Crescent Nebula
| Polish version is here |
The Crescent Nebula, designated NGC 6888, is an emission nebula located in the constellation Cygnus. In Photo 1, I’ve indicated where to find this object in relation to the entire constellation.
The Crescent Nebula was discovered on September 15, 1792, by William Herschel. It is situated at a distance of about 4,700 light-years from the Sun in the Orion Arm of the Milky Way.
I have already written about Cygnus as a constellation here (the photograph above, in fact, comes from a session similar to the one described there). This time, however, I would like to focus more on its stars. It’s worth mentioning that some of Cygnus’s brighter stars form an asterism sometimes called the Northern Cross, analogous to the Southern Cross (Crux), which in earlier times was often considered an asterism within the constellation Centaurus.
Cygnus lies in a star-rich region of the Milky Way, making it a hub of intense astronomical research. In particular, the Kepler telescope conducts observations here in search of extrasolar planets.
Within the boundaries of the constellation, there are many intriguing deep-sky objects, and one of them is the Crescent Nebula.
Observations
May 3, 2024, around 11:30 PM – Katowice (Poland)
urban conditions, extremely high level of light pollution
That night, the sky conditions were less than ideal: cloudy, with reports of Saharan dust lingering in the upper atmosphere. Despite this, I managed to gather enough light to capture the nebula’s recognizable shape.
NGC 6888 formed as a result of the evolution of the Wolf-Rayet star WR 136, located at its center. This star was born as a luminous and hot O-type supergiant around 4.5 million years ago. The nebula’s formation began about 250,000 years ago, when the massive central star transitioned from the red giant phase to that of a Wolf-Rayet star.
At this stage of the star’s evolution, its outer envelope was ejected by a powerful stellar wind—its estimated intensity can be illustrated by noting that, every 10,000 years, WR 136 lost an amount of mass comparable to that of the Sun. This stellar wind impacted the surrounding gas left over from a previous phase, compressing it into a series of complex shells and further ionizing it via intense ultraviolet radiation.
It is projected that, as WR 136 nears the end of its life cycle, it will explode as a supernova within the next million years.
It is also worth mentioning here what Wolf-Rayet stars are. These are very large and extremely hot stars, characterized by broad emission lines in their spectra rather than the narrow absorption lines typical of ordinary stellar populations. This is explained by the presence of a vast, low-density gaseous envelope expanding at high speed.
Wolf-Rayet stars are among the most luminous stars, with absolute magnitudes estimated between –4m and –8m. Their diameters are about twice that of the Sun, yet their masses exceed 20 solar masses. Their surface temperatures range from 25,000 to 50,000 Kelvin.
Photo 1 Parameters:
- Total exposure time: 5 minutes (stack of 15 RAW frames at 20s each, using an appropriate number of dark, bias, and flat frames)
- Xiaomi Redmi Note 8 Pro + DeepSkyCamera (beta)
- ISO: 800
- Mount: photographic tripod
Photo 2 Parameters:
- Total exposure time: 15 minutes (stack of 30 RAW frames at 30s each, using an appropriate number of dark, bias, and flat frames)
- Canon EOS 60D
- ISO: 1500
- Maksutov-Cassegrain telescope (100/1400), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system.
Further readings:
- Substyk M., Atlas nieba 2000.0, AstroCD, 2021, str. 22
- Seligman C., NGC 6888, w: Celestial Atlas [08.05.2024]
- Złoczewski K., Kosmos. Przewodnik obserwatora, Amermedia Sp. z o.o., 2013, str. 20-23
- Ridpath I., Gwiazdy i planety. Przewodnik Collinsa, MULTICO Oficyna Wydawnicza Sp. z o.o., 2010, str. 112-113
Marek Ples