Brain and Bubble (120° - 110°)

This sector is the dustiest in the Cassiopeia arc (the brightest region of the Perseus arm as seen from Earth), and contains a 2 million solar mass complex of molecular clouds within the Perseus arm. [1] It also includes two of the most energetic OB associations in the Perseus arm, the brightest radio source in the galaxy as seen from Earth (except for the Sun), and the easternmost major star formation region in the local Cepheus clouds.

Cas OB2

Blaha and Humphreys list 5 O-stars in the Cas OB2 association [2], and more recent research has added at least two more. Widely varying different estimates are available for many objects in this region. One of the major puzzles for astronomers studying the Perseus arm is whether Cas OB2 is really one major star association or if, alternatively, the various star, cloud and nebula complexes observed in this direction are spread out at many different distances across several thousand parsecs.

There are at least seven HII regions or groups of HII regions in this direction, each associated with one dominant ionising star.

The hottest O-star known in the direction of Cas OB2, the O 6.5 III giant BD+60 2522, is one of the most photographed O-stars in the Milky Way because its powerful winds have created the dramatic Bubble nebula (NGC 7635) which is just a part of the larger Sh 2-162 complex. Sh 2-162 contains two larger bubbles that also surround BD+60 2522 [3] as well as the the infrared cluster [BDS2003] 44. [4]

A range of distances have been estimated for Sh 2-162, including 3500 parsecs. [3] A more recent estimate is 2500 parsecs [5], which is closer to Blaha/Humphreys's estimate of 2630 parsecs for Cas OB2 as a whole. [2]

Sh 2-157 (Sim 13) is a ring nebula around the Wolf-Rayet multiple star system WR 157, in the 7.5 million year old Basel 3 (Markarian 50) cluster. [6] WR 157 is a close double with the other component being a B1 II giant [7] and a period of about 2 days. [8] Sharpless associates Sh 2-157 with a number of other stars, including the B0 Ia supergiant HD 219287, which Blaha/Humphreys identifies as the most luminous member of Cas OB2 by bolometric magnitude. [2] One study says that this cluster is part of Cas OB2. [9] However, Van der Hucht gives the distance to WR 157 as 3390 parsecs [10] and a recent study estimates the distance to this system as 4200 parsecs [8]. Both estimates would appear to place it well beyond Cas OB2 and the Cassiopeia arc into the region between the Perseus and Outer arms.

Sh 2-157 is accompanied by several compact HII regions including Sh 2-157A [6], Sh 2-157B [11] and IRAS 23138+5945 (at 2500 parsecs) [12] as well as the infrared cluster [BDS2003] 43, also at 2500 parsecs. [4]

Two star formation regions in the direction of Cas OB2 are associated with two major molecular cloud complexes in this direction (G and C), which in turn are both part of the larger and more diffuse Cloud B. [1] Both are energised by obscured O6-7 class stars embedded in dust clouds and fully visible only in infrared.

There are at least 11 high luminosity infrared sources in the direction of the 595 thousand solar mass molecular cloud [UUT2000] Cloud G associated with Sh 2-158 (NGC 7538). [1] The brightest of these, NGC 7538 IRS 6, contains a class O6-7 star that is likely the main ionising source for the entire HII region. If Sh 2-190, the nebula within the Cas OB6 association, looks like a human heart, then Sh 2-158 looks like a human brain. This prominent nebula needs a popular name, and as it currently does not have one, the Brain nebula seems appropriate.

The same giant molecular cloud complex, the second largest known in the Cassiopeia arc, includes the much fainter HII region Sh 2-159 [13] and is surrounded by the much larger diffuse HII region Sh 2-161B. Russeil agrees that these nebulae are in the same complex. Sh 2-158 is located at a distance of 2270 ± 150 parsecs and Sh 2-159 at a distance of 2970 ± 310. The average distance for the complex is 2400 ± 140 parsecs. [14]

Also called IC 1470, the Sh 2-156 HII region is part of the star cluster NGC 7510. It appears to be ionised primarily by an O7 class star deeply embedded in a dust cloud. This bright core is only one of five distinct and apparently independent emission sources within the 392 thousand solar mass giant molecular cloud [UUT2000] Cloud C. [1] The distance is estimated to be about 3500 to 4000 parsecs. [15], [16], [17] However, Kharchenko estimates the distance to NGC 7510 as 2075 parsecs [18], which would put the cluster (and possibly the nebula as well) in front of Cas OB2.

Russeil says that the nebulae BFS14, BFS15, BFS16, BFS17 and BFS18 are also part of the Sh 2-156 star formation region and argues that the ionising star is actually class O8 V at 2870 ± 750 parsecs. She adds that BFS17 is also ionised by the B1.5 V star WRAM 17 at 3660 ± 1700 parsecs and that the average distance to the complex as a whole is 3000 ± 700 parsecs. [14] Two infrared clusters [BDS2003] 39 and [BDS2003] 42 are also in this direction although with a distance of 6100 parsecs that would place them in the Outer arm. [4]

In addition to these four major regions, the Cas OB2 region also contains four less prominent HII regions: Sh 2-163, Sh 2-164, Sh 2-165 and Sh 2-166.

According to Adedisova, Sh 2-163 is ionised by the O 9.5 V class star LSI +60 8 at 2800 parsecs [17], which Blaha and Humphreys list as one of the members of Cas OB2. [2] A more recent study gives an even closer estimate of 2500 parsecs for Sh 2-163 [5], which seems consistent with membership in Cas OB2. On the other hand, Russeil gives a distance of 4000 ± 220 parsecs, much further away. [14] Sh 2-163 contains the infrared cluster [BDS2003] 45. [4]

Sh 2-164 is ionised by the B1 Ib supergiant LS I +59 10 with two distance estimates of 3500 ± 1500 parsecs [17] and 2820 parsecs [19]. Russeil says that this star is actually a B0.2 III giant at a distance of 2870 ± 260 parsecs. [14]

Sh 2-165 is ionised by the B0 V star BD 61 +2494 and has a distance of about 2400 [17], 1600 [20] or 2020 [19] parsecs and contains the infrared cluster [BDS2003] 46. [4] The 7700 year old supernova remnant SNR 114.3+0.3 and its associated pulsar PSR 2334+61 appear in front of Sh 2-165 at a distance of about 700 parsecs. [21]

Sh 2-166 is ionised by the O9 V star BD+60 2607 with a distance as 2800 parsecs.[19] This star is missing from the BH catalog completely and SIMBAD gives no spectral class or luminosity for this star.

Russeil says that Sh 2-163, Sh 2-164 and Sh 2-166 are all part of the same star formation complex.[14] It is not clear what she means by this, however, as she also gives widely different distance estimates for these nebulae that scatter them across about 1500 parsecs.

The recently discovered supernova remnant SNR 113.0+0.2 lies at a distance of 3100 parsecs in the same direction as Cas OB2 and is near a 10 million year old pulsar that may or may not be associated with SNR 113.0+0.2. [22]

The B0 II giant BD+63 1964 is the second most luminous member of Cas OB2 according to BH. It is located much further north than the traditional boundary of Cas OB2 and has been identified as a high velocity runaway star in the Perseus arm. [23] It seems possible that this star may have been ejected from Cas OB2 by a supernova explosion or stellar collision.

Cassiopeia A

Cassiopeia A (W81, SNR G111.7-02) is the second youngest known supernova remnant in the Milky Way and the brightest radio object in the Galaxy as seen from Earth (except for the Sun). It was the last of the three supernovae known to have exploded during the European renaissance. Unlike Tycho's and Kepler's supernovas, Cassiopeia A exploded inside a dense dust cloud south of the Cas OB2 association and was not observed until the advent of radio astronomy in the mid-20th century.

A detailed examination of the exploding gas dates Cassiopeia A to 1681 ± 19 years. [24] The supernova remnant was first observed by Martin Ryle and colleagues at Cambridge University, and announced in 1948. [25]

The Chandra X-ray Observatory's first observation of Cassiopeia A discovered its central object, apparently a neutron star [26].

In the 20 years between its discovery in 1948 and 1999, the radio emission from Cas A has dropped by 26%. The rate of drop seems to have slowed in recent years. [27]

Cas OB5

Cas OB5 is very different from its neighboring association Cas OB2. While many of Cas OB2's ionising star clusters and O-stars are visually obscured and embedded in massive and dusty molecular clouds, the region around Cas OB5 is much clearer.

Blaha and Humphreys list 5 O-stars as members of Cas OB5 [2] and Kharchenko catalogs 6 ionising clusters in this direction. [18] The hottest star is the peculiar O6 class HD 108 and the most luminous by far is the F8p Ia hypergiant rho Cassiopiae. Blaha and Humphreys give a distance estimate for Cas OB5 of 2510 parsecs. [2]

Cas OB5 is surrounded by a huge 750 thousand solar mass expanding shell of both atomic hydrogen (630 thousand solar masses) and molecular gas (120 thousand solar masses) at a distance of about 2500 parsecs. [28] Supernovae and solar winds from the central clusters of Cas OB5 seem to have driven most of the gas out into the expanding shell. Consistent with this shell structure, Cas OB5's ionising clusters are largely found towards the centre of the association, and the rather faint nebulae associated with this association are on the outskirts, within the expanding shell.

Cas OB5 was the subject of an early study that argued strongly that the various OB stars in this direction form a single OB association. [29]

Ionising stars

Four of the six known visually bright ionising clusters in Cas OB5 have been reported in the past. Two (ASCC 1130 and ASCC 1001) were recently discovered by Kharchenko and her colleagues. [30]

The cluster NGC 7790 is best known for harbouring 3 Cepheid variables: the F8 Ibv supergiant CF Cas and the binary Cepheid system containing the F9 Ib class CE Cas A and the F8 Ib CE Cas B. [31] These pulsating F-class hypergiants, like similar such Cepheids in the Milky Way and other galaxies, can be used to measure distance because their period is related to their true luminosity. [32] Given their rarity, the presence of a Cepheid binary in HC 7790 is especially interesting. Kharchenko gives a distance of 2944 parsecs and an age of 81.2 million years for NGC 7790, and lists two ionising stars: HIP 118163 (B2III-IV), and TYC 4281-1230-1 (B 1 V). [18] A more detailed photometric study determined a distance of 3300 ± 230 parsecs and an age of 120 ± 20 million years [33], which is essentially the same given the error bounds.

The remaining three known clusters contain one ionising star each. Kharchenko gives a distance of 2374 parsecs and age of 30.1 million years for the cluster NGC 7788 and lists the class B2 ionising star: DM +60 2644 = BD+60 2644. [18] The cluster Frolov 1 [34] has a distance of 2560 parsecs, an age of 45.7 million years and contains the ionising star TYC 4281-550-1 (B 1.5V). [18]. A 1984 study found a distance of 2490 parsecs for the cluster King 12. [35] Kharchenko finds a similar distance of 2378 parsecs, an age of 13.1 million years and the ionising star DM +61 2554 (B 1.5V). [18]

Curiously, none of the 5 O-stars identified by Blaha and Humphreys as members of Cas OB5 are located in the four known clusters. Kharchenko adds two new clusters in this direction. ASCC 1001 is 177.8 million years old and contains the O9 star TYC 4018-2878-1 (SIMBAD, however, says that this is a B-class star). At an estimated distance of 4000 parsecs, ASCC 1001 lies behind Cas OB5. [30]

The 10.7 million year old ASCC 1130 also likely lies behind Cas OB5 with an estimated distance of 3400 parsecs. It contains the O9 V star BD+61 2559. [18]

This last O-star is on the Blaha and Humphreys list, leaving the remaining 4 O-stars in the list outside star clusters. These are HD 240464 (O9 V), BD +62 2299 (O 8. ), HD 225146 (O 9.7 IB) and HD 108 (O 6. F). HD 108 is a peculiar erratic variable star. Even the spectral type seems to shift between O6 and O8. There is no obvious explanation for the variations seen, although there are many possibilities. [36]

There is also a Wolf-Rayet star in this direction, WR 158. Van der Hucht assigns it a distance of 7940 parsecs [10], placing it in the Outer arm.

Other stars of note

Blaha/Humphreys lists 15 luminous supergiants in the direction of Cas OB5. [2] By far the most studied of these is one of the most massive known stars in the Milky Way, the F8-G2 Ia0e semiregular pulsating variable rho Cassiopiae. This 4-6 million year old hypergiant has a radius 400 to 500 times larger than the Sun and is about 40 times as massive. [37]

Supernova remnants

There are two supernova remnants visible in the direction of Cas OB5. A recent review concludes that both of these remnants lie within the same atomic hydrogen bubble at a distance of 1600 parsecs. [21] Confusingly, this review describes this distance as "local", although 1600 parsecs appears to be well beyond the Cepheus clouds which mark the outer boundary of the Orion spur in this sector and places these supernova remants in the region in between the Orion spur and the Perseus arm.

Nebulae

Avedisova says that BD +60 39 (O9 V) is the ionising star of Sh 2-173 and the distance is 2900 parsecs. She suggests a connection to the star cluster NGC 103. [17] Sh 2-173 is bright in hydrogen-alpha but less visible at infrared and radio frequencies.

No ionising star is known for the HII region Sh 2-172, however, Russeil argues that it forms a complex with Sh 2-173 and Sh 2-177 [14], and these three do seem to lie together in an area of enhanced hydrogen-alpha emission. Sh 2-172 contains two distinct cores of carbon monosulphide. [38] CO velocity gives a distance estimate of about 3500 parsecs. [39] Russeil gives a distance estimate of 3120 ± 340 parsecs for the complex also containing Sh 2-173 and Sh 2-177. [14]Sh 2-172 contains the loose infrared star cluster [BDS2003] 48. [4]

The nebulae Sh 2-167, Sh 2-168 and Sh 2-169 all appear in the same direction as the hydrogen shell surrounding Cas OB5. [28] Very little has been published in the scientific literature about any of these nebulae. Sh 2-168 is by far the brightest of the three in infrared and radio and Sh 2-169 appears close to it in the sky.

Sh 2-167 is an HII region that was once misclassified as a planetary nebula. It is associated with the infrared region IRAS 23331+6435. [40]

Avedisova gives the ionising star of Sh 2-168 as the O9 V LS I +60 50 at a distance of 3100 parsecs. [17] Blaha and Humphreys say this star is a member of Cas OB5. [2] Blaha/Humphreys and SIMBAD both give the class for this star as B0 V. Sh 2-168 contains the loose infrared star cluster [BDS2003] 47 at a distance of 3800 parsecs. [4] The bright infrared region IRAS 23504+6012 appears in the same direction.

An analysis of the CO associated with Sh 2-169 finds an 8800 solar mass molecular cloud. [41] CO velocity in this direction gives a distance estimate of 1500 parsecs [39], and a more recent estimate is 2300 ± 200 parsecs [5].

Sh 2-172 and Sh 2-173 fall in this sector of the sky but they appear to be part of a complex connected to the Cas OB4 association to the east, and are discussed in the section on the Tycho (130°-120°) sector.

Sh 2-170

The HII region Sh 2-170 is located north of Cas OB5.

The sole exciting star is DM +63 2093, an O8 V star of 31 solar masses, which has created a cavity in the surrounding hydrogen-alpha emission and is located at a distance of 2300 parsecs according to one study. [38] Avedisova gives the class as O9 V and estimates the distance to this star as 2800 ± 700 parsecs. Blaha/Humphreys gives the class as O 9.5 V, estimates the distance as 2290 parsecs and says that this star is not a member of any OB association. [2] SIMBAD lists DM +63 2093 as a G5 class star, but this seems incorrect. Russeil associates Sh 2-170 with the star cluster Stock 18 and gives a distance of 2600 ± 270 parsecs [14]. All three distance estimates are roughly consistent given the error bounds.

Cep OB4

Blaha and Humphreys identifies 4 O-stars in the OB association Cep OB4, the easternmost major star formation region in the Cepheus clouds. [2] Kharchenko adds a fifth. [18] A 1977 study suggests the presence of a sixth O-star. [42] At least two of these O-stars (DM +66 1674 (O9.5 V) and DM +66 1675 (O7 V)) are located in the ionising cluster Berkeley 59. [18] Kharchenko gives a distance estimate of 1000 parsecs and an age of 6.3 million years for Berkeley 59. [18] A detailed study of the region estimates a distance of 850 parsecs. [43]

The HII region Sh 2-171 is the expanding shell of gas and dust created by the original star cluster at the heart of Cep OB4, which has now dispersed. It is now lit and ionized by Berkeley 59 at its south end, and it is expanding into the dark nebula NGC 7822 at its north end. Berkeley 59 also generates the W1 radio source. [44]

SIMBAD identifies Sh 2-171 with a supernova remnant, SNR 118.3+04.8. Although it was once believed that W1 was energised by a supernova remnant [42], no such remnant is contained in the April 2006 version of Green's list. [45]

Cep OB3

Blaha and Humphreys lists five O-stars in the OB association Cep OB3, which is northwest of the Cepheus B molecular cloud. [2] The hottest is the multiple HD 217086 (O7 V). Blaha and Humphreys estimates a distance of 870 parsecs. [2] Kharchenko gives a slightly closer estimate of 700 parsecs for Cep OB3 but locates two of Blaha/Humphreys's O-stars in a separate cluster much further away. [18]

Cep OB3 is divided into two optically visible subgroups. The younger and hotter subgroup, Cep OB3b, contains the stars around HD 217086 and lies closer to Cepheus B. [46] The older association, Cep OB3a, is believed to be 8 to 12 million years old and is surrounded by an expanding atomic hydrogen shell. Cep OB3b is believed to be about 3 million years younger. Star formation in Cep OB3b may have been driven by shock waves from Cep OB3a in an example of sequential star formation. [47] A third generation of stars now seems to be forming near Sh 2-155, the Cave nebula, which is ionised by Cep OB3b and is a good example of the ionisation front between a giant molecular cloud (Cepheus B) and an OB association. [48]

The HII region Sh 2-160 is located to the northwest of Cep OB3 at 900 parsecs. [39] Sharpless lists three possible ionising stars including the B0 III giant HD 218323, which Blaha/Humphreys assigns to Cep OB3.

The newly discovered cluster ASCC 1125 contains two of the Blaha/Humphreys O-stars originally assigned to Cep OB3 (HD 216532 (O8V) and HD 216898 (O8.5V)). Kharchenko says that ASCC 1125 is a 10 miilion year old cluster located at a distance of 1500 parsecs. [30]

Nebulae in this sector

Notes

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 3. ^ Christopoulou, P. E., Goudis, C. D., Meaburn, J., Dyson, J. E., & Clayton, C. A. 1995, Astronomy and Astrophysics, The kinematics of an ionized shell, NGC 7635, in the S162 complex

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 5. ^ Foster, T. & Routledge, D. 2003, Astrophysical Journal, A New Distance Technique for Galactic Plane Objects

 6. ^ Baume, Gustavo, Vázquez, Rubén A., & Carraro, Giovanni 2004, Monthly Notices of the Royal Astronomical Society, The young open cluster Markarian 50

 7. ^ Turner, D. G., Moffat, A. F. J., Lamontagne, R., & Maitzen, H. M. 1983, Astronomical Journal, The WN4.5 component of HD 219460 in the open cluster Markarian 50

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24. ^ Fesen, Robert A., Hammell, Molly C., Morse, Jon, Chevalier, Roger A., Borkowski, Kazimierz J., Dopita, Michael A., Gerardy, Christopher L., Lawrence, Stephen S., Raymond, John C., & van den Bergh, Sidney 2006, Astrophysical Journal, The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant

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27. ^ Stankevich, K. S., Ivanov, V. P., & Stolyarov, S. P. 1999, Astronomy Letters, Fifty years of radio observations of the discrete source Cassiopeia A

28. ^ Moór, A. & Kiss, Cs. 2003, Commmunications of the Konkoly Observatory Hungary, Multiwavelength Study of the Cas OB5 Supershell

29. ^ Reddish, V. C. 1961, Monthly Notices of the Royal Astronomical Society, UBV photometry of the association Cassiopeia V

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32. ^ Feast, Michael 1999, Publications of the Astronomical Society of the Pacific, Cepheids as Distance Indicators

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