PLEIADES OR THE "SEVEN SISTERS CLUSTER" (M45)

2015 February 21, 10:37 p.m. EST (Feb. 22, 03:37 UT)
Rosemary Hill Observatory • Bronson • Florida

Brief Object Notes: The Pleiades in Taurus (also known as the "Seven Sisters") is easily the most spectacular open star cluster visible with amateur telescopes or binoculars. (Visually, M45 is the largest, brightest Messier object.) Although many people can only see six stars in this cluster even under dark skies, binoculars easily show this cluster contains many more stars. With a telescope dozens of exceedingly bright stars can fill the entire view if the field of view is wide.

Visually the cluster spans about 1.6 degrees (more than three moon diameters) corresponding to a linear diameter of more than 14 light years (ly). Nevertheless, the actual cluster diameter is probably much larger, possibly spanning nearly 100 light years. Among the nearest star clusters (abt. 444 ly), this cluster has been known since ancient times since at least six or seven stars can be discerned with the naked eye (measuring 7 ly across). M45 is also a middle age star cluster with an estimated age of about 100 million years. The actual star population probably exceeds 500 though not all stars in the field are necessarily cluster members. (Some could be either foreground or background stars.)

Under dark skies with a telescope having good contrast, one can sometimes see faint, bluish reflection nebulosity around the brightest stars. (The bluish color results from starlight scattering off the nebula's dust particles.) This nebula was originally thought to be left over material from the formation of the cluster. However, modern research shows the dust is from an unrelated cloud in the interstellar medium through which the stars are now moving.

Photograph Below: Taken with a 5-in., f/5.2 refractor. Mimics what might be seen visually through a small (127 mm) to moderate size telescope (300 mm) if skies very dark and clear. (Limiting visual magnitudes are roughly +13.5 and 14.5 respectively for these apertures if naked eye limit is mag. +6.5.) However, sky transparency was mediocre when this photo taken. Still, the limiting magnitude in this photograph is likely fainter than what can one can see visually, possibly fainter than magnitude +15.

The Pleiades requires a wide field of view to appreciate all stars. Click here to see how a more restricted field hinders the view making the overall appearance of the cluster less impressive. (Opens in a new window.) Some telescopes cannot show a sufficiently wide field so many recommend binoculars for the Pleiades. However, the 127 mm refractor used for this photo has high contrast and produces breathtaking visual views using wide-field, low power eyepieces. This also often allows one to glimpse the faint, bluish nebulosity around the brighter stars. (Hazy skies limited the visibility of the nebulosity in this photo.)

PUT CURSOR OVER IMAGE FOR LABELED VIEW — ALSO CLICK IMAGE TO ENLARGE
(Principal stars labeled with their magnitudes in parentheses)

Photo Details 2015 Feb. 21, 10:37 p.m. EST TeleVue 127 mm, f/5.2 APO Refractor, Foc. Len. 660 mm Camera: Canon DSLR EOS 5D II, Exposure 30 sec, f/5.2 (ISO 1600)

OBSERVATIONAL CHALLENGES How many stars can you count with the naked eye using your own telescope? How many stars are in this photo? (Count if you have time on your hands!) Use a good star atlas to estimate limiting magnitudes Find which eyepiece with your scope gives best view (low power, wide-field essential) Can you see the faint, bluish nebulosity around the brighter stars with your telescope? QUESTION: The Pleiades is easily seen by the naked eye. People unfamiliar with the sky often mistake this grouping of stars with a well-known asterism. Which asterism is this? ANSWER HERE

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Images © 2004–2015 H.L. Cohen
Email Cohen followed by @astro.ufl.edu
Last updated 2015 May 18