Celestial hemisphere:  Northern  ·  Constellation: Cassiopeia (Cas)

Image of the day 06/28/2017

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    Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae, Mario Tuernich
    Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae
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    Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae

    Image of the day 06/28/2017

    Getting plate-solving status, please wait...
      Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae, Mario Tuernich
      Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae
      Powered byPixInsight

      Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae

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      Description

      It is a capital mistake to theorize before one has data.

      Arthur Conan Doyle - A scandale in Bohemia

      Having collected many data of this and other objects during the last 9 month - going as deep as 12h per chanel -, I was of course very curious for the integrated result but unfortunately I was missing the time and the opportunity to process them properly in time.

      While still developing my necessary skill set, I feel the highly addictive and enjoyable nature of the process of creating these photos and sharing them with others, even that at times it is frustrating having to start over and over again.

      I hope to be able to release more processed data of other targets soon.

      The object here is Sh2-188. Sharpless 188 is located in Cassiopeia and the data were collected from 09/2016-01/2017.

      It is a somehow unusual type of planetary nebulae with an interesting morphology.

      Sh2-188 is crescent shaped with a steep brightness gradient to the southeast displaying very fine filaments.

      Different interpretations of this object arose during the last 60 years.

      When Sh2-188 was observed in 1951 from the Simeiz Observatory in Russia, astronomers believed it to be a HII region due to its high red emissions. Sharpless added it to his catalogue of galactic HII regions in 1959.

      In the 60ties and 70ties the theory had changed.

      Most emission nebulae with a filamentary structure had been identified with old supernova remnants. Spectrophotometry revealed that the relative line intensities varied strongly between the filaments as it is typical for excited line emissions induced by collisional processes. Sh2-188 was then therefore considered to be a supernova remnant.

      Observations with the Westerbork Radio Telescope in the middle of the 70ties then revealed a very low flux density in the hydrogen 21cm line. This low flux In combination with the bright optical appearance ruled out the theory of Sh2-188 being a supernova remnant. Additional support for the falsification of the SNR theory was provided by kinematical studies.

      The low mass of Sh2-188 suggested that most likely Sharpless 188 is due to a mass loss of a star into a interstellar medium.

      From the 80ties till today Sh2-188 was then considered to be a planetary nebulae.

      PN are important in stellar evolution studies and for their dominant role in mass exchange between stars and the interstellar medium.

      In a simplified way, most PN’s arise following the asymptotic red giant branch stage of low or intermediate mass stars. During this phase the star exhibits a strong mass loss. The ejected stellar envelope interacts with the stellar winds released from the core and becomes visible as a PN (interacting stellar wind model).

      In the beginning of a movement through interstellar medium the PN can still expand freely.

      When the density of such a nebulae becomes sufficiently low, the interaction with the ISM will slow down part of the material. A kind of reversed shock will be driven into the PN shell and increases the surface brightness of this region.

      A nearly spherically symmetric PN becomes asymmetric and distorted.

      So, the filaments of Sh2-188 can be explained by shock compression which is supported by the fact that most of them are aligned along the crescent shaped outer rim.

      Sh2-188 might be an example of an advanced stage of interaction with the ISM where the compression took place and the central star has already moved to the edge of the PN.

      Deep observations reveal a much fainter part in Ha-light to the Northwest. Here Sh2-188 appears nearly spherically again. The part is very faint but can be seen on this photo here.

      Assuming that the central star is moving with a relatively high velocity of 125kms-1, detailed hydrodynamical numerical simulations based on a stellar wind model actually predict and explain the morphology of Sh2-188 very well - including the vortices in the left part of the nebulae (they are due to so called hydrodynamical Rayleigh-Taylor instabilities - a phenomena sometimes observed in cloud formation as well).

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      Sh2-188, Sharpless 188, Simeiz 22, Dolphin Nebulae, Mario Tuernich