The Ghost of Cassiopeia seen by the Hubble Space Telescope

IC 63 (Image ESA/Hubble, NASA)
IC 63 (Image ESA/Hubble, NASA)

An image captured by the Hubble Space Telescope shows a nebula known as IC 63 and nicknamed the Ghost Nebula or the Ghost of Cassiopeia for its ghostly appearance generated by the transparencies in the gas and dust it contains. It looks suitable for the upcoming Halloween celebrations but it will not last forever because a star called Gamma Cassiopeiae is slowly wiping out that gas with its powerful emissions.

At a distance from the Earth estimated at about 550 light years, the nebula IC 63, also known as VdB 141 or Sh2-136, is a reflection nebula because it reflects the light of nearby stars, in particular Gamma Cassiopeiae. At the same time it’s an emission nebula because the those stars’ ultraviolet radiations ionize its gas and consequently it emits light of various colors. It’s often associated with another nebula called IC 59 because they’re close and surround Gamma Cassiopeiae. Along with other gas clouds they’re included into a larger reflection and emission nebula cataloged as Sh2-185.

The star Gamma Cassiopeiae isn’t visible in the image of the IC 63 nebula but is recognizable in the sky because it’s the central point of the W shape of the Cassiopeia constellation. Classified as a blue subgiant, it has a mass estimated at just over 19 times the Sun’s and a brightness 65,000 times greater than the Sun’s.

The most notable feature of Gamma Cassiopeiae is its rotation speed, about 1.6 million km/h (0.93 million mph), over 200 times higher than the Sun’s rotation speed. That rotation generates powerful eruptions with the consequent ejection of considerable amounts of materials that form a disk around it. For this reason its brightness varies over time and is also classified as a variable star.

The star Gamma Cassiopeiae has an age estimated at about 8 million years but has already consumed all of its hydrogen or almost all and is beginning a series of changes that will be increasingly violent until it ends its life in a supernova. Its influence on neighboring nebulae is already important and will increase because a supernova emissions violently compress interstellar gas over a wide radius. If there’s enough gas, that can trigger the birth of new stars.

It’s impossible to predict when Gamma Cassiopeiae will explode so it’s also impossible to know if at that point the nubula IC 63 will have already dissipated. Astronomers can follow the star’s agony from its early stages, in the meantime they have also obtained what’s perhaps the most detailed image of IC 63.

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