Details: Located at 444 light years away, the Pleiades, also known as the Seven Sisters, are popularly referred to among astronomers as Messier 45 or just M45. We calculate a very accurate distance to M45 by using an old but comprehensible technique called parallax, which calculates an object's apparent shift against the background stars when the Earth is at opposite sides of the Sun. Like blinking your eyes back and forth can sometimes cause close objects to seem to shift, blinking eyes or sensors placed 200 million miles apart (the diameter of the Earth’s orbit) greatly exaggerates this effect. At 444 light years away, M45 moves back and forth each year by 0.00734 arcseconds or 2 millionths of a degree. Modern techniques involving space-based instruments can accurately measure distance to objects throughout much of our galaxy! The core dozens of stars of the Pleiades are within a region 16 light-years across, although the entire cluster contains more than 1000 stars, including dwarf stars, and its gravitational influences spans some 80 light years across, over 10° or 20 times the size of the full moon!

The Pleiades cluster has many young hot class B stars which are blue in color, very young in astronomical terms, and very hot. These stars are likely to burn up their nuclear fuel much more quickly and die relatively young. The clouds surrounding these young hot B stars are reflection nebula which reflect light from nearby stars. Since the stars here are bluish in color, so are the reflection nebulae. These nebulae were thought to be left over material from the star cluster’s formation, but recent evidence suggests the cluster of stars is merely passing through the already existing but unrelated nebula.

Many of us easily recognize the Pleiades and some even confuse it with the little dipper due to its shape (see the above upper left inset). At about 2 degrees in apparent diameter, the Pleiades is visually 4 times larger than the full moon! The beautiful reflection nebulosity of the Pleiades star cluster helps to exhibit a well known astronomical observing technique called averted vision which works as follows. Look directly at the Pleiades star cluster and you can make out 5-10 stars depending on location and vision. Now if you slightly “avert” your eyes and look away from the cluster about 20 degrees or so, you notice that the stars seem to have clouds or nebulae around them. This is not an illusion; looking straight at the object uses our eye’s natural photoreceptor cells called cones as detectors. Cones are great for brighter objects and for sensing color, but they are not as good for faint details in dark areas. Cones are most concentrated in the center of our eye, therefore instead of looking straight on, look at the object with averted vision (20 degrees off) to use the eye’s photoreceptors called rods more present off center from the eye. Rods are much better at detecting fainter objects.

The name Pleiades, comes from name used for the seven divine sisters and daughters of Pleione although the term is more likely far earlier and references the time of the year in the Mediterranean when it was safe to sail when the Pleiades rises. The brightest stars in the cluster are within the core and were given the names of the seven sisters and daughters of Atlas and Pleione. In order of descending brightness, they are Alcyone, Electra, Maia, Merope, Taygeta, Celaeno, and Sterope. Shown here is a very close up view of Merope!

The Star Merope. In the image above, Merope is one of the dozen or so bright, B-type stars of the Pleiades. At magnitude 4.18, it is barely 5 times brighter than the faintest stars typically visible in a dark night sky. Merope is also designated as 23 Tauri in the Flamsteed designation of cataloging stars. The ancient Greeks labeled stars by their constellation. The brightest star were labeled alpha, then beta and so on. Alpha Tauri would then be the brightest star in Taurus and most of these bright stars also had Arabic names assigned during the Islamic Golden Age and kept to this day! In this case Alpha Tauri is also the Arabic word Aldebaran, which means follower since it seems to follow the Seven Sisters! When astronomers ran out of Greek letters, they had to derive a different system. This is the Flamsteed designation which uses neither letters nor brightness to classify a star. It uses numbers starting with 1 and going in order of right ascension, the celestial coordinate system roughly equivalent to longitude here on Earth, and is one of the two coordinates needed to identify an object’s position in the night sky.

Surrounding Merope is a significant amount of reflection nebula shown here as blue-like waves. As the star passes through this independent nebula, it’s intense light and stellar winds actively reshapes the nebula. The Pleiades is home to many nebula, including the tiny Merope Nebula visible here as a small ghostly object just to the upper left of Merope itself. Note also that both the cross like spikes as well as other inner spikes emanating from the star Merope are caused by the optics and are not present in the real star and surrounding nebula.

Annotations. In the image above, hover a mouse or curser over the image to show annotations of Merope, with several enlarged insets identifying interesting features! Starting with the rectangle at the far upper left is a zoomed-out view of the whole Pleiades cluster. The red box indicates the extents of the entire image above. Merope is the brightest star in the image. Also labeled 23 Tauri, it has a small nebula called the Merope Nebula, just above and slightly left of the star as seen in the inset. At the right edge of the image is an enlarge inset of the waves caused by Merope’s light and stellar wind. Just above that is a close-up of the star Tycho 1800-1579-1. While some sources declare this star is not part of the Pleiades cluster, I believe it is, since it has a very similar proper motion or angular speed of travel as that of Merope. For comparison, Tycho 1800-1579-1 has a proper motion of +21.32 milliarcseconds (abbreviated mas) per year in Right Ascension and -45.44 mas in Declination while Merope is +13.07 mas/yr and -48.40 mas/yr, respectively. The star is an F class, not a B class like Merope, but still it is likely gravitationally connected to Merope and the gas and dust surrounding them both.

Below Images: In the image below, is the same image as above, but processed to remove all stars, except for the two stars believe to be within the Pleiades cluster. This is what the view would appear like if no stars existed along our line of sight. Using a mouse to hover over the image brings all the stars back.

Object Statistics: Constellation: Taurus, Merope Right Ascension: 3h 46m 20s, Declination: +23° 56' 54”, Magnitude: 4.18, Pleiades Size: 2-4°, Distance 444 light-years, Size of core 16 light-years.