The Dolphin Head Nebula is an H II region located 4,530 light years away in the constellation Canis Major (the Great Dog). It surrounds the massive Wolf-Rayet star EZ Canis Majoris. The nebula is catalogued as Sharpless 308 (Sh2-308) in the Sharpless catalogue of H II regions.
The Dolphin Head Nebula has an apparent magnitude of 7.0 and an apparent size of 35 arcminutes. It has a bubble-like shape with a distinctive “nose” in long-exposure images, which has earned it its popular nickname, the Dolphin Head.
Sharpless 308 surrounds EZ Canis Majoris (WR 6), a Wolf-Rayet star in the late stage of its evolutionary cycle. The massive star will go out as a supernova relatively soon (astronomically speaking) and is already losing a lot of material through its stellar wind.
Dolphin Head Nebula (Sh2-308), image credit: Wikimedia Commons/Jimdelillo (CC BY-SA 4.0)
The Dolphin Head nebula is about 60 light-years across. It was formed around 70,000 years ago, when EZ CMa threw off its outer hydrogen layers into space.
The strong stellar winds from the evolved massive star have created the expanding bubble. Blowing at 1,700 km/s, the winds clear away the material expelled from the star in an earlier stage of its evolution. The star’s intense ultraviolet radiation ionises the hydrogen in the nebula, making it glow.
The Wolf-Rayet nebula is composed mainly of ionised hydrogen. Like all H II regions, it is a site where hot, young, massive blue stars recently formed, and their intense ultraviolet light ionises the surrounding gas. The nebula will eventually disappear, once its gas is dispersed by the strong stellar winds of the young stars that form in the vicinity.
EZ Canis Majoris has an apparent magnitude that varies from 6.71 to 6.95 over a period of 3.766 days. The star’s spectrum indicates that the hot, luminous star is devoid of hydrogen at its surface.
Even though it is one of the brightest Wolf-Rayet stars in the sky – along with Regor (Gamma Velorum) in the constellation Vela, Theta Muscae in Musca, and WR 140 in Cygnus – EZ CMa is invisible to the unaided eye.
EZ Canis Majoris is really a binary star system. The Wolf-Rayet star has a neutron star companion that orbits the primary component with a period of 3.63 days, completing a rotation every 100 days.
The primary star has 23 times the Sun’s mass and a radius of 3.25 solar radii. With a surface temperature of 89,100 K, it is 620,000 times more luminous than the Sun.
This entrancing image shows a few of the tenuous threads that comprise Sh2-308, a faint and wispy shell of gas located in the constellation of Canis Major (The Great Dog). Sh2-308 is a large bubble-like structure wrapped around an extremely large, bright type of star known as a Wolf-Rayet star — this particular star is called EZ Canis Majoris. These type of stars are among the brightest and most massive stars in the Universe, tens of times more massive than our own Sun. Thick winds continually poured off the progenitors of such stars, flooding their surroundings and draining the outer layers of the Wolf-Rayet stars. The fast wind of a Wolf-Rayet star therefore sweeps up the surrounding material to form bubbles of gas. EZ Canis Majoris is responsible for creating the bubble of Sh2-308 — the star threw off its outer layers to create the strands visible here. The intense and ongoing radiation from the star pushes the bubble out further and further, blowing it bigger and bigger. Currently the edges of Sh2-308 are some 60 light-years apart! Credit: ESA/Hubble & NASA (CC BY 4.0)
The companion has a mass of 1.5 solar masses. It is the remnant of a star that was more massive than the Wolf-Rayet star and evolved faster due to its higher mass.
EZ Canis Majoris is a probable member of the open cluster Collinder 121. The cluster is centred around the K-type red supergiant Omicron1 Canis Majoris, which appears to be interacting with the nebula surrounding EZ Canis Majoris.
However, various estimates of the distance of Omicron1 CMa place the supergiant much closer to us (2,673 ly), which would mean that the two stars are unrelated.
The distance to the Dolphin Head Nebula is also uncertain. The value of 4,520 light-years (1,389 pc) comes from the 2007 validation of the astrometric data obtained by the Hipparcos satellite. However, some sources propose that the nebula may be as close as 1,875 light-years (575 pc) from the Sun, while other place it at a distance of 5,870 ly (1,800 pc).
Dolphin Head Nebula (Sh2-308), image credit: Wikimedia Commons/Lucky Budd (CC BY-SA 4.0)
Facts
The Dolphin Head Nebula was catalogued as Sharpless 308 by the American astronomer Stewart Sharpless in the 1950s. Sharpless surveyed and catalogued H II regions at the United States Naval Observatory Flagstaff Station in Arizona, using photographic plates from the Palomar Observatory Sky Survey (POSS).
The Dolphin Head Nebula was among the first three ring nebulae discovered around Wolf-Rayet stars. The other two are the much brighter Crescent Nebula (NGC 6888) in the constellation Cygnus) and Thor’s Helmet Nebula (NGC 2359) in Canis Major.
The open cluster Collinder 121 was catalogued by Swedish astronomer Per Collinder in the Collinder catalogue of open clusters, published in 1931.
This image shows part of a bubble-like cloud of gas — a nebula named Sh2-308 — surrounding a massive and violent star named EZ Canis Majoris. It uses observations from Hubble’s Advanced Camera for Surveys, and is the parallel field associated with another view of the nebula produced by Hubble’s Wide Field Camera 3. EZ Canis Majoris is something known as a Wolf-Rayet star, and is one of the brightest known stars of its kind. Its outer shell of hydrogen gas has been used up, revealing inner layers of heavier elements that burn at ferocious temperatures. The intense radiation pouring out from EZ Canis Majoris forms thick stellar winds that whip up nearby material, sculpting and blowing it outwards. These processes have moulded the surrounding gas into a vast bubble. A bubble nebula produced by a Wolf-Rayet star is made of ionised hydrogen (HII), which is often found in interstellar space. In this case, it is the outer hydrogen layers of EZ Canis Majoris — the bubble — that are being inflated by the deluge of radiation — the air — coming from the central star. The fringes of these bubbles are nebulous and wispy, as can be seen in this image. Credit: ESA/Hubble & NASA (CC BY 4.0)
Location
The Dolphin Head Nebula is relatively easy to find because it lies about 8 degrees south of Sirius, the brightest star in the sky. The nebula appears in the centre of the Great Dog, near the relatively bright Omicron1 Canis Majoris (mag. 3.87).
Omicron1 CMa appears about a third of the way from Mirzam, the brightest star near Sirius, to Wezen, the star at the top of the triangle that marks the Great Dog’s hind quarters and tail.
Sirius and the constellation figure of Canis Major can be found using the three stars of Orion’s Belt. A line drawn from the Belt to the southeast leads towards Sirius.
The location of the Dolphin Head Nebula (Sh2-308), image: Stellarium
Sh2-308 location, image: Stellarium
At declination -23° 55’, the Dolphin Head Nebula never rises for observers north of the latitude 66° N. It is best observed from the southern hemisphere, but also easily seen from most locations in the northern hemisphere.
The best time of the year to observe Sh2-308 and other deep sky objects in Canis Major is during the month of February, when the constellation appears higher above the horizon in the early evening. The Great Dog is prominent in the night sky from December to April.
Dolphin Head Nebula – Sh2-308
| Constellation | Canis Major |
| Object type | H II region |
| Right ascension | 06h 54m 13.0439608392s |
| Declination | −23° 55′ 42.023319852″ |
| Apparent magnitude | 7.0 |
| Apparent size | 35′ × 35′ |
| Distance | 4,530 light-years (1,389 parsecs) |
| Radius | 30 light-years |
| Names and designations | Dolphin Head Nebula, Sharpless 308, Sh2-308, Henize 3-20, Hen 3-20, LBN 1052, LBN 234.57-10.00, RCW 11, LMA IC 1613 10, Collinder 121 4, [BNJ2003] XMM 1, BRAN 2, CEL 1426, WRAY 15-2 EZ Canis Majoris, EZ CMa, HD 50896, HR 2583, HIP 33165, SAO 172546, WR 6, ALS 98, GC 9061, GCRV 4522, CD-23 4553, CSV 6527, PPM 251223, JP11 4532, LS 98, MR 6, CMC 203719, CMC 172546, CPD-23 1588, 2E 1743, 2E 0652.1-2351, GSC 06522-03270, TD1 8528, SKY# 12250, GEN# +5.11210004, 1RXS J065413.8-235543, N30 1504, UBV 6842, UBV M 12581, UBV M 40625, UCAC4 331-017802, uvby98 511210004, 2XMM J065413.0-235542, YZ 113 4156, IRAS 06521-2351, 2MASS J06541303-2355422, TIC 78959225, TYC 6522-3270-1, Gaia DR1 2922367972368451072, Gaia DR2 2922367976673391232, Gaia DR3 2922367976673391232 |