emitting roughly 550,000 times more energy than the Sun and an absolute magnitude of -7.5.[6] It is thought the star's mass is roughly 40 solar masses and a radius of 4.3 AU, making it 450 times larger than the sun.[7] Its surface has a temperature of 7,300 kelvin, a little hotter than sol.[8] The star belongs to the spectral class G2Ia0e and the sun, most of the radiation it emits is in the visible region of the electromagnetic spectrum. Due to a large cloud of dust in between earth and the star, the star appears fainter by two orders of magnitude, but is still sufficiently bright to be seen with the unaided eye.

The star is a semi-regular variable star, meaning its intrinsic brightness varies slowly over time. Several periods have been observed including multiple periods of 820, 350, 510, and 645 days.[8] These periods appear to themselves vary, meaning the star is quite unpredictable.

The star is thought to be loosing mass at a rate of 10-4 solar masses per year or equivalently, 33 earth masses.[7] This is lost in a solar wind being projected from the star at a speed of 10 km/s. However every 50 years, the star undergoes an "outburst" event in which up to 10,000 earth masses of material may be lost.[6] Such outbursts occurred in the summer of 1946 and more recently in June-September 2000.[8][9] In 1946, the stars brightness decreased by two magnitudes to the sixth magnitude. Its surface also reduced its temperature from around 7,000 kelvin to only 3,000 kelvin.[10] Curiously the spectral class also changed to M, though a year later it had returned to its usual G.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Rho Cassiopiae. Simbad Astronomical Database. simbad.u-strasbg.fr. Retrieved on June 29, 2020.
  2. 2.0 2.1 2.2 2.3 2.4 Gaia Collaboration (2018). "VizieR Online Data Catalog: Gaia DR2". VizieR Online Data Catalog I/345: I/345. Bibcode2018yCat.1345....0G.  arXiv:0708.1752
  3. 3.0 3.1 3.2 3.3 3.4 Rho Cassiopeiae (7 Cassiopeiae) Star Facts. universeguide.com. Retrieved on June 29, 2020.
  4. Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters 32 (11): 759-771. doi:10.1134/S1063773706110065. Bibcode2006AstL...32..759G.  arXiv:1606.08053
  5. Rho Cassiopeiae. daviddarling.info. Retrieved on June 29, 2020.
  6. 6.0 6.1 6.2 Cassiopeia Constellation. constellation-guide.com. Retrieved on June 29, 2020.
  7. 7.0 7.1 Gorlova, Nadya; Lobel, Alex; Burgasser, Adam et al. (2006). "On the CO Near-IR Band and the Line Splitting Phenomenon in the Yellow Hypergiant Rho Cassiopeiae". The Astrophysical Journal 651 (2): 1130-1150. doi:10.1086/507590. Bibcode2006ApJ...651.1130G.  arXiv:astro-ph/0607158
  8. 8.0 8.1 8.2 Jim Kaler (March 7, 2000). Rho Cassiopeiae. Stars. stars.astro.illinois.edu. Retrieved on June 29, 2020.
  9. Lobel, Alex; Dupree, Andrea; Torres, Belanna et al. (2003). "The Millennium Outburst of the Cool Hypergiant rho Cassiopeiae: Spectroscopy and Modeling". Modelling of Stellar Atmospheres, Poster Contributions. Proceedings of the 210th Symposium of the International Astronomical Union. Bibcode2003IAUS..210P.F10L.  arXiv:astro-ph/0211508
  10. David Tytell (July 23, 2003). A Star Prepares To Blow Its Top. skyandtelescope.org. Retrieved on June 29, 2020.