231008 - Sh2-185 "Ghost of Cassiopeia" nebula
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click image to enlarge
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ASTRO:
type=Emission Nebula const=Cassiopeia mag=13.3 dist=600 ly size=~10 ly IMAGE: location=EB Driveway BrtlCls=4 exposure=CMOS OSC: 64x300s (5.3h) EQUIPMENT: camera=ZWO ASI2600MC-Pro optics=Explore Scientific ED102 CF corrector=Stellarview 1.0x flattener, FL=714mm, f/7 filter=Radian Triad Ultra QuadNB mount=Celestron AVX EQ guiding=Orion 60x240mm, ZWO ASI224MC SOFTWARE: acquisition=Stellarium, APT, PHD2 processing=PixInsight, RCAstro, PhotoshopCC, Lumenzia, APF-R, LrC |
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Many see these flowing shapes as appearing ghostly on a cosmic scale. A close look toward the constellation Cassiopeia includes the colorful skyscape with swept-back clouds of Sh2-185, a nebula catalogued in 1953 by American astronomer, Stewart Sharpless, but also known more commonly as "The Ghost of Cassiopeia". Professor Sharpless' modern contemporaries have since broken the profile of clouds into smaller and more detailed constituent parts, known as IC 59 (upper) and IC 63 (lower).
About 600 light-years distant, the clouds aren't actually ghosts at all; but, they are slowly disappearing under the constant barrage of energetic stellar winds radiating away from the hot, luminous star Ɣ Cas. The Greek letter Ɣ ('gamma') - the 3rd letter of the Greek alphabet - is assigned to this star due to its being the 3rd brightest among the stars residing inside the recognized boundaries of the Cassiopeia constellation. Ɣ Cas is also known by the common name, "Navi".
Navi is the bright star with starburst in the lower left quadrant of the frame. It is a monstrous, type B0 IV, magnitude 2.15 blue supergiant, physically located only 3 to 4 light-years away from the nebulae. It is 40,000 times more luminous, and 15,000 times more massive than our Sun; and, it rotates at about 470km/s or roughly 150 times more rapidly than our Sun does.
Slightly closer to Navi, IC 63 is dominated by red hydrogen-alpha light, emitted as hydrogen atoms ionized by the star's ultraviolet radiation engage in the process of recombining with electrons. Slightly further from the star, IC 59 shows less Hα emission, but exhibits some of the characteristic blue tint caused by reflected star light.
There is ongoing discussion among professional astronomers as to whether the Hα signal we pick up in our images is actual emission from the nebulae, or a reflection of Hα light emitted by Navi, and then scattered by the dust of IC59 and IC63. This latter process of reflecting and scattering red Hα light is called Extended Red Emission (or "ERE"). Despite its well measured output of ultraviolet radiation and its apparent blue color, Navi is also known to emit significant Hα. But the wavelength is slightly higher than "normal" main sequence Hα, placing it somewhat further outside the visible spectrum of light, and indicating Navi likely has an extended photosphere (stellar atmosphere). Also, because Navi is somewhat cooler than a type B0 V star, it is thought to be only marginally capable of ionizing molecular hydrogen in the nearby vicinity of space. Thus, it is possible that the Hα we pick up in our images is a mixture of both processes; 1) direct Hα emission from the process of ionization within the nebulae, and 2) ERE reflected back from Navi. The analytical tool of-choice used by scientists to investigate this question is multiwavelength spectroscopic analysis. Much of the work lies outside the capabilities of amateurs, in the realm of professional equipment and analysis; but, it is truly fascinating to image these objects with modest backyard equipment while knowing that a firm understanding of their physics is still under scientific investigation.
The field of view of this image spans about 1 degree, or appx 10 light-years across at the estimated distance of Navi and friends. Due to its faint luminosity, the nebula is generally not visible with the unaided eye. The close proximity and brightness of Navi contribute to the difficulty of visually resolving the nebula, but are also why amateur astro-imagers enjoy pursuing it. Producing an image that successfully captures both the brightness of Navi and the faintness of the nearby nebula, while preserving the detailed uniqueness of each is both challenging and rewarding.
About 600 light-years distant, the clouds aren't actually ghosts at all; but, they are slowly disappearing under the constant barrage of energetic stellar winds radiating away from the hot, luminous star Ɣ Cas. The Greek letter Ɣ ('gamma') - the 3rd letter of the Greek alphabet - is assigned to this star due to its being the 3rd brightest among the stars residing inside the recognized boundaries of the Cassiopeia constellation. Ɣ Cas is also known by the common name, "Navi".
Navi is the bright star with starburst in the lower left quadrant of the frame. It is a monstrous, type B0 IV, magnitude 2.15 blue supergiant, physically located only 3 to 4 light-years away from the nebulae. It is 40,000 times more luminous, and 15,000 times more massive than our Sun; and, it rotates at about 470km/s or roughly 150 times more rapidly than our Sun does.
Slightly closer to Navi, IC 63 is dominated by red hydrogen-alpha light, emitted as hydrogen atoms ionized by the star's ultraviolet radiation engage in the process of recombining with electrons. Slightly further from the star, IC 59 shows less Hα emission, but exhibits some of the characteristic blue tint caused by reflected star light.
There is ongoing discussion among professional astronomers as to whether the Hα signal we pick up in our images is actual emission from the nebulae, or a reflection of Hα light emitted by Navi, and then scattered by the dust of IC59 and IC63. This latter process of reflecting and scattering red Hα light is called Extended Red Emission (or "ERE"). Despite its well measured output of ultraviolet radiation and its apparent blue color, Navi is also known to emit significant Hα. But the wavelength is slightly higher than "normal" main sequence Hα, placing it somewhat further outside the visible spectrum of light, and indicating Navi likely has an extended photosphere (stellar atmosphere). Also, because Navi is somewhat cooler than a type B0 V star, it is thought to be only marginally capable of ionizing molecular hydrogen in the nearby vicinity of space. Thus, it is possible that the Hα we pick up in our images is a mixture of both processes; 1) direct Hα emission from the process of ionization within the nebulae, and 2) ERE reflected back from Navi. The analytical tool of-choice used by scientists to investigate this question is multiwavelength spectroscopic analysis. Much of the work lies outside the capabilities of amateurs, in the realm of professional equipment and analysis; but, it is truly fascinating to image these objects with modest backyard equipment while knowing that a firm understanding of their physics is still under scientific investigation.
The field of view of this image spans about 1 degree, or appx 10 light-years across at the estimated distance of Navi and friends. Due to its faint luminosity, the nebula is generally not visible with the unaided eye. The close proximity and brightness of Navi contribute to the difficulty of visually resolving the nebula, but are also why amateur astro-imagers enjoy pursuing it. Producing an image that successfully captures both the brightness of Navi and the faintness of the nearby nebula, while preserving the detailed uniqueness of each is both challenging and rewarding.