NGC 6891 is a planetary nebula located in the northern constellation of Delphinus, approximately 7,200 light-years away from Earth. Planetary nebulae are formed during the late stages of a star’s life when it sheds its outer layers into space, creating a nebulous shell of gas and dust surrounding the central remnant star.
The nebula was discovered by the German-British astronomer William Herschel in 1784. Here’s an overview of this celestial object:
Composition
NGC 6891 exhibits the characteristic features of a planetary nebula, with a central star surrounded by a shell of ionized gas and dust. The nebula was formed during the final stages of a medium-mass star’s life, when the star expelled its outer layers, leaving behind a dense core. The ultraviolet radiation from this core, now a white dwarf, ionizes the surrounding gas, causing the nebula to glow with distinct colors.
Like other planetary nebulae, NGC 6891 is primarily composed of hydrogen and helium, with traces of other elements such as oxygen and nitrogen. The colors observed in the nebula arise from specific emission lines: hydrogen emits a reddish hue (H-alpha), while doubly ionized oxygen (OIII) contributes a bluish-green glow, making it visually striking through certain telescopic filters. This interplay of colors provides insight into the chemical composition and ionization states of the gases.
The central white dwarf of NGC 6891 represents the star’s remaining core, with a surface temperature exceeding 100,000 K, though it will gradually cool over billions of years. The nebula surrounding it will continue to expand and dissipate into the interstellar medium over the next 10,000 to 20,000 years. Studying objects like NGC 6891 gives astronomers valuable information about the evolution of stars similar to our Sun and the processes by which planetary nebulae enrich the interstellar environment with heavy elements.
Size
NGC 6891 has an apparent size of about 0.3 arcminutes (or 18 arcseconds) across. Due to its compact nature, it can be challenging to observe in small telescopes without sufficient magnification.
In terms of physical size, it is approximately 0.8 light-years in diameter, based on its distance of 7,200 light-years. This size is typical for planetary nebulae, which tend to expand over tens of thousands of years before dissipating into the interstellar medium.
Observation
The apparent magnitude of NGC 6891 is around 11.5, making it faint for viewing with binoculars and requiring a telescope with an aperature of atleast 4.5 inches to observe effectively. In small telescopes it will appear as nothing more than a small faint patch. Larger telescopes will reveal more detail.
NGC 6891 is best observed from the northern hemisphere during summer and early autumn. The months most suitable for observing the nebula would typically be from July to October. During these months, its host constellation Delphinus, is prominently positioned in the night sky, making it more easily visible and accessible for observation.
However, the exact visibility of the nebula may vary depending on your location, local weather conditions, and light pollution levels. Therefore, it’s always a good idea to check astronomical charts and plan your observation sessions during clear, dark nights for the best chance of seeing the nebula clearly.
