NGC 1432/5

Description: The Pleiades is a well-known open star cluster in which many of the individual stars may be distinguished with the naked eye. The brightest star in the group is h (= 25) Tauri, also known as Alcyone, at magnitude 2.9.

The obvious grouping of stars subtends about 1° of sky at 410 light years distance (Þ 7 light years diameter). However, the visible "cluster" is only the core of a much larger volume up to 30 light years across and containing as many as 500 member stars, including many blue giants.

The cluster began forming within the past 50 to 60 million years and the youngest members may be as young as 2 million years. Thus it is a very young open cluster, hence the many blue giants. The brightest stars are class B white giants which rotate about their axes very rapidly; up to 100 times faster than the sun. One of the naked eye member stars, BU (= 28) Tauri, also known as Pleione, is a shell star which throws off rings of gas at irregular intervals, causing it to fluctuate unpredictably between magnitudes 5.0 and 5.5.

The cluster also contains clouds of dust, visible as reflection nebulae.  The reflection nebula surrounding the Pleiades displays a wispy, "fibrous" texture resulting from the alignment in the interstellar magnetic field of the tiny particles responsible for scattering the light. The blue colour the result of Rayleigh scattering of starlight by small particles.  Merope is the star closest to an invisible molecular cloud (NGC 1435) whose presence has been detected by radio observations. The star seems to be illuminating particles more numerous and perhaps larger than those elsewhere. The reflected light is therefore more like that of the illuminating star, so it is less blue than the rest of the nebula (Malin 1993, pp. 96-97).

The nebula was long presumed to be material left over from star formation, now illuminated by the bright young stars, but this view has changed. Among the evidence for the new belief is an infrared map prepared from a mosaic of images from IRAS (the Infrared Astronomical Satellite) which reveals a triangular emission cavity with bright rims extending 5° east of the cluster and tapering from 2° to 1° in height.  This structure "delineates the wake of the Pleiades as it moves supersonically through the interstellar medium…. The analysis indicates that the interstellar matter in the Pleiades is undergoing a chance collision with the cluster at ~18 km s^-1; it is unrelated to the birth of the cluster" (White & Bally 1993, p. 234). The interstellar cloud, approaching from the west, passes around and through the cluster. Either directly by radiation pressure, or indirectly by heating the interstellar gas, ultraviolet light from the young stars diverts material from the cloud, forming the wake.

Tracing the cloud’s motion backwards through space and time, White & Bally conclude that the cloud may have been launched toward the Pleiades 15 million years ago by an energetic supernova about 750 light years away, associated with that part of Gould’s Belt passing through Vulpecula. A possible candidate for the supernova remnant is the pulsar PSR 1919+21 which is estimated to be of about the right age: 15 million years. The pulsar lies about 1100 light years from Earth and is not visible in small telescopes. (Also see: Sky & Telescope, July 1993, p. 12.)

Observation: The visibility of the Pleiades is a useful test of viewing conditions. Under ideal conditions, an observer with good eyesight can discern all nine named stars with the naked eye. From a lighted suburban area, it is more normal to distinguish only five or six stars.  Dozens are visible in binoculars.

It is claimed that under the most ideal conditions the nebulae, also, are visible to the naked eye. However, this seems unlikely.

h (= 25) Tauri (Alcyone) and the shell star BU (= 28) Tauri (Pleione) are readily identifiable in binoculars. In the best conditions, binoculars will also reveal NGC 1435 – the brightest part of the nebula around 23 Tauri (Merope).

The Pleiades contain some double stars that can be split with binoculars. 25 Tauri (Alcyone) and 20 Tauri (Maia) both have 8th magnitude companions separated by 3'. Alcyone’s companion is itself a multiple star comprising three magnitude 9 components forming an approximately equilateral triangle 1' on a side. Notice also the delicate chain of stars extending about 25' SE from Alcyone.

Fig 1: Finder chart for NGC1432 showing stars of magnitude 5 and greater. North is down (southern hemisphere view). Pan north west from the belt of Orion via a Tauri (Aldebaran) and the Hyades.

Fig 2:  Map of NGC1432 showing stars of magnitude 8 and greater.  North is down (southern hemisphere view).