MIZAR THROUGH THE GALILEAN TELESCOPE

The image at left is a 2 second exposure of Mizar taken at 11:11 pm PDT on April 28, 2004. The photo has been enlarged to a scale of 0.37 arc-seconds per pixel, twice its original scale.

Mizar, also known as Zeta Ursa Major, consists of two components. The bright star, Mizar A, is of visual magnitude 2.3, while the dimmer star at 7 o'clock from it, Mizar B, is magnitude 4.0. Their current separation is 14.4 arc-seconds, a value which changes very slowly with time. The faint arcs of light around Mizar A are diffraction rings, an optical artifact created by the limited aperture of the telescope. Diffraction artificially enlarges the size of point images. As viewed through a one-inch aperture telescope, Mizar B lies approximately on the second diffraction ring of Mizar A, the first bright ring falling in the gap between the stars. Mizar B should not be confused with Alcor, a star of similar brightness about 12 arc-minutes from Mizar, far outside the small field shown here. Under favorable conditions, Alcor can be seen with the naked eye. Mizar B can be seen only through a telescope.

In an undated manuscript (National Edition, Vol. 3, page 877) pointed out in a 1949 article by Umberto Fedele (Coelum, Vol. 17, pp. 65-69), and recently brought back to the world's attention by Czech amateur astronomer Leos Ondra (A New View of Mizar), Galileo estimated the separation between the centers of Mizar A & B as 15 arc-seconds and the diameters of the images as 6 and 4 arc-seconds with a gap of 10 arc-seconds between them. Ondra's webpage includes an English language translation of Galileo's Latin text by University of Nebraska Classics Professor Thomas Winter. Galileo erroneously assumed that the size of the images he saw in his telescope represented the true size of the stars; and since the angular diameter of the larger image was about 1/300th the angular diameter of the Sun he speculated Mizar A might be about 300 times as far as the Sun. Although this conclusion was wrong (modern astronomy places both components of Mizar at 78 light-years from Earth -- 4.9 million times as distant as the Sun), the fact that the apparent diameters recorded by Galileo are close to those in the present photo indicates the image quality in his telescope must have been similar to that of the present replica.

Mizar was among the objects viewed by Abetti and Hale in their 1923 sky tests of the telescopes preserved in the museum at Florence. They report that due to deformation of the image in Galileo's 14-power telescope Mizar B could be detected only with difficulty, but that it was separated much better in the (smaller) 20-power telescope. It should be noted that although the telescopes in Florence are thought to be associated with Galileo, we are unaware of any evidence they were ones he used for his own astronomical observations. We were able to see Mizar B easily through the present 20X replica, much as shown in the photo above. Galileo's description of the gap or interval between Mizar A and B being as large as the combined diameters of the two stars indicates he split them even better than us. Such higher resolution would be possible only if he were using a somewhat larger aperture telescope.

An extensive history of observations of Mizar, including a facsimile and English language translation of Galileo's notebook page (National Edition, Vol. 3, page 877) about Mizar are given on Leos Ondra's webpage. Ondra also calls attention to an observation of Orion (National Edition, Vol. 3, page 880), mentioned in Fedele's article, and notes that based on Galileo's description of how the apparent spacing between a pair of stars near the Trapezium as seen through his telescope exactly matched the spacing of a pair of stars in Orion's belt as seen with the unaided eye, Galileo's telescope at that time (the manuscript is dated February 4, 1617), and possibly the same telescope he used to observe Mizar, must have been closer to 27 power. As pointed out by Fedele, Galileo was also at that time easily able to resolve the three brightest stars of the Trapezium itself, which, at least currently, have spacings slightly smaller than that between Mizar A and B. He may possibly have been using the large cracked objective preserved in Florence, or something similar to it, but it is clear his resolution was significantly better than the 20 arc-seconds estimated by Greco, Molesini and Quercioli. A ray-tracing simulation of the resolution we would expect on Mizar from a 38 mm diameter singlet refractor, plus further examples of Galileo achieving similarly high resolution on other astronomical targets, are shown on our Photo-Drawing Comparison page. Even more accurate simulations of the images of Mizar expected at a variety of apertures are shown on our Focus page.

The observation of Mizar described in Galileo's 1617(?) manuscript may not have been his first. In a famous letter to Kepler, Martin Horky, a rather vicious critic of Galileo, describes a "star party" held in Bologna on the evening of April 25, 1610. Horky thought that the four moons Galileo had reported around Jupiter were illusions produced by the telescope. If Horky's letter is to be believed, during the party, at which Galileo was attempting to demonstrate his new discoveries to the leading scientific men of the city, Horky suggested pointing the telescope at Alcor, the naked-eye companion of Mizar. He says the observation placed Galileo in a state of dejection, for they "saw four very small stars nearby," and since this was similar to what Galileo reported seeing around Jupiter all present were convinced both were illusions. Galileo's true reaction or interpretation of what he saw (if the story is true) is unknown. There are indeed three stars easily visible in this region within the field of the Galilean telescope (Mizar A, Mizar B and Alcor) and under dark skies it should be possible to glimpse an additional 8th-magnitude star known as Sidus Ludoviciana ("Ludwig's Star") bringing the total to four. However, in fairness it should be noted that Horky also claimed Galileo's telescope made the bright star Spica appear double on the same night (National Edition, Vol. 3, p. 142; Vol. 10, pp. 358 & 387 [Letters 314 and 347]), and Spica is not double.

According to Leos Ondra's research, which in this regard is mostly a recounting of Fedele's 1949 article, Galileo's interest in the spacing between Mizar A and B (and the possibility that it might change during the year) stems from his wish to prove to his critics the motion of the Earth through space by the well-known effect called parallax -- the apparent change in the position of a nearby object relative to a more distant one when one changes one's point of observation. In his famous Dialog Concerning the Two Chief World Systems (1632) Galileo explains the then already well understood idea that if the stars are at finite distances and if the Earth moves in a huge orbit around the Sun, then the apparent positions of the closer stars should move relative to the more distant ones. Tycho Brahe had searched in vain for examples of stellar parallax that could be detected by the naked eye. Apparently Galileo thought the effect might be within the range of the added twenty to thirty times greater precision provided by his telescope, and perhaps that the fainter stars revealed by the telescope would provide a more distant backdrop against which the motion of the brighter stars could be seen. However, Galileo's note to himself regarding the possible distances to these stars seems strange since Tycho had already concluded the fixed stars must be much further away. If Mizar A were indeed only 300 solar distances away its annual wobble relative to a truly distant star should have been an easily observable ±12 arc-minutes; and Mizar B, at 450 solar distances (3/2 times farther than Mizar B), would have been expected to show an annual parallax of ±8 arc-minutes. Their motion relative to one another would be about 4 arc-minutes (Mizar A would appear to move in a yearly circle of this radius with respect to the more distant Mizar B). The complete absence of any detectable change at the level of a few arc-seconds must have been very puzzling indeed to Galileo and Castelli. Since they continued to believe the Earth moved, they must have had to discard either the idea that these stars were close to the Earth or the idea that they were at significantly different distances.

Images © Tom Pope and Jim Mosher
Last modified: July 14, 2006