The Sky Guide Chal­lenge

Count­ing stars in the Pleiades.

Sky at Night Magazine - - CONTENTS -

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.

There are 22 stars of mag. +7.0 or brighter in the Pleiades

Our rec­om­mended count­ing se­quence; 10 or more is ex­cel­lent

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