Seven Sisters Of The Pleiades

Looking At The Pleiades: A Practical Guide

The Pleiades is one of the easiest star clusters to find in the night sky, but getting anything useful out of it requires a bit more than just pointing your eyes upward. It sits in the head of Taurus the bull, near the brighter stars of the Hyades cluster, which can actually confuse newcomers because both groups are nearby in the sky but physically unrelated. The cluster is about 444 light-years away from Earth, which means when you look at it you are seeing light that left those stars roughly four centuries ago. That is a long time to wait for an observation. Everyone knows the story of the seven sisters. Alcyone, Electra, Maia, Merope, Taygeta, Celaeno, and Sterope. That is the traditional Greek mythological grouping. In practice, the cluster contains over a thousand stars bound together by gravity, so calling it seven sisters is a shorthand that works for folklore but falls apart under actual scrutiny. The brightest member, Alcyone, is about 1,800 times more luminous than the Sun. It is a blue giant roughly 10 to 12 times the mass of our star and only about 100 million years old, which is basically an infant in stellar terms. The whole cluster is young, maybe 100 million years old, and it is drifting away from us at about 22 kilometers per second. It will disperse completely in another billion years or so. I spent most of my early amateur observing career trying to count the sisters with the naked eye. You can see six of them fairly easily on a dark night with steady air, maybe seven if the seeing is good and your eyes are rested. But here is the thing nobody tells beginners: light pollution makes this much harder than it should be. I live in a suburban area where the skyglow washes out everything below about magnitude 5.5, and even Alcyone looks dimmer than it really is because the surrounding sky background is elevated. When I drove two hours to a dark site once, the cluster just exploded into visibility with way more stars showing up than I had ever seen before. That is not your eyesight improving. It is the cluster actually having more visible members than you thought.

Binoculars are the single best tool for this. I recommend anything from 7x50 to 10x50. The wider field of view shows the cluster in context with the surrounding stars, and the light gathering power reveals additional members. A 10x50 pair will typically show about 20 to 30 individual stars in the cluster core. Larger apertures help but introduce other problems like hand shake and narrower fields of view. The trick is finding the balance between aperture and usability for your particular situation. Telescopes are less ideal than you might expect. A typical 80mm refractor at low power might show the same number of stars as good binoculars, but at higher magnifications the cluster spreads out too much and you lose the big picture. I once tried to observe the Pleiades through a 200mm reflector at high power and ended up with a sparse field of stars that told me almost nothing about the cluster itself. Low power eyepieces are your friend here. A 40mm Plössl gives you roughly 30x magnification on a standard 1000mm focal length scope, which is close to what binoculars give you but with better resolution on individual stars. There is a common misconception that the blue haze around the Pleiades is reflection nebula directly connected to the cluster. It is not, not entirely. Some of the dust is related to the cluster passing through a diffuse interstellar cloud, but a lot of what you see in photographs is background nebula at different distances. The nebulosity is very faint visually. Even with a 200mm telescope you will barely detect it, and it requires narrowband filters and long exposures in imaging to show up properly. I wasted months trying to see the nebula with my eyes through various telescopes before someone explained that the human eye simply cannot detect the surface brightness levels involved. This is a standard example of the difference between what cameras capture and what your retina can actually perceive.

If you want to photograph the cluster, start with a simple setup. A DSLR or mirrorless camera on a tripod with a 50mm lens at ISO 1600 and a 15-second exposure will show noticeably more detail than the naked eye view. Stacking multiple exposures using free software like DeepSkyStacker will improve the signal-to-noise ratio substantially. You do not need expensive tracking mounts for short exposures like this, though a basic equatorial mount helps if you want to go longer. I initially bought a motorized tracker and spent more time aligning it than actually observing, which is a frustrating pattern I recommend avoiding until you have the basics down. The cluster is visible from most inhabited places on Earth between late summer and early spring. It reaches its highest point in the northern sky around December, which is typically when atmospheric seeing is at its best at mid-latitudes. During summer months in the north, the cluster appears low in the northwest after sunset and the atmospheric extinction and turbulence make it significantly harder to observe clearly. Plan your observations accordingly and do not waste time in June trying to resolve fine detail when December will give you twice the clarity. One specific problem I ran into that most guides do not mention: the Pleiades is bright enough to cause significant glare in finderscopes and red dot finders, which makes locating it can surprisingly difficult if you are scanning the sky blindly. The solution is to star-hop from nearby brighter stars. Start from Aldebaran, which is just across the sky to the southeast, or use the handle of the Big Dipper and follow the arc to Arcturus and then continue to Spica, noting the Pleiades is positioned well north of that path. This is standard astronomical navigation technique but it is easy to overlook when you are just trying to point at something bright.

For spectroscopic details, Alcyone is a binary system with a primary star of spectral type B7III and a secondary that is much fainter and harder to resolve. The other bright members are similarly hot blue-white stars ranging from B to A spectral types. This means the cluster has very little evolved red giant population, which makes sense given its youth. Most of the massive stars that would have evolved off the main sequence already died in supernovae millions of years ago, and the remaining massive stars are still burning through their hydrogen fuel rapidly. What you are seeing is a snapshot of a cluster in its energetic youth. There are also brown dwarf candidates in the cluster that are far too faint for casual observation. These objects are important for studying stellar formation boundaries but require infrared instrumentation to detect. The WISE satellite has identified several of these objects in the Pleiades field, and this kind of data is freely available through NASA's archive if you want to dig into it. If you are planning a session, bring warm clothing, let your eyes adjust for at least fifteen minutes, and start with binoculars before moving to any telescope. The cluster is accessible to virtually anyone with a clear view of the northeast sky on a dark night. The main barrier is usually not equipment but expectation management. It looks impressive but it is not going to fill your entire field of view with thousands of glittering points of light unless you are at a truly dark site with optimal equipment. Accept what it actually is and the experience will be good enough.