THE PLEIADES THROUGH THE GALILEAN TELESCOPE

The image at left is a composite of approximately 20 star fields photographed through the Galilean telescope on January 24, 2005. Each photograph, covering a diameter of approximately 14 arc-minutes, was a 16 second exposure using a new Olympus C-5050 digital camera with automatic noise reduction. The image at left is Galileo's impression of the Pleiades as published in Sidereus Nuncius. Galileo's engraving is also a composite of many small impressions and appears to suffer from considerable distortions of scale. The photographic composite has similar, but smaller, distortions in scale, and may contain holes where an observation was omitted. To display at a reasonable size, the photograph has been reduced to 1/10th its original size. In the process of reduction some of the fainter stars have been lost. Click here or on the photo to see the composite at half its original size. It should be noted that neither the photo nor Galileo's print give any hint of the dramatic veils of wispy light that envelop the Pleiads in most conventional astrophotographs of this region. That nebulosity is simply too faint to be seen by either the eye or the camera through a telescope with this small aperture and long focal length. The many photographs showing it are typically longer exposures through much faster (lower f/ratio) systems and may have been additionally enhanced to show it.

In the following, the preceding pictures have been numbered to aid in identification of stars common to the two. In this case, the photographic composite has been "posterized" before reduction. This makes similarly-visible all the stars above a certain threshold brightness, which are then shown on a uniform black background. The numbers correspond to Galileo's engraving and an attempt has been made to preserve the distinctions in magnitude he makes by representing stars with differently sized symbols. Galileo appears to distinguished four ranks of magnitude: First are the six naked eye Pleiads listed in Ptolemy's Star Catalog. These are numbered in red. Next come seven stars represented by larger than average star symbols. These are numbered in green, followed by twenty-one average sized star symbols, numbered in blue, and two smaller than average star symbols numbered in magenta, for a total of thirty-six stars.

Galileo's stars 7, 19, 9, 12, and 13 appear to fall outside the field of the photo as does star 31 which appears to have fallen in one of the gaps between photos. On the other hand, above stars 17 and 18 the photo shows a number of stars not included in Galileo's drawing. In addition to these differences in field and distortions in scale there are number of other differences between the two images which might be noted. The most striking of these is that the relative magnitudes observed by Galileo do not always accord well with the magnitudes as they appear in the photo. For example, Galileo called attention to star 8 as being brighter than average, yet it is now one of the dimmer stars in the field. Similarly, at position 33 he indicates an average brightness star, yet we see nothing above the threshold of the posterized composite near that location. Conversely, he does not seem to have called special attention to star 17, yet it is now the seventh brightest of the Pleiads. In addition to these there are a number of lesser-brightness stars which though prominent in the photo have no numbers beside them because Galileo did not record them at all. Since Galileo was, in general, a very careful observer it seems likely that at least some of these discrepancies represent real changes in brightness over the last 400 years. Finally, the average brightness star which Galileo depicts at location 29 is clearly resolved as a pair of stars in the photo; and the very faint star he shows at location 35 shows as a group of three stars in the photo. The two stars at location 29 are currently separated by about 40 arc-seconds. Based on their currently understood proper motions that spacing would have been about 33 arc-seconds in 1609.

Several of Galileo's hand drawn sketches of the Pleiades, along with a diagram of Cassiopea, can be found on Volume 3, page 962 of the National Edition of Galileo's works. With a few exceptions, the Pleiades sketches are very similar to the much better known plate from Sidereus Nuncius that is reproduced above. The exceptions include two extra stars above numbers 14 and 15, and two extra stars in the long string from number 22 to number 27. One of these fills the gap between 26 and 27. A total of eight evenly spaced stars are seen from 22 to 27, compared to the six in the final print. Also, the different magnitude classes are not distinguished as clearly in the original sketch as they are in the final print. In the sketches, all the "new" stars are depicted by small star symbols or dots of approximately equal size.

The following table provides a cross-identification of Galileo's star designations with the modern and traditional nomenclature, and also gives the normal modern visual magnitude of each. Click on the Henry Draper Catalog Number to bring up additional information about each star, including its identity in numerous other catalogs, via the SIMBAD database, operated by the CDS in Strasbourg, France. In the cases indicated with "?" the modern catalog number of Galileo's star is highly uncertain. It appears that Galileo's limiting magnitude at the time of this observation was a little over 8.

For additional examples of Galileo's drawings of extended star fields see our Photo-Drawing Comparison page.

Images (unless otherwise credited) © Tom Pope and Jim Mosher
Last modified: July 14, 2006