Mizar and Alcor

This brightest of naked-eye double stars was known in antiquity and attracted the attention of the early telescopic observers. Alcor (magnitude 4.2) is 11.8' distant, making the pair easy to see. In 1617 Castelli noted that Mizar, the brightest of the two stars (V = 2.0) was again double and so Mizar has the distinction of being the first double star discovered at the telescope.

Bradley, in 1755, was the first to measure its relative position at 143?1, 13'.'9. Lewis,1 using positions up to 1903, found that the annual motion in position angle was +0?025 and from this he estimated a period of 14,000 years. The physical connection between Mizar

and Alcor was established when the proper motions were found to be similar. In fact, there is a stronger connection since a number of the other bright stars in the Plough are moving through space in a loose association - the nearest star cluster to us, in fact twice as close to us as the Hyades. The exceptions to this are a and n.

In 1857 Mizar emerged into prominence once more as it became the first double star to be imaged photographically. Bond used the 15-inch refractor at Harvard College Observatory for this purpose. Agnes Clerke2 quotes "Double star photography was inaugurated under the auspices of G.P. Bond, Apr 27, 1857 with an impression, obtained in eight seconds, of Mizar, the middle star in the handle of the Plough."

With the advent of photographic spectroscopy, plates of Mizar A taken at Harvard College Observatory in 1886 showed the Calcium K line, leading to an announcement by Pickering in 1889.3 Mizar A had also become the first spectroscopic binary to be found, beating the discovery of Algol4 by a few months. In 1906 Frost5 and Ludendorff6 independently announced that Mizar B was also a spectroscopic binary, this time a single-lined system of low amplitude, making radialvelocity measurements rather difficult. The period was not determined correctly until relatively recently when Gutmann7 found a value of 175.5 days.

In the 1920s, with the 20-foot stellar interferometer Frederick Pease8 carried out two sets of observations, in April 1925 and May/June 1927, calculating a period of 20.53851 days for the orbit of Mizar A (Figure 9.1).

The Hipparcos satellite showed that the parallax of Mizar is 41.73 mas whilst that of Alcor is 40.19 mas, corresponding to distances of 23.96 and 24.88 parsecs, thus giving a formal difference in the distance to the two stars of about 3 light years.

In the 1990s the spectroscopic pair Mizar A became one of the first stars to be observed using the Mark III optical interferometer on Mount Wilson in California. An improved instrument, the Navy Prototype Optical Interferometer (NPOI), was then constructed in Arizona. A product of the collaboration between the United States Naval Observatory, the Naval Research Laboratory and Lowell Observatory, the instrument uses phase-closure to build up an optical image of the two components. An interesting consequence of observing pairs with such short periods is that the orbital motion over one night is substantial and has to

Figure 9.1. The components of the 20.3 day spectroscopic binary Mizar showing motion over a 24-hour period (below). Above is plotted the apparent orbit from NPOI observations. The minimum separation is 4 mas. Note the size of the error ellipse for each observed point. The error ellipse arises because the errors in position angle and separation are not the same magnitude and also depend on the orientation of the interferometer optics with respect to the star being observed. (Courtesy of Dr Christian Hummel, USNO.)

be allowed for. The NPOI data are more accurate than that from the Mark III and allows the dimension of the orbit to be determined without an independent measure of parallax.

The orbit was found to have a semimajor axis of 9.83 mas and the maximum observed separation was 11 mas and the minimum 4 mas. Combined with the data from the spectroscopic orbit, the masses have been determined with great accuracy. The distance has also been derived since both the linear and angular sizes of the orbit are known.

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