Features:

  1. Probably the best known example of an open cluster (of stars) is the Pleiades in constellation Taurus. The Pleiades are well known in many human societies because they form a distinct group of stars on the sky.

    In Europe, they have also been called the Seven Sisters and their Japanese name is Subaru (Pleiades) like the Subaru car and the Subaru telescope (1998--). Geoffrey Chaucer (c. 1343--1400) alludes to them in his The Nun's Priest's Tale (AKA Chanticleer Tale, 1390s).

  2. The stars in the image are finite in size because of finite resolution of the imaging system. They are actually unresolved.

  3. The points on stars are part of star diffraction patterns caused the imaging system. The overexposure of the stars brings out the diffraction patterns which would otherwise be faint compared to the bright round part of the diffraction patterns.

    In the diffraction patterns, the circles are imposed by the circular symmetry of the HST aperture and the 4 points by the 4-fold symmetry of the arms of the secondary mirror (one guesses).

  4. The Pleiades stars themselves have a bluish glow around them because their light is being reflected from clouds of interstellar dust. Such reflecting clouds are called reflection nebulae. The interstellar dust preferentially scatters blue light.

    The bluish glow is actually rather faint and a long exposure and/or high sensitivity was needed to bring it out. This, of course, means that the brightest stars get overexposed.

  5. The 9 brightest Pleiades in apparent V magnitude (see Wikipedia: Pleiades: Brightest Stars) are named in the image. The Pleiades has about 1000 known members, but this number does NOT include unresolved binaries (see Wikipedia: Pleiades: Composition).

  6. How many Pleiades can be seen with the naked eye? Well this depends a bit on your particular scotopic vision and skill, and on the astronomical seeing. Most people under reasonably good seeing can see the brightest 5 in apparent brightness right off the bat. Numbers 6 and 7 in apparent brightness are just a bit harder. After that it gets harder and harder. However, observing 14 seems to be limit for a skilled observer NOT making a colossal effort. Under best astronomical seeing, observing 20 Pleiades has been claimed. For information on observing the Pleiades with the naked-eye astronomy, see Sky & Telescope: How Many Pleiades Can YOU See? Bob King, 2014 October 22.

  7. The 10 brightest Pleiades in order by apparent V magnitude: