SH2-200
Description by Sakib Rasool
This exquisitely detailed image is a portrait of the planetary nebula HDW 2, which is located towards the royal constellation of Cassiopeia. Consisting of iridescent shades of purple and blue, this ethereal bubble represents the ghostly remains of a dying star. The category of planetary nebulae was given its name due to telescopic observations by 18th century astronomers such as William Herschel and others being described as similar in appearance and brightness to planets. In astronomy, many things are named after their resemblance to something.
HDW 2 is a very faint ancient planetary nebula of low surface brightness, therefore making telescopic observations very difficult. Faint planetary nebulae such as this one are best appreciated in detailed astrophotographs such as this one and even then, long exposures of many hours are still required. Whilst in the past planetary nebulae might have been discovered through direct visual observations, the many discoveries made in the 21st century have been through photographic means. Some discoveries have been made by professionals as part of large scale surveys such as the Ear Nebula and some discoveries have been due to amateur astrophotographers such as the Soap Bubble Nebula.
This nebula was first discovered by the astronomer Stewart Sharpless in 1959 who included it in his catalogue of HII regions under the name of Sh2-200. His catalogue was compiled after analysing photographic plates taken at the Palomar Observatory. The misidentification of HDW 2 (or Sh2-200) arose from the criteria of assembling the catalogue, which compared the separate red and blue photographic plates of the same region. If a nebula was more prominently visible on the red plate, it was deduced that it was more likely to be an ionized nebula. The other criteria was the presence of bright stars in the vicinity, which might be the source of ionization.
Despite increasing the number of catalogued and known deep sky objects, the method of analysing monochromatic photographic plates displayed its limitation with the incorrect classification of nebulae with a number of entries in the Sharpless catalogue of HII regions subsequently being found to belong to different categories of objects such as planetary nebulae, supernova remnants, Wolf Rayet nebulae, high latitude dust clouds and even galaxies.
In 1983, Sh2-200 was included in the HDW catalogue of possible planetary nebulae by the astronomers Herbert Hartl, Johann Dengel and Ronald Weinberger. However it wasn't confirmed to be a true genuine planetary nebula until 2017 when spectra was taken as part of professional observations. In 1987, further narrowband observations by Herbert Hartl and Ronald Weinberger detected a large faint outer halo extending further than the central shell.
A feature not fully appreciated or clearly defined in the scientific literature is something known as ISM-type haloes. These are a type of structure that represent ambient interstellar gas in the vicinity of the planetary nebula but not physically related to it. Despite being very old and highly evolved, low surface brightness planetary nebulae such as HDW 2 contain very hot and energetic central stars, which are the remnant cores of the progenitor star that died and ejected its outer gaseous layers. The energy output of some planetary nebula central stars is so prodigious that it is capable of ionizing unrelated gas in the surrounding vicinity causing it to glow. The most popular example of this phenomenon is the large halo around NGC 3242.
Taken with a Planewave 17" f6.7 on a Planewave HD Mount and a SBIG 16803 camera. Imaged from "Dark Sky New Mexico" in Animas.
LRGB- 240 each, HA- 780m, O3- 600m, S2-780