The Emerald Nebula is a young planetary nebula located approximately 6,030 light-years away in the equatorial constellation of Ophiuchus. It has an apparent magnitude of 8.1 and an apparent size of 0.1 arcminutes. The nebula is catalogued as NGC 6572 in the New General Catalogue.
NGC 6572 is known by many different names. Other than the Emerald Nebula, they include the Emerald Eye Nebula, Blue Racquetball Nebula, Turquoise Orb Nebula, Green Nebula, and Planet Krypton Nebula.
NGC 6572 is still a relatively young nebula and the material expelled from the central star is concentrated around the progenitor. For this reason, the expanding bubble of gas is quite bright for an object of its type. A 1994 study found an expansion age of around 2,600 years for the nebula.
The NASA/ESA Hubble Space Telescope has turned its eagle eye to the planetary nebula NGC 6572, a very bright example of these strange but beautiful objects. Planetary nebulae are created during the late stages of the evolution of certain stars that eject gas into space and emit intense ultraviolet radiation that makes the material glow. This picture of NGC 6572 shows the intricate shapes that can develop as stars exhale their last breaths. Hubble has even imaged the central white dwarf star, the origin of the dazzling nebula, but now a faint, but hot, vestige of its former glory. NGC 6572 only began to shed its gases a few thousand years ago, so it is a fairly young planetary nebula. As a result the material is still quite concentrated, which explains why it is abnormally bright. The envelope of gas is currently racing out into space at a speed of around 15 kilometres every second and as it becomes more diffuse, it will dim. NGC 6572 was discovered in 1825 by the German astronomer Friedrich Georg Wilhelm von Struve, who came from a family of distinguished stargazers. The name planetary nebula is left over from the time when the telescopes of early astronomers were not good enough to reveal the true nature of these objects. To many, the discs looked like the outer planets Uranus and Neptune. The application of spectral analysis, later in the 19th century, first revealed that they were glowing gas clouds. Image credit: ESA/Hubble & NASA (CC BY 3.0)
Planetary nebulae are produced by stars with masses of up to about 8 solar masses at the end of their evolutionary cycle. When they reach the asymptotic giant branch (AGB) phase, these evolved giants start to lose mass at an accelerated rate. The expelled material blows away at a few kilometres per second.
With the gaseous envelope expelled, the central stars first grow hotter as they continue to contract. When their exposed cores reach a temperature of more than 30,000 K, the stars become hot enough to ionize the surrounding nebulae.
Planetary nebulae are visible for only about 10,000 years. As the nebular material disappears into the surrounding interstellar medium, the central stars begin to cool and eventually become dim white dwarfs.
The central star of the Emerald Nebula has the spectral class Of-WR(H). It has an effective temperature of 68,000 K and is around 5,700 times more luminous than the Sun. The intense ultraviolet light from the star excites the expanding gases, making them produce their own light.
The central star of NGC 6572 started expelling its gases only a few thousand years ago. The ejected envelope of gas is expanding at around 15 km/s. As it speeds away from the central star, the nebula will gradually become dimmer.
The nebula’s structure is composed of two bipolar shells that appear slightly misaligned and a toroidal structure around the shells at the waist. The nebula has multiple lobes within a spiral ring-like structure. The bipolar shells have closed and opened lobes, and there are concentric rings and arcs around the central region.
A 2023 study of the nebula proposed a mass of around 1.25 solar masses for the progenitor star and 0.565 solar masses for the stellar remnant.
Emerald Nebula (NGC 6572), based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA) (CC BY-SA 3.0)
Facts
The Emerald Nebula was discovered by the German astronomer Friedrich Georg Wilhelm von Struve on July 18, 1825. Struve used the 9.6-inch Fraunhofer refractor at Dorpat Observatory in Estonia.
Danish astronomer John Louis Emil Dreyer, who included the nebula in his New General Catalogue (1888), described the object as “a planetary nebula, very bright, very small, round, a little hazy.”
Location
The Emerald Nebula lies in the northeastern part of Ophiuchus. It appears close to the imaginary line connecting Cebalrai (Beta Ophiuchi) in Ophiuchus and Okab (Zeta Aquilae) in Aquila, about a third of the way from Cebalrai to Okab.
Okab is part of the distinctively bird-like constellation figure of the celestial Eagle, and Cebalrai is part of the large polygon that dominates the constellation Ophiuchus. Rasalhague, the constellation’s brightest star, marks the top of the polygon and appears roughly halfway between the bright Vega in the constellation Lyra and Antares in Scorpius.
Location of the Emerald Nebula (NGC 6572), image: Stellarium
NGC 6572 lies about 5 degrees north-northeast of the Bull of Poniatowski, an asterism formed by five fainter stars in Ophiuchus: 66, 67, 68, 70, and 73 Ophiuchi. The nebula appears above the V-shaped star pattern.
Several star clusters lie in the same region of the sky. The bright, large open cluster IC 4756 in the constellation Serpens and NGC 6633 in Ophiuchus both have apparent magnitudes of 4.6 and can be seen without binoculars on a clear night. They are known as the Tweedledee Cluster (IC 4756) and the Tweedledum Cluster (NGC 6633). The clusters appear east of the Emerald Nebula.
The open cluster IC 4665 (mag. 4.2) appears north of Cebalrai, and the globular cluster NGC 6426 lies south of the star. Another globular cluster, NGC 6535, appears south of Poniatowski’s Bull.
Emerald Nebula and Poniatowski’s Bull, image credit: Stellarium
The best time of the year to see the Emerald Nebula and other deep sky objects in Ophiuchus is during the month of July, when the constellation is prominent above the horizon in the early evening. The nebula is best seen in larger telescopes at high magnification.
At declination +06° 51’, the Emerald Nebula is visible from virtually anywhere for at least part of the year.
Emerald Nebula – NGC 6572
| Constellation | Ophiuchus |
| Object type | Planetary nebula |
| Right ascension | 18h 12m 06s |
| Declination | +06° 51′ 13″ |
| Apparent magnitude | 8.1 |
| Apparent size | 0.1’ |
| Distance | 6,030 light-years (1,850 parsecs) |
| Radius | 0.05 light years |
| Names and designations | Emerald Nebula, Blue Racquetball Nebula, Turquoise Orb Nebula, Green Nebula, Emerald Eye Nebula, Planet Krypton Nebula, NGC 6572, PN VV’ 370, PN G034.6+11.8, PK 034+11 1, PN ARO 7, PN VV 159, SCM 192, HD 166802, AG+06 2201, BD+06 3649, PLX 4174, RAFGL 5206S, NSV 24329, NVSS J181206+065112, JCMTSE J181206.2+065114, PLX 4174.00, PPM 165362, TYC 443-1482-1, IRAS 18096+0650, 2MASX J18120627+0651129, EM* CDS 964, JCMTSF J181206.2+065114, GCRV 10650, WB 1809+0650, GB6 B1809+0650, 87GB 180940.7+065051, MITG J181207+0651, PCCS2 143 G034.61+11.84, VSOP J1812+0651, WEB 15153, Gaia DR2 4472051344537031808, Gaia DR3 4472051344537031808 |