Steve Gottlieb
July 31st, 2013, 07:36 PM
4) Sextant Arc
The "Sextant Arc" of the LMC consists of NGC 1955, 1968, 1974 and associations LH 51, 54, 60, 63. Nail and Shapley (1953) called this region "Constellation III", one of several "Constellations" of blue supergiant stars in the LMC . This complex is part of a supergiant shell of HI and HII regions called LMC 4 and is thought to be a result of supernovae and O-type stars near the center of LMC 4.
NGC 1955 (30"): this cluster and emission nebula is near the western end of a beautiful curved chain of bright clusters involved with prominent nebulosity (referred to as the "Sextant Arc") that extends 17' WSW to ENE and includes NGC 1966 and NGC 1974 to the NE and S-L 456, a group of stars and nebulosity 4' W of NGC 1955. The cluster, which is part of stellar association LH 54, includes as many as 40 stars in a 4' region including a half-dozen mag 11.5-12.5 stars in a 3' gently curving arc elongated E-W. The cluster is immersed in a large, irregular haze that is brightest on the eastern side in a 30" circular glow. This is a locally brighter portion of a large irregular loop bowed out to the east and extending N-S for 6'-7' to a mag 9.5 star 3.5' S of the cluster. A fainter group of stars and haze lies 4' W (S-L 456 within association LH 51) and the DSS reveals both halves form an 8' bubble (LHa 120-N51D) like a Wolf-Rayet shell or supernova remnant. NGC 1968 lies ~8' ENE and NGC 1974 11' NE.
S-L 456 (30"): faint group of stars and nebulosity that lies 4' W of NGC 1955 (S-L 456 within association LH 51) and forms the western end of the "Sextant Arc" with NGC 1955, 1968 and 1974.
NGC 1968 (30"): third in a great chain of clusters involved in extensive nebulosity oriented southwest to northeast. The cluster is bright and very elongated 3'x1' E-W with ~20 stars including a number of mag 12-13 stars. The cluster (part of association LH 60) is surrounded by nebulosity (Henize N51C) that brightens on the east end in a large, round knot and extends beyond the cluster on the south side for several arc minutes in the direction of NGC 1955 to the west. NGC 1968 is connected to NGC 1974, another nebulous cluster 3' NE and NGC 1955 lies 8' WSW. The entire complex is nicknamed the LMC "Sextant Arc".
NGC 1974 (30"): fourth in a great looping chain of clusters and nebulosity (collectively called the "Sextant Arc") including NGC 1955 and NGC 1968 to the SW. This group is virtually attached to NGC 1968, only distinguished by less nebulosity and stars. There are roughly three dozen stars resolved in a 3' circular group (stellar association LH 63) including a number of mag 12-13 stars. The cluster is involved in fairly bright nebulous haze (Henize N51A) and is part of LH 63 association.
The "Sextant Arc" of the LMC consists of NGC 1955, 1968, 1974 and associations LH 51, 54, 60, 63. Nail and Shapley (1953) called this region "Constellation III", one of several "Constellations" of blue supergiant stars in the LMC . This complex is part of a supergiant shell of HI and HII regions called LMC 4 and is thought to be a result of supernovae and O-type stars near the center of LMC 4.
NGC 1955 (30"): this cluster and emission nebula is near the western end of a beautiful curved chain of bright clusters involved with prominent nebulosity (referred to as the "Sextant Arc") that extends 17' WSW to ENE and includes NGC 1966 and NGC 1974 to the NE and S-L 456, a group of stars and nebulosity 4' W of NGC 1955. The cluster, which is part of stellar association LH 54, includes as many as 40 stars in a 4' region including a half-dozen mag 11.5-12.5 stars in a 3' gently curving arc elongated E-W. The cluster is immersed in a large, irregular haze that is brightest on the eastern side in a 30" circular glow. This is a locally brighter portion of a large irregular loop bowed out to the east and extending N-S for 6'-7' to a mag 9.5 star 3.5' S of the cluster. A fainter group of stars and haze lies 4' W (S-L 456 within association LH 51) and the DSS reveals both halves form an 8' bubble (LHa 120-N51D) like a Wolf-Rayet shell or supernova remnant. NGC 1968 lies ~8' ENE and NGC 1974 11' NE.
S-L 456 (30"): faint group of stars and nebulosity that lies 4' W of NGC 1955 (S-L 456 within association LH 51) and forms the western end of the "Sextant Arc" with NGC 1955, 1968 and 1974.
NGC 1968 (30"): third in a great chain of clusters involved in extensive nebulosity oriented southwest to northeast. The cluster is bright and very elongated 3'x1' E-W with ~20 stars including a number of mag 12-13 stars. The cluster (part of association LH 60) is surrounded by nebulosity (Henize N51C) that brightens on the east end in a large, round knot and extends beyond the cluster on the south side for several arc minutes in the direction of NGC 1955 to the west. NGC 1968 is connected to NGC 1974, another nebulous cluster 3' NE and NGC 1955 lies 8' WSW. The entire complex is nicknamed the LMC "Sextant Arc".
NGC 1974 (30"): fourth in a great looping chain of clusters and nebulosity (collectively called the "Sextant Arc") including NGC 1955 and NGC 1968 to the SW. This group is virtually attached to NGC 1968, only distinguished by less nebulosity and stars. There are roughly three dozen stars resolved in a 3' circular group (stellar association LH 63) including a number of mag 12-13 stars. The cluster is involved in fairly bright nebulous haze (Henize N51A) and is part of LH 63 association.