Orwell Astronomical Society (Ipswich)
Pleiades Observing Project, 18 October - 09 December 2002
During the winter months, the constellation Taurus is prominently high in the sky throughout much of the night. The constellation culminates at midnight on 01 December and it is conveniently placed for early evening observation from early November until the end of February. Its three most prominent features make it immediately recognisable to the naked eye:
During early evening in the middle of winter, with Taurus high in the southern sky near culmination, Orion lies to the south and Auriga to the north-east, with Gemini to the east and Cetus to the west. The visibility of so many fine constellations presents a magnificent spectacle and a plethora of interesting objects for the amateur observer. The chart below shows Taurus in relation to its neighbours.
The Pleiades is arguably the most prominent object in the field of view of Taurus and its neighbours. It is a cluster of some 500 stars located approximately 370 light years (ly) from Earth. Some nine stars at the centre of the cluster are visible to the naked eye under good conditions; the remainder span the range of magnitudes from just beneath naked eye visibility down to extremely faint stars visible in only the largest telescopes. The entire cluster is located within an area of space some 20 ly in diameter, although the nine bright central stars are bounded by an area only seven ly across. Nearly all the stars in the Pleiades are very young, a mere 20 million years old, compared with some 4.5 billion years for the Sun. The cluster has been a rich source of observations for professional astronomers (see below). For amateur astronomers, the challenge in observing the Pleiades is to determine the visibility of particular stars within the cluster, to observe occultations (the Pleiades lies close to the ecliptic and thus is subject to occasional occultations by the Moon and planets) or simply to enjoy the spectacle that it offers.
A star's brightness or apparent luminosity is represented by its magnitude. The brightest star in the sky, Sirius, shines at magnitude -1.5 while the faintest stars visible to the naked eye are typically circa magnitude 6.0. The magnitude scale extends via higher numbers to fainter stars that are visible only via binoculars or a telescope. The magnitude scale is defined so that a difference in magnitudes of 1.0 corresponds to a difference in brightness of 2.5; thus for example a star of magnitude 2 is 2.5 times brighter than a star of magnitude 3. I proposed an observing project in the November 2002 OASI Newsletter to determine the faintest star that could be seen in the Pleiades using the naked eye, binoculars or a telescope.
Mankind has recognised the Pleiades as a defined stellar grouping since ancient times. Early civilisations associated the grouping with the passage of the Sun through the vernal equinox from south to north of the ecliptic heralding the onset of spring. The Pleiades were named in mythology for Atlas and Pleione and their seven daughters, supposedly the half sisters of the Hyades (offspring of Atlas and Aethra). In modern times, professional astronomers have found the Pleiades a rich observing target. The following gives a flavour of the current state of knowledge of the Pleiades, obtained after many years of research by professional astronomers.
1. Dynamic Evolution
The Pleiades cluster has low density - circa three stars per cubic parsec on average, compared to 83 for M11 and 12 for M36. Gravitational attraction is unable to hold the Pleiades together for more than perhaps 1000 million years, and professional astronomers use computer simulations of stellar dynamics to predict the future evolution of the cluster.
2. Stellar Evolution
The nine brightest stars in the Pleiades are all B-type giants. The largest is Alcyone, approximately 1000 times as luminous as the Sun and 10 times the size. The remainder of the cluster straddles the whole extent of the Main Sequence, down to faint red dwarfs at magnitude 16. (By way of comparison, the Sun at the distance of the Pleiades would shine at magnitude 15.0 - barely visible in the Tomline Refractor!) Bright stars in the Pleiades are all spinning very fast, with surface velocities up to 150-300 km/sec. Such high surface velocities result in wide, blurred spectral lines as one limb approaches while the opposite limb recedes. Due to fast rotation, the stars are oblate spheroids rather than spheres. The star with the fastest rotation speed, Pleione, is rotating approximately 100 times as fast as the Sun: a revolution of Pleione takes only six hours!
Pleione is in fact a very unstable star. In addition to its high rotation speed, it is a variable star with a range of ~0.5 magnitudes, occasionally ejecting shells of material. Pleione suffered an outburst in 1938, coinciding with a drop in brightness by ~0.5 magnitudes thought to be associated with absorption of light by a shell of ejecta. Another outburst occurred in 1972. Pleione is perhaps the lost Pleiad of legend?
3. Flash Stars
The Pleiades contains several red dwarf variable stars which are subject to sudden, irregular "flash" outbursts varying from one - four magnitudes of durations from several minutes to around three hours. All such stars are faint: magnitudes 13 - 17. Astronomers discovered seven such stars in the early 1960s. Current theories suggest that these stars are still undergoing gravitational contraction and are not yet stable.
4. White Dwarfs
The Pleiades contains several white dwarfs. This poses a significant problem of stellar evolution: how can white dwarfs exist in such a young star cluster? (The Pleiades is thought to be only some 20 million years old.) There are several white dwarfs so it is likely that they are original cluster members and not captured field stars (in any case, capture does not work effectively in a loose open cluster such as the Pleiades). From the theory of stellar evolution, the white dwarfs cannot have masses above about 1.4 solar masses (the Chandrasekhar limit), as they would collapse due to their own gravitation if they were more massive. But stars with such low mass evolve so slowly that it takes them billions of years to evolve into dwarfs, not the 20 million year age of the Pleiades. The only possible explanation seems to be that these dwarf stars were once massive so that they evolved fast, but due to some reason (perhaps strong stellar winds, mass loss to close neighbours, or fast rotation) they lost the greatest part of their mass bringing the cores of the original stars below the Chandrasekhar limit to permit entry into the stable white dwarf state in which they are now observed.
5. Brown Dwarfs
Observations of the Pleiades since 1995 have revealed several candidates for an exotic type of star, or star-like body, the so-called brown dwarf. These hitherto hypothetical objects are thought to have a mass intermediate between that of giant planets (like Jupiter) and small stars. (Theories of stellar structure indicate that the smallest stars, i.e. bodies that produce energy by fusion at some point in their lifetime, must have at least about six - seven percent of one solar mass, i.e. 60 to 70 Jupiter masses). Therefore brown dwarfs should have 10 to about 60 times the mass of Jupiter. They are visible in infrared light, have a diameter of up to that of Jupiter (143,000 km) and a density 10 to 100 times that of Jupiter, as their much stronger gravity compresses them.
The nine bright central stars of the Pleiades are surrounded by faint, wispy nebulosity. It is caused by interstellar dust shining by reflected starlight - the stars and nebulosity have the same spectrum. The nebulosity has a delicate blue colour, caused by reflecting the light of the stars and also because of preferential scattering of blue light by the tiny interstellar particles of the nebula. The brightest portion of the nebula occurs around Merope. The nebula and cluster just happen to be crossing one another: their radial velocities differ by 11 km/sec. Nebulosity around Merope was first noticed by Professor W Temple in 1859 using a 100 mm refractor - he compared the nebulosity to breath on a mirror. The nebula is very difficult to see and indeed it is frequently impossible to see even with a very large telescope unless observing from an exceptionally dark-sky location.
There are 20 stars in the Pleiades which an observer could, in principle, glimpse with the naked eye under ideal conditions (using the generally accepted limit of naked eye visibility under ideal conditions as magnitude 6.5). However, conditions are seldom ideal (especially under UK skies!) and glare from the brighter stars in the cluster obscures the fainter stars. In practical terms, only the nine brightest stars are typically visible by naked eye to experienced UK observers under good conditions - the following table lists them.
|Asterope (double)||5.8 & 6.4|
Table 1. Nine brightest stars in the Pleiades.
Figure 1 shows the sky vista around Taurus. Figures 2 and 3 show the magnitudes of respectively the brighter and some fainter members of the star cluster. In figure 3, the line of faint stars running roughly west of Merope, for which magnitudes are show, is a feature easily recognised in binoculars or a telescope.
Observers participating in the Pleiades Observing Project were asked to proceed as follows:
A total of 13 members of OASI plus one visitor to Orwell Park Observatory participated in the project and reported their observations, as below:
Table 2. Reported observations.
Note especially the observations by Paul Whiting under the clear skies of Woomera, South Australia. Paul travelled to Australia to observe the total solar eclipse on 04 December. His observations transformed a purely local observing project into a truly international effort!
All 14 observers reported naked eye observations. The most immediately obvious aspect of the observing reports was the wide variety of descriptions of the Pleiades, summarised below. This does highlight the significantly different interpretations that observers can place on observations even of an object which is very well known.
Most observers looking out on a night with average weather conditions reported seeing some five - seven Pleiads, corresponding to a faintest magnitude of 4.1 - 5.1. Under good sky conditions, observers reported typically nine Pleiads visible to the naked eye, corresponding to a faintest magnitude of 5.8. Where up to nine Pleiads were visible, observers were more likely to report seeing the double star Asterope as apparently elongated, reflecting the fact that it consists of two components (mags 5.8 and 6.4) separated by a distance of 150 arcsec.
Observing on 04 December at the relatively dark site of Orwell Park, I was initially able to discern seven Pleiads but after more than 30 minutes of dark adaption I managed to glimpse 10, comprising the nine brightest forming the heart of the cluster plus the magnitude 5.4 star marking the end of the line of stars west of Merope. The latter star is in fact 0.4 magnitudes brighter than the faintest of the nine bright central Pleiads; however its isolation from the rest of the group of bright stars means that it is much more difficult to locate. No other observer reported seeing more than nine Pleiads by naked eye.
Dark adaption, a Moon not close to full and the absence of light pollution are important factors in discerning the maximum number of Pleiads by naked eye. On 18 October, under clear, frosty skies but with a 95% waxing moon in Pisces (only 60° distant from the Pleiades) I was able to observe only eight Pleiads and indeed needed averted vision to see the eighth faintest (Calaeno). This compares with 10 Pleiads seen from Orwell Park some six weeks later on a moonless night (as noted above). However, Alice Longhurst, observing on 15 November with an 85% waxing moon in Cetus, reported being able to count nine Pleiads by naked eye - so clearly the Moon does need to be very close to full before it materially reduces the number of Pleiads visible.
Thirteen observers reported observations with binoculars, using instruments ranging from 8x24 to 11x80. Binoculars in fact are the ideal instrument to observe the Pleiades as a cluster, because their wide field of view enables the observer to see the cluster in its entirety and thus appreciate it in its full glory. Many of the observers commented on the superb view offered by binoculars as follows:
All binocular observers could separate Pleione and Atlas. Three observers commented that the stars were separated easily. All but two of the binocular observers could separate the components of Asterope, although there was uncertainty in two cases where observers held the binoculars by hand and suffered from image shake. Binocular observers were typically able to detect stars down to magnitude 8.8 directly, with stars circa one magnitude fainter accessible using averted vision. This meant that binocular observers were able to discern the brightest stars in the line of stars west of Merope.
Eight observers reported observations with telescopes, ranging from a 75 mm reflector to the 260 mm Tomline Refractor. A telescope provides a much smaller field of view than binoculars and is capable of separating stars in close proximity to one another. This is reflected in the following comments from telescopic observers:
One aim of the observing project was to determine telescopic visibility of three close, faint companions of Alcyone. The table below lists data on the stars.
|Alcyone (25 Tau, η Tau, Tycho 1800-2202-1)||2.8||-|
|24 Tau (Tycho 1800-2201-1)||6.3||117|
|V647 Tau (Tycho 1800-1607-1)||8.2||181|
Table 3. Faint stars close to Alcyone.
Seven telescopic observers were able to identify the three companions; one telescopic observer did not check.
Telescopic observers typically reported the faintest star visible in the Pleiades as being circa magnitude 12-13. In this range, a skilled observer could hope to see roughly half of the stars in the cluster. Unfortunately, I have been unable to find a star chart that identifies all the members of the Pleiades down to very faint magnitudes so there is no obvious means of determining which very faint stars are members of the Pleiades and which are not. No telescopic observers reported any trace of visibility of nebulosity in the Pleiades - this is unsurprising for observations from the UK where poor skies and light pollution are endemic problems!
Neil Morley took the following photograph of Taurus at 22:38 UT on 06 November 2002, using a Casio QV-2800UX digital camera (1600 x 1200 pixel resolution) on a fixed mount with a 60 second exposure.
Star trails are visible because the camera was fixed; however the photograph shows clearly Aldebaran, the Hyades and the Pleiades. The faintest star visible in the image is of magnitude approximately 6.5 (i.e. the very best that a naked eye observer could hope to see under ideal conditions).
For those who would like to explore the Pleiades further, the following star chart of the central region provides a great deal of detail and lists stellar magnitudes over the range 2.87 - 12.89. it is based on the chart in .
Many of the great observers of history counted the number of Pleiads visible to the naked eye. Some of these observers had exceptional eyesight and, of course, observed without the curse of light pollution from which we suffer so badly today. The most impressive historical observers of the Pleiades were as follows:
One record of the modern era is by O'Meara who claimed to discern 17 Pleiads by naked eye in 1978 at Cambridge, MA, USA.
G B Airy reported on the visibility of stars in the Pleiades in the Monthly Notices of the RAS for 1863  as follows:
Thanks are due to all observers who supported the project.
W Schlosser, T Schmidt-Kaler and E F Milone, "Challenges of Astronomy", Springer Verlag Inc., 1991.
G B Airy, "On the Visibility of Stars in the Pleiades to the unarmed Eye", MNRAS, Vol. 23, p. 17 (1863).