The Sky Guide Challenge
Counting stars in the Pleiades.
The Pleiades open cluster is one of the more iconic sights of the autumn and winter night skies. Easily visible to the naked eye, it is large enough for you to be able to make out a number of its key members and its overall shape. This month’s challenge is to push your eyesight to the limit and see how many stars you can count within it.
An alternative name for this cluster is the Seven Sisters, so you might be forgiven for thinking you have a shortcut to the answer. However, it is possible to see more than seven stars, and as a number of these lie close to the threshold of visibility you may find yourself becoming obsessed with trying to outdo your previous best.
One danger of describing the stars is that it pre-programmes your mind with what it should be seeing. The mere suggestion that a star may exist in a particular position may be enough to make you think
you’ve seen it. This is a common issue with threshold observations. Ultimately it’s up to you to make an assessment as to how likely it is that you’re seeing is actually real!
Sky transparency, darkness and allowing your eyes to the dark are all important. Another often overlooked factor is your comfort. If you are standing uncomfortably, the likelihood of a good count is reduced. It’s a good idea to sit or lie back on a chair and relax before starting the count. Give your eyes at least 20 minutes to acclimatise too.
The brightest cluster star is Alcyone (Eta (d) Tauri), which at mag. +2.9 should be easy to spot. Alcyone is part of a box shape that extends to the west, which also includes mag. +3.9 Maia (20 Tauri), mag. +3.7 Electra (17 Tauri) and mag. +4.1 Merope (23 Tauri). The box appears to have an east-pointing ‘handle’, the end of which is marked by mag. +3.6 Atlas (27 Tauri).
So far, none of the targets should have presented too much trouble if you have reasonable eyesight, giving you a count of five. Now things start to get harder. First, look immediately above Atlas where you should be able to see mag. +5.1 Pleione (28 Tauri). The separation of Atlas and Pleione is one-fifth the length of the ‘handle’ (Alcyone to Atlas). Mag. +4.3 Taygeta (19 Tauri) should help you get the count up to seven; it is relatively easy spot, located just northwest of Maia.
Mag. +5.5 Celaeno (16 Tauri) is harder because it’s relatively faint, but it is better positioned that Taygeta, slightly west of the mid-point between Maia and Electra. If you’ve made it this far you’re up to eight and ready for the next level. Using our charts, see how many of the additional cluster stars you can spot.