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Clamshell (110° - 100°)

Submitted by Kevin Jardine on 22 February, 2008 - 09:55

The scalloped edges of Sh 2-142 are visible in this image along with its ionising cluster NGC 7380. This region forms an important part of the Cep OB1 association in the Cassiopeia arc of the Perseus arm.

Source: Davide De Martin -

The Cassiopeia arc, the brightest part of the Perseus arm visible from Earth, extends into the constellation Cepheus in this sector, in the form of the large star association Cep OB1. The Cep OB2 association appears to the north in the much closer Cepheus molecular clouds. The sector also contains 5 Wolf-Rayet stars, including the two closest binary pairs known in the Milky Way.

Cep OB1

Humphreys lists 11 O-stars within the large Cep OB1 association, which she says is located at a distance of 3470 parsecs. [1] Gerismenko notes that the Perseus arm tends towards longer distances from the Sun as the galactic longitude decreases through the second quadrant, and finds the following distances:

100°:   3200 ± 300 pc
110°:   2800 ± 300 pc
120°:   2300 ± 300 pc
130°:   1800 ± 200 pc
140°:   1700 ± 100 pc [2]

The Humphreys estimate for Cep OB1 is consistent with this.

Kharchenko identifies 3 ionising star clusters in the direction of Cep OB1: NGC 7380, IC 1442, and the newly discovered cluster ASCC 1121. [3],[4]


Kharchenko gives a distance of 2222 parsecs for NGC 7380, an age of 5.2 million years, and lists three ionising stars: HD 215835 (O6), HD 215806 (B0IB), and TYC 3996-1327-1 (B0). [3] Other sources usually give a larger distance for NGC 7380, which is associated with the beautiful HII region Sh 2-142. For example, a detailed 1994 study estimates the distance as 3200 parsecs [5], and Avedisova gives a distance of 3730 ± 650 parsecs. [6] Both of these estimates seem more consistent with estimates for the overall distance of Cep OB1. Sh 2-142 does not appear to have a common name. The "Clamshell nebula" might be appropriate given the scalloped edges of some of the clouds visible in this region.

The Clamshell region contains 4000 solar masses of gas and is associated with a molecular cloud, NGC 7382E, containing an additional 6-15 thousand solar masses of gas, as well as the star cluster NGC 7382, which is also part of the Cep OB1 association. The major ionising star is the eclipsing binary star DH Cephei (HD 215835) (O6+O6). [5]

The much fainter Sh 2-143 appears nearby and appears to be at a similar distance of 3700 parsecs. [7], [6] According to Avedisova, Sh 2-143 is ionised by the O9.5V star LS III +57 93. [6]

IC 1442

Kharchenko gives a distance of 2346 parsecs, an age of 51.2 million years and lists one ionising star, DM +53 2833 (B1III). [3] A 1970 study found an even closer distance of 1810 parsecs. [8] These distance estimates imply that IC 1442 is part of the Cassiopeia arc but lies in front of the Cep OB1 association. However, Humphreys gives a distance modulus of 12.77 for DM +53 2833, implying a distance of 3580 parsecs and putting this star within Cep OB1. [1]

Most of the molecular clouds in this sector fall within the Cep OB2 association, which is part of the Orion spur. The Cep OB1 association to the south forms the western most major section of the Cassiopeia arc in the Perseus arm. This image shows the known ionising stars, clusters and associations in this sector.

Source: Galactic Plane Explorer microwave image

ASCC 1121

Kharchenko identifies a new star cluster in the direction of Cep OB1, ASCC 1121, with a distance of 2500 parsecs, an age of 53.7 million years and the ionising star DM +54 2790 (B0IVn). [4] Humphreys lists this star as a member of Cep OB1 and gives a distance modulus of 12.49, or 3148 parsecs. [1]


In addition to the three Kharchenko ionising clusters, there is another major concentration of bright ionising stars in the direction of Sh 2-132, which also lies within the Cep OB1 region [9] at a distance of 3180 parsecs. [10] This giant HII region, which in some images looks like an arcing celestial dragon, is ionised by two WR stars (WR 152 and WR 153) and about 11 OB stars. The binary star HD 211853 (WR 153) and an O 8.5 star lie near the middle of the radio shell associated with this nebula. [10]

Also in Cep OB1

There are two other Wolf-Rayet stars in the direction of Cep OB1 beside the two associated with Sh 2-132. These are HD 213049 (WR 154) and the binary DQ Cep (WR 155). Both of these stars are believed to lie at the same distance as Cep OB1. [11] The two components of DQ Cephei (WN6 + O9) have the shortest orbital period (1.64 d - less than two days) and distance between components (20-25 solar radii) of any known Wolf-Rayet binary. [12] DQ Cephei is believed to lie within Cep OB1 at a distance of about 3500 parsecs. [12].

Cep OB2

The OB association dominating the galactic plane north of Cep OB1 is the much closer Cep OB2, which lies within the Cepheus molecular clouds. Humphreys lists 10 O-stars in this association and gives a distance estimate of 910 parsecs. [1] The hottest of these O-stars is the O6Iab supergiant Lambda Cephei (HD 210839). [1]

Cep OB2's central cluster Trumpler 37 ionises the spectacular nebula Sh 2-131 (IC 1396), which is part of an expanding shell of gas called the Cepheus bubble. These objects will be discussed in the section on the sector to the west.

Kharchenko lists three ionising clusters in this sector that lie in the direction of Cep OB2: NGC 7235, NGC 7281 and Alessi-Teutsch 5. Of these, only Alessi-Teutsch 5 lies at the same distance (900 parsecs) as Cep OB2. This 10.4 million year old cluster includes the ionising star and eclipsing binary HD 210478 (B1V). [3], [SIMBAD]

The nebulae Sh 2-134, Sh 2-135, Sh 2-138, Sh 2-139, Sh 2-141, and BFS10 all appear in the direction of Cep OB2 in this sector.

Of these, only Sh 2-134 lies within Cep OB2. It is ionised by the the 40 solar mass O6Iab runaway supergiant Lambda Cephei at a distance of 920 ± 200 parsecs, according to Avedisova. [6] Most of the great beacon stars of the Milky Way are too distant or too obscured by dust clouds to be visible to the unaided eye. Lambda Cephei is an exception. With an apparent magnitude of about 5.0, it is visible on dark nights near Zeta Cephei, the star that marks the left shoulder of King Cepheus. In clear sky conditions, the star is easily visible with binoculars every evening throughout the northern hemisphere. Detailed observations reveal that Lambda Cephei is moving at high speed and is creating a visible bow shock as it ploughs through the dust and gas of Sh 2-134. [13]

The origin of Lambda Cephei is controversial. The influential 1987 paper by the Hungarian astronomers Kun, Balazs and Toth proposes that Lambda Cephei was blasted out of the central cluster of Cep OB2, most likely NGC 7190, in a supernova explosion about 3 million years ago. This explosion played a key role in forming the Cepheus bubble. [14] This view was endorsed by a paper published in 2000 by Abraham, Balazs and Kun with a reduction of the time of the supernova explosion to 1.7 million years ago. [15]

However, a 2001 paper by Dutch astronomers Hoogerwerf, de Bruijne, and de Zeeuw disagrees. Using new data from the Hipparcos astrometry satellite that finds that the distance to Cep OB2 is 615 ± 35 parsecs [16] and that the distance to Lambda Cephei is 505 (+153 / -95) parsecs, they conclude that an origin in Cep OB2 is impossible and that instead, Lambda Cephei is more likely to have been blasted out of Cep OB3a in a supernova explosion 4.5 million years ago. The paper does support a supernova origin for the Cepheus bubble, however, and suggests that the runaway B5 class star HIP 102274 (HD 197911) may have been knocked out of the centre of Cep OB2 by this explosion. [17] Sharpless associates HD 197911 with Sh 2-130, which is located well above the galactic plane at a latitude of 12.64°. [18]

In addition to Lambda Cephei, Sh 2-134 contains the bright infrared source IRAS22134+5834, which appears to be a hot B-class protostar with about 8 solar masses. [19]

Other objects in the direction of Cep OB2

Many objects appear in the direction of Cep OB2 but are actually located well beyond the Cepheus molecular clouds.

The young (5.7 million years) cluster NGC 7235 is located at a distance of 2823 parsecs and contains the ionising giant HIP 109603 (B0II), according to Kharchenko. [3] Paunzen, give a slightly higher distance estimate of 3330 ± 400. [20]

The older (72.4 million years) and closer (1620 parsecs) star cluster NGC 7281 contains the ionising star DM +57 2525 (B1.5IV-V:pe). [3]

Van der Hucht says that the binary Wolf-Rayet system WR 151 (CX Cephei) (WN4+O5V) lies in the direction of Cep OB2, but at the much greater distance of 5700 parsecs. [21] CX Cephei has the second shortest orbital period known for a Wolf-Rayet binary (2.12 d - just over two days). [22]

Sh 2-135 is a small nebula with a pronounced ionisation front. It is probably a blister at the edge of a dust cloud [23] and is ionised by the O9.5V star BD+57 2513. [6] There are numerous distance estimates for Sh 2-135, including 1220 ± 160 [6], 1400 [7] and 1900 [23] parsecs. These estimates all suggest that Sh 2-135 lies beyond the Cepheus molecular clouds.

A large group of nebulae to the southeast are associated with the molecular clouds surrounding the Cas OB2 association further east. The supernova remnant CTB 109 is also bright in this radio image.

Source: Galactic Plane Explorer radio image

Sh 2-138 and Sh 2-139 both appear to lie well beyond Cep OB2 at or beyond the distance to Cep OB1 and the Cassiopeia arc. Sh 2-138 is a compact HII region at a distance of about 5000 parsecs, containing a large star cluster resembling the Orion Trapezium cluster: it is centrally peaked around several massive stars (including at least 4 ionising stars), and is dense with more than 550 stars at its centre. [24]

According to Avedisova, Sh 2-139 is ionised by two giant stars: LS III +57 38 (O9III) and LS III +58 37 (B2III) at a distance of 3200 ± 800 parsecs. SIMBAD disagrees with the spectral classification of LS III +57 38, however, and gives it a much cooler spectral class of B5. [SIMBAD] Blitz, Fich and Stark give a similar distance estimate of 3300 parsecs. [7]

Russeil argues that Sh 2-138 and Sh 2-139 are part of the same star-forming complex and gives a distance estimate with a large uncertainty: 2910 +/ 1400 parsecs. [25]

The faint circular HII region Sh 2-141 lies far away at a distance of 8340 ± 600 parsecs according to Russeil. [25] Pineault and Joncas give a distance estimate of about 7000 parsecs and suggest that Sh 2-141 may be ionised by a single O8 class star. [26] There is a 1996 Chinese study that examines the ionising star of Sh 2-141 but unfortunately it is not available through the ADS. [27]

BFS10 is part of a large star formation region that also includes the HII region DA568. Russeil gives a distance estimate for the complex of 5250 ± 290 parsecs. [25]

Cep OB5

Humphreys locates the Cep OB5 association at a distance of 2090 parsecs and mentions two O-stars - HD 240165 (O 9.5 V) and HD 240160 (O 9.5 II). Sh 2-151, which appears in the same area of sky, is apparently much further away (5800 parsecs) [28]. Five molecular clouds are part of the Sh 2-151 complex and have a total of 29 thousand solar masses. About 700 solar masses has been ionised by an unseen star that is likely obscured by dust in one of the molecular clouds. [28] The hypergiant HR 8752 (G0-5Ia+) is also located in this direction (although Humphreys do not list it as a member of the association) at a distance of 2600 parsecs. [29]

Other regions

A number of nebulae appear in the direction of [UUT2000] Cloud A and Cloud B [30]. These are part of the larger complex of dusty molecular clouds surrounding the Cas OB2 association [30] and described in the Brain and Bubble section.

The two cores of the molecular cloud [UUT2000] Cloud A are visible in this full resolution MSX infrared image.

Source: Galactic Plane Explorer infrared image

The Sh 2-147, Sh 2-148, Sh 2-149, Sh 2-152 and Sh 2-153 are all parts of the same giant molecular cloud [UUT2000] Cloud A [30] located at a distance of about 4500 pc. This cloud has two cores - one around Sh 2-152 and Sh 2-153 and one around Sh 2-147, Sh 2-148, Sh 2-149. [31] These cores are both visible in infrared. Some astronomers have suggested that the 9000 year-old supernova remnant CTB 109 (SNR G109.1-01.0) may be associated with this molecular cloud [32] and that its expansion may be blocked by the cloud. [33] This theory seems to be contradicted by the closer distance estimate of 3000 ± 500 parsecs suggested for CTB 109, however. [32]

Both Sh 2-144 and Sh 2-146 appear in the direction of the 377 thousand solar mass [UUT2000] Cloud B [30].

Sh 2-146 is split in half by a dust cloud and is excited by a highly obscured O7 star. [34] Crampton estimates a distance of 4700 parsecs [35] and Paladini estimates a distance of about 5500 parsecs (personal communication). Both of these are kinematic estimates based on the gas velocity measured around the nebula and may be overestimated because of known streaming movements in the Perseus arm. [36]

Despite its location in the direction of [UUT2000] Cloud B, Sh 2-144 appears to be in the foreground of Cas OB2 and is associated with the local Cepheus clouds, with distance estimates of 800 [37] and 830 [38] parsecs.

Essentially nothing appears in the scientific literature for BFS12 beyond a distance estimate of 5200 parsecs [37] and an identification with LBN 107.8-0.34 (LBN 513). [7]

Sh 2-154 is ionised by the giant star LS III +60 28 (B0III) at 1400 ± 300 parsecs, according to Avedisova. [6] Foster and Routledge have a similar distance estimate of 1500 ± 400 parsecs. [39] Sh 2-154 is associated with a giant molecular cloud that appears to contain 700 solar masses of carbon monoxide and perhaps 10 thousand solar masses in total (viral mass). It is possible that this cloud is found within the Cepheus molecular clouds and only appears to be further away because the gas has been accelerated by star formation within the cloud. [30]

The 10 thousand year old supernova remnant SNR 106.3 +2.7 lies at a distance of 800 parsecs. [40] It incorporates the Boomerang nebula (the pulsar wind nebula G106.6+2.9 [41]) which surrounds the pulsar PSR J2229+6114. [SIMBAD]

Sh 2-155, the Cave nebula, is part of the Cep OB3 association and is discussed in the section on the Brain and Bubble (120°-110°) sector to the east. BFS 14 is also discussed in the Brain and Bubble section as part of Cas OB2.

Nebulae in this sector


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