- 4 Cassiopeiae Equals
David A. Aguilar, CfA
Who Lives at 723 S Casino Center Blvd, Las Vegas, NV 89101 .... Larger illustration.
Located around 10,000
light-years away, Rho
Cassiopeiae is a
sometimes reddish,
yellow hypergiant that
recently has been ejecting
about 5.4 percent of a
Solar-mass annually and
may be close to exploding
as a supernova (more from
CfA and Lobel et al, 2003).
Eta Cassiopeiae is the nearest star in Cassiopeia to our solar system, only 19.4 light-years away. It is similar to our Sun, being a yellow-white G-class hydrogen fusing dwarf, slightly cooler than the Sun with a surface temperature of 5730 Kelvin. Eta Cassiopeiae has a magnitude of 3.45. Another fainter star is Zeta Cassiopeiae.
Yellow Hypergiants
Rho Cassiopeiae (Rho Cas) belongs to an unusual class of stars called hypergiants, which are much brighter and many times more massive than Sol. Despite being located some 10,000 light-years (ly) away, the star is visible to the naked eye because it is over 500,000 times more luminous than Sol. Like all extremely massive stars, however, hypergiants are very short-lived with a total life of only a few million years. Rho Cas is a yellow hypergiant, which are particularly rare objects as only seven (including HR 8752 and IRC+10420) have been found in the Milky Way. With surface temperatures between 3,500 and 7,000 °K, yellow hypergiants appear to be stars that are at a very evolved stage of their life and may be close to exploding as supernovae.
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).
- The remarkable Gamma Cassiopeiae is a blue-white subgiant variable star that is surrounded by a gaseous disc. This star is 19 times more massive and 65 000 times brighter than our Sun. It also rotates at the incredible speed of 1.6 million kilometres per hour — more than 200 times faster than our parent star.
- 4 Cassiopeiae is a wide binary star system in the northern constellation of Cassiopeia, located approximately 790 light-years away from the Sun. It is visible to the naked eye as a faint, red-hued star with a baseline apparent visual magnitude of 4.96.
Yellow hypergiants are peculiar because they display an uncommon combination of brightness and temperature that places them in a so-called 'Yellow Evolutionary Void,' a part of the Hertzsprung-Russell diagram where post-red supergiants exhibit atmospheric unstability while evolving blueward (Cornelius de Jager, 1998; de jager and Nieuwenhuijzen, 1997; and Nieuwenhuijzen and de Jager, 1995). Unlike luminous blue variables (such as Eta Carinae and the Pistol Star) which are so massive that they are able to keep their photospheres stable, yellow hypergiants may become particularly unstable with larger pulsations when rapidly evolving towards the Void to return towards the blue supergiant phase, with increasing variability in brightness and spectral emissions while ejecting massive amounts of gas and dust. According to one theory, they cannot cross the Void unless they have lost sufficient mass, but they can quickly run out of core nuclear fuel and implode to become supernovae.
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.
Rho Cassiopeiae
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.
