Rho Cassiopeiae

Lobel et al, 2003, CfA



Larger image from animation.




Before its 2000-2001 eruption,
the star was a yellowish-white
hypergiant with a surface
oscillation of 320 days (more).

Lobel et al, 2003, CfA



Larger image from animation.



During 2000-2001, the star's
outer gas envelope contracted
then expanded, brightening
then dimming and reddening
over several months (more).

In theory, hydrogen-fusing dwarf stars of 10 to around 60 Solar-masses first evolve as spectral type O to become blue supergiants and then progress to become red supergiants (type M). Stars with 30 to 60 Solar-masses then "loop back" from swollen and cooler, red supergiant phase back into much hotter but smaller blue supergiants (Stothers and Chin, 2001); in contrast, those starting with more 60 Solar-masses remain as blue supergiants. Rho Cas appears to be changing back from being a red supergiant, when it may have been five times larger (James Kaler, 2002; Israelian et al, 1999; and Cornelius de Jager, 1997). In which case, Rho Cas may be "bouncing against" the Yellow Evolutionary Void where such stars become unstable -- as evidenced by its high, if irregular, variability -- and soon explode as a Type-II supernova (like Supernova 1987 A).

NASA Observatorium


Some astronomers believe
that Rho Cas may soon run
out of core nuclear fuel
and become a supernova.


See a discussion of "the
Burning of Elements Heavier
than Helium" and "Supernova
Explosions" as part of stellar
evolution and death.

Based on estimates of interstellar absorption and absolute visual magnitude, Rho Cassiopeiae may be located around 10,100 +/- 1,600 ly from Sol (Lobel el al, 2003). It lies in the southwestern part (23:54:23.0+57:29:57.8, ICRS 2000.0) of Constellation Cassiopeia, the Lady of the Chair -- southwest of Caph (Beta Cassiopeiae), south of Tau Cassiopeiae, north of Sigma Cassiopeiae, northwest of Schedar (Alpha Cassiopeiae), and east of M52, the Bubble Nebula, and Delta Cephei. It is located in the Cassiopeia OB5 Association (James Kaler, 2001).

The star is a slowly pulsating, post-main sequence yellow hypergiant of spectral and luminosity type F8-G2 Ia0pe, with an atmospheric abundance of Nitrogen and Sodium and strong emission lines of Iron (Fe I), Nickel (Ni I), and Calcium (Ca I). It may have around 40 Solar-masses (James Kaler, 2002). According to Robert Burnham, Jr. (1931-93), the star ranges in brightness from a normal range of magnitude of 4.4 to about 5.1 but dimmed to 6th magnitude on 1946. Although no real periodicity has been evident, the interval between some peaks has been around 100 days. When near maximum, its spectral type has been classified as F8, but the light is redder than normal for a F-type star. When varying in brightness, its spectral type has fluctuated between F8 and K5, and reached M5 in June 1946, mostly through atmospheric cooling rather than dimming (James Kaler, 2002). While its spectrum appears to be that of a supergiant, its luminosity is around 100 times greater.

Gabriel Pérez Díaz,
Instituto de Astrofísica de Canarias



Larger illustration.



During its 2000-2001 eruption, Rho Cas
ejected about 10 percent of a Solar-
mass, dimmed by more than a full
magnitude, and changed its spectral
type from late F to early M (more
from ING and Lobel et al, 2003).

Neither a cool red or a hot blue like most hypergiants, Rho Cassiopeiae is particularly unusual. With a surface temperature of around 7,300 °K, it radiates substantially in visible light. The star has an absolute bolometric magnitude of -9.6, radiating with a luminosity of about 550,000 times that of Sol. It appears to have a diameter between 400 and 500 times Sol's, where at 450 times the Solar diameter it would extend 4.3 AU or about 40 percent more than the orbit of Mars (James Kaler, 2002; and Alex Lobel, 2001). The star is emitting a 10-km-per-second stellar wind that is ejecting around 5.4 percent of a Solar-mass of its gas and dust annually (Lobel et al, 2003). It is classified as a variable star (Percy et al, 2000). Useful catalogue numbers and designations for this star include: Rho Cas, 7 Cas, HR 9045, HIP 117863, HD 224014, BD+56 3111, SAO 35879, FK5 899, and SV* HV 194.

Lobel et al, 2003, CfA


Larger image from animation.



After the star dimmed,
dark bands in its spectrum
appeared indicating that
molecules such as Titanium
Oxide formed as its outer
atmosphere cooled (more).

During the recent 2000-2001 eruption, Rho Cas brightened briefly, then dimmed for a period of months. Astronomers believe that the initial brightening occurred because gases fell in towards the star and were compressed and heated. Subsequently, some of that material was blasted outward in a powerful, circumstellar shock wave, which dimmed the star by a factor of six (from 4th to 6th magnitude) and altered its spectral type from F to M, indicating a drop in surface temperature from 12,000 °F (7,000 °K) to 5,000 °F (3,000 °K). As when Rho Cas dimmed around 1945-46 and 1985-86, dark bands appeared in the optical spectrum of Rho Cas, indicating that molecules not normally present (particularly that of titanium oxide) were able to form in the star's cooler outer atmosphere (Lobel et al, 2003). Rho Cass ejected unusually high amounts of mass in 1893 and around 1945-47 that appeared to be associated with a decrease in surface temperature and the formation of a dense envelope, which suggest that the star has a major eruption about once every 50 years and that the current eruption is leading to such an extreme event.

Gabriel Pérez Díaz,
Instituto de Astrofísica de Canarias



Another animation of the
star's recent eruption.




More discussion and images.

Since its eruption in 2000, the star's atmosphere has been pulsating in a strange manner. Its outer layer now seems to be collapsing again, as occurred prior to the last outburst. As a result, astronomers believe that an even larger eruption may be imminent (AAS, 2003).