The Blue Flash Nebula is a small but relatively bright planetary nebula located in the northern constellation Delphinus. It is listed as NGC 6905 in the New General Catalogue. With an apparent magnitude of 10.9 and an apparent size of 1.2 arcminutes, it can be spotted in a small telescope.
NGC 6905 is one of three deep sky objects in Delphinus listed in the Herschel 400 catalogue of objects visible in amateur telescopes. The other two are the globular clusters NGC 6934 (Caldwell 47) and NGC 7006 (Caldwell 42).
The Blue Flash Nebula (NGC 6905), image credit: Hunter Outten, Outten Astrophotography (CC BY 4.0)
Structure and formation of NGC 6905
Like all planetary nebulae, the Blue Flash is the remnant shell of a Sun-like star that reached the end of its evolutionary cycle and expelled its outer gaseous envelope into space. Once most of the star’s atmosphere was ejected, the stellar remnant left behind in the centre grew hotter and developed a strong ionizing ultraviolet flux.
The intense ultraviolet radiation from the star’s luminous core started to ionize the expanding shell of gas, causing it to glow as a brightly coloured nebula. At the same time, a fast stellar wind from the evolved star kept sweeping and compressing the expelled material, carving out the intricate shape of the young nebula.
A bipolar nebula with fast-moving knots
The Blue Flash Nebula consists of a central cavity with a radius of about 19 arcseconds and a pair of V-shaped bipolar structures extending in the northwest and southeast directions. The nebula has a total mass of gas between 0.31 and 0.47 solar masses.
The bipolar structures contain low-ionization knots at their ends, dense clumps of gas that have not been stripped of as many electrons as the rest of the nebula. These knots are moving outward faster than the nebula’s central region, a pattern believed to result from the collimated outflows that gave the Blue Flash its bipolar structure in the first place.
The Blue Flash Nebula (NGC 6905) – Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 088.D-0514(A) (CC BY 4.0)
Discovery and naming
The Blue Flash Nebula was discovered by the German-born British astronomer William Herschel on September 16, 1784. Herschel listed the object as WH IV 16 in his catalogue. The Roman numeral IV marked his class of planetary nebulae.
The nickname Blue Flash is believed to have been first used by John Mallas, an amateur astronomer who wrote a series of articles about planetary nebulae in The Review of Popular Astronomy in the 1960s. Mallas was the first to describe NGC 6905 as “the Blue Flash.”
NGC 6905 is sometimes also called the Caged Spirit Nebula. The name comes from Steve O’Meara’s Herschel 400 Observing Guide (2013).
The central Wolf-Rayet-type star
The central star of the Blue Flash Nebula is catalogued as HD 193949 and has an apparent magnitude of 14.0. It belongs to the [WO] subclass of [Wolf-Rayet] stars, which means that the star’s spectrum shows the same kind of broad emission lines seen in true Wolf-Rayet stars, but the brackets indicate that it is not one. The spectral type [WO2] indicates an oxygen-rich [WR] star, one with prominent emission from highly ionized oxygen.
[WR] stars have similar spectra to true Wolf-Rayet stars, with broad emission lines of helium, carbon and oxygen, but they are much older and less massive. True Wolf-Rayet stars are exceptionally massive, young and unstable, while central stars of planetary nebulae like NGC 6905 are modestly sized and live much longer lives because they consume their hydrogen fuel more slowly. These stars are not massive enough to go out as supernovae. Instead, they eject their envelopes to form planetary nebulae and gradually cool and fade as white dwarfs.
Around 10 percent of planetary nebula progenitors are Wolf-Rayet-type stars. Like HD 193949, these stars have a typical present-day mass of 0.6 solar masses.
HD 193949 has a surface temperature of 150,000 K and gives off more than enough energy to ionize the expanding expelled envelope. When it was still on the main sequence, the star had a mass of around 1.07 solar masses but lost much of that material through a strong stellar wind once it evolved into a giant.
A star with a mysterious heartbeat
A 2025 study found variations in the star’s brightness with a period of around 652 days, one of the longest such periods found among central stars of planetary nebulae. However, the mechanism causing the variations remains uncertain. The central star may be a binary system, which may account for the periodic dimming and brightening, or the variability may be caused by strong stellar winds from the star itself. A 2021 study by Chornay et al. lists the star as a candidate binary.
HD 193949 is sometimes referred to as a PG 1159 star or a GW Virginis variable. These are hot, pulsating stars in transition between being central stars of planetary nebulae and white dwarfs. The pulsations cause slight variations in the stars’ luminosities.
Here is a 15-minute combined exposure of the Blue Flash Nebula in the constellation Delphinus (also known as NGC 6905). The inset has the brighter surrounding stars removed and adjusted to bring out some details. Focus slipped out on this somewhat – back on the target list for next season. Tech Specs: This image is composed of 60 x 15 second images at ISO 3,200 with 5 x 15 second darks and 5 x 1/4000 second bias frames using a Meade LX90 12” telescope and Canon 6D camera mounted on a Celestron CGEM-DX mount. Imaging was done on October 5, 2016 from Weatherly, Pennsylvania. Image: Tom Wildoner (CC BY-SA 4.0)
Distance to the Blue Flash Nebula
The Blue Flash Nebula lies approximately 8,862 light-years (2.7 kiloparsecs) away. Astronomers previously placed it 5,500 to 8,500 light-years (2,300 to 1,700 parsecs) away, but more recent sources place it at a somewhat greater distance of around 2.6 to 2.8 kiloparsecs.
The distance of 2.7 kpc comes from a detailed 2021 multiwavelength study of NGC 6905 and its central star. The value is based on data obtained from the Gaia Early Data Release 3 (Gaia EDR3) and a 2021 study by Bailer-Jones et al.
Location and observing guide
The Blue Flash Nebula is located in the northwestern corner of Delphinus, close to the border with Sagitta and Vulpecula. Delphinus is a small but distinctive constellation located near Altair, easily recognizable for its dolphin-like pattern.
NGC 6905 appears in the region between the familiar constellation figures of the Dolphin (Delphinus) and the Arrow (Sagitta), close to the imaginary line connecting Tianjinnan (Zeta Cygni) in the constellation Cygnus and Tarazed (Gamma Aquilae) at the Eagle’s shoulder in Aquila.
The nebula can be found along the line extended from Delta Delphini through Sualocin (Alpha Delphini). These stars mark the sides of the Dolphin’s body.
Location of NGC 6905 (the Blue Flash Nebula), image: Stellarium (annotated for this article)
NGC 6905 is situated in a dense star field and framed by a trapezoid of 11th magnitude and fainter stars. It can be seen in 4-inch telescopes in exceptionally good viewing conditions, but because of its small angular size, it is best observed in larger telescopes at high magnification. In 10-inch telescopes, the Blue Flash appears bright, large and oblong in shape, with well-defined edges.
The best time of the year to observe the Blue Flash Nebula and other deep sky objects in Delphinus is in September, when the constellation is high overhead in the early evening.
Explore other planetary nebulae in the northern sky:
- Cat’s Eye Nebula (NGC 6543) in Draco
- Ring Nebula (Messier 57) in Lyra
- Owl Nebula (Messier 97) in Ursa Major
- Blue Snowball Nebula (NGC 7662) in Andromeda
- Dumbbell Nebula (Messier 27) in Vulpecula
Blue Flash Nebula – NGC 6905
| Constellation | Delphinus |
| Object type | Planetary nebula |
| Right ascension | 20h 22m 22.9919225472s |
| Declination | +20° 06′ 16.285052088″ |
| Apparent magnitude | 10.9 |
| Apparent size | 1.2′ |
| Distance | 8,862 ± 675 light-years (2,717 ± 207 parsecs) |
| Size | 0.848 light-years (0.26 parsecs) |
| Names and designations | Blue Flash Nebula, NGC 6905, GCRV 12720, GSC 01639-01907, HD 193949, Hen 2-466, PK 61-9.1, PLX 4855.00, BWE 2020+1956, CSI+19-20201, 2MASX J20222299+2006162, GB6 B2020+1956, 87GB 202009.1+195631, IRAS 20201+1956, MITG J202222+2006, PLX 4855; PN G061.4-09.5, PN ARO 75, PN VV 257, PN VV’ 535, WD 2020+199 |