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Westerlund 1 (also called Ara Cluster, designated Wd1) is a compact young super star cluster in the Milky Way galaxy, about 3.5–5 kpc away from Earth. It is one of the most massive young star clusters in the Milky Way, and was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster. The cluster contains a large number of rare, evolved, high-mass stars, including: 6 yellow hypergiants, 4 red supergiants including Westerlund 1-26, one of the largest known stars, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB() star which has been proposed to be the remnant of a recent stellar merger. In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.2-455216, a slow rotating neutron star that must have formed from a high-mass progenitor star. Westerlund 1 is believed to have formed in a single burst of star formation, implying the constituent stars have similar ages and compositions. Aside from hosting some of the most massive and least-understood stars in our galaxy, Westerlund 1 is useful as a relatively nearby, easy to observe super star cluster that can help astronomers determine what occurs within extragalactic super star clusters. ==Observations== The brightest O7-8V main sequence stars in Wd1 have V-band photometric magnitudes around 20.5, and therefore at visual wavelengths Wd1 is dominated by highly luminous post-Main Sequence stars (V-band magnitudes of 14.5–18, absolute magnitudes −7 to −10), along with less-luminous post-Main Sequence stars of luminosity class Ib and II (V-band magnitudes of 18–20). Due to the extremely high interstellar reddening towards Wd1, it is very difficult to observe in the U- and B-bands, and most observations are made in the R- or I-bands at the red end of the spectrum or in the infra red. Stars in the cluster are generally named using a classification introduced by Westerlund, although a separate naming convention is often used for the Wolf-Rayet stars. At X-ray wavelengths, Wd1 shows diffuse emission from interstellar gas and point emission from both high-mass, post-Main Sequence and low mass, pre-Main Sequence stars. The Westerlund 1 magnetar is the most luminous X-ray point source in the cluster, with the sgB() star W9, the (presumed) binary W30a and the Wolf–Rayet stars WR A and WR B all strong X-ray sources. Approximately 50 other X-ray point sources are associated with luminous optical counterparts. Finally, at radio wavelengths the sgB() star W9 and red supergiants W20 and W26 are strong radio sources, while the majority of the cool hypergiants and a few OB supergiants and Wolf–Rayet stars are also detected. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Westerlund 1」の詳細全文を読む スポンサード リンク
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