Cassiopeia's Heart

Cassiopeia's heart1)Not to be confused by the heart nebula, a massive star-forming region on the cassiopeia border , the nameless middle star of the conspicuous 'W' in the sky, shows a surprise in this deep image.

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The bright star in the center of this 3° x 2° region of the sky (about 8 full moons diagonally) is γ Cas, the nameless middle star of the constellation Cassiopeia, the middle star of the 'W'.

The big questions of astronomy these days are: what is the nature of dark matter and dark energy? How short after the big bang did the forces of nature decouple? And how will it all end?

But besides these big riddles, there are many more unsolved smaller mysteries much closer to home. 550 light years close, to be precise. The first little mystery is relatively benign. The brightest stars of Cassiopeia are Schedar, Caph, γ-Cas, Ruchbah, Segin, and Achird. Why doesn't the third (γ) star have its own name? Even more striking considering that it is the middle star of the 'W', and quite prominent at magnitude +2.47. This serious omission was amended only in recent times, when during the Apollo era it was christened Navi, to emphasize its prominent role in space navigation, apparantly because γ Cas is instantly recognized from outer space.

Of much more serious concern is the fact that γ Cas, optically the brightest member of a special class called Be stars, has an unseen low mass (~1.0M) companion of unknown nature, orbiting γ Cas in 204 days in a tight, nearly circular orbit. Be-stars are 2)A thorough introduction into the nature of Be-stars is given here here a class of very hot, very large stars of spectral type B that spin so fast that they almost break up. In the process of almost breaking up, they flatten out along the equator to form a thin, gaseous disk. This disk is formed by matter breaking free from the stellar surface due to the high centrifugal force. In contrast to the more commonly known accretion disks (formed around black holes), these disks are called decretion disks, and these disks add their own features to the star's spectrum in the form of pronounced Ballmer-lines with narrow emission profile.

γ Cas is also an X-ray source, as much as 10% of the sun's total emission in the entire spectrum is being poured out by γ Cas as hard X-rays. Since this is not normal behavior for a Be-star, it has been theorized for a long time that γ Cas' unseen companion must have something to do with it, probably being a white dwarf (WD) or even neutron star (NS) scooping up matter from its host star and releasing it as pure X-rays when the matter comes crashing down with near light-speed as it hits the surface of the degenerate companion. The only problem with this model, is that the spectral properties of the X-rays do not match at all with that of known typical compact binary systems. A recent paper 3)A propelling neutron star in the enigmatic Be-star γ~Cassiopeia link, however, argues that under certain conditions it is very hard for accreted star-stuff from γ Cas to actually reach the NS' surface, rather, it collides with the NS magnetosphere producing a different kind of X-rays that match observations really well. If confirmed for other systems, this would lend credible evidence to the theory that the unseen companion is indeed a NS after all.

IC 63 & IC 59
γ Cas is itself embedded in a rather large 2° bubble of relatively dense interstellar matter. With spectral type B0, γ Cas emits most of its energy in the UV range, but being an additional strong X-ray source, it is bound to have an ionizing effect on the larger area. As a result, within this ISM, only two high density clumps are left over, called IC 63 and IC 59 4)See: A multiwavelength study of IC 63 and IC 59 link, the rest no doubt long gone. The direction of cometary nebula IC 63 is clearly coincident with the line of sight of γ Cas. Where γ Cas shines directly on the matter, it is ionized, forming an ionization front (visible as a thin red edge around the brighter, blue part of the nebula). Because the ionization front appears very thin, it is reasonable to assume that we are looking at it edge-on. This would imply a nearly identical distance to IC 63 and γ Cas.

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IC63 (left) and IC59 (right). The bright core of IC63 spans about 10'x10'. At about 19.5' E of γ Cas, IC 63 is projected to lie 3ly distant from γ Cas. At the distance of IC63, γ Cas would shine at magnitude -9, 100 times brighter than Venus!
Right between IC 63 and IC 59 sits another small, unrelated, nebula called Ced 4a, slightly yellowish in color and just 25" wide, caused by a dissociating star. The other blueish reflection nebula is also associated with the complex, but does not have a catalog name.

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A prominent, yet relatively obscure dark nebula lurks in the γ Cas region. It has been dutifully labeled DOBASHI 3753, after going over the 2 Micron images in search for dark nebulae. The horizontal base measures about 12', while the darkest lanes measure 6' and 3' respectively. Nothing else is known about this object, except for it being a candidate molecular cloud.

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Unmarked, faint nebulosity, probably associated with the γ Cas nebula complex, 50' directly to the east.

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The strikingly red object in the center is a another rare class of stars called Carbon Stars. This one is called V721 Cas, and can be found exactly 1° south of γ Cas. In the entire view there are about a dozen more of these carbon stars to be found.

References   [ + ]

1. Not to be confused by the heart nebula, a massive star-forming region on the cassiopeia border
2. A thorough introduction into the nature of Be-stars is given here here
3. A propelling neutron star in the enigmatic Be-star γ~Cassiopeia link
4. See: A multiwavelength study of IC 63 and IC 59 link

Image information

Dark nebulae, Emission nebulae, Open clusters, Reflection Nebulae, Stars