This post describes the stars Mizar and Alcor as part of our series of interesting objects to observe with binoculars or a small telescopes. For information about this series please see our feature, “Summer Observing List“.
Mizar and Alcor
Mizar and Alcor are an easy pair to find, but there is much more going on with this pair than first meets the eye. Mizar is the Zeta star in Ursa Major, but it is more easily recognized as the second to last star in the handle of the Big Dipper.
Just 11.8 arcminutes to the northeast of Mizar is the star Alcor. On a clear dark night those with good eyes will be able to see the pair as two distinct stars. Otherwise, many people see them as a single star. The two are visible year round from the mid-northern hemisphere, although from mid-October to early February they will be very low on the horizon around an 8 PM reference time. Depending on your local horizon (buildings, hills, trees, etc.) you may not see them in the early evening until the late spring months.
Mizar has a visual magnitude of 2.4 and is definitely a naked eye object. Alcor has a magnitude of 4.02 so given its proximity to Mizar it is a little bit of a challenge to see it with naked eyes. In fact, it is said that Mizar and Alcor were used as a vision test for Greek soldiers in ancient times, those who could see Alcor were considered to have excellent vision. In binoculars and even the smallest telescope Mizar and Alcor are easily separated as distinct bright dots.
Mizar and Aclor form what is known as a Double Star, which is simply any pair of stars that appear to be close together. There are two distinct types of Doubles. Optical Doubles are stars that just happen to be aligned close together when viewed from Earth. One of the pair may be many light years, even thousands of light years, behind the other. Binary Stars are two (or more) stars that are gravitationally bound to each other. They each orbit the center of mass of the combined system, which may be external to both stars. It turns out that, in the universe, binary star systems are actually more common than single stars such as our sun.
An interesting twist is that Mizar itself is a Double Star, it has a binary companion known as Mizar B with a magnitude of about 4.0 and a separation of about 14 arcseconds. As such Mizar B can be found with a decent quality small telescope. Mizar is, in fact, the first binary star discovered and that was way back in 1650. It takes this pair over 5,000 years to orbit the center of their system. Making the Mizar system even more interesting is the fact that both Mizar and Mizar B each have their own binary companion, which makes Mizar a double-double star (or quadruple system). Mizar’s individual binary companion is too close to resolve visually with amateur equipment but the pair are known to orbit each other in about 20.5 days. Mizar B’s companion also is very close and the Mizar B pair orbit their center with a period of about 180 days.
For a long time it was thought that Mizar and Alcor were Optical Doubles because the motion of Alcor was not consistent with it being gravitationally bound to Mizar. Distance measurements of Mizar are made difficult by the complexity of its double-double star system so it was unclear wether Mizar and Alcor were in close enough proximity to actually be gravitationally bound. In March of 2009 astronomers were studying Alcor searching for evidence of planets. They found no planets but discovered that Alcor has its own binary companion, a red dwarf star given the name Alcor B. This makes the Mizar-Alcor system a sextuple system! With knowledge of this companion star, astronomers found that the gravitational effects of Alcor B on Alcor somewhat explain the inconsistencies in Alcor’s motion with regard to Mizar suggesting they are a true Binary pair. Knowledge of Alcor B also allowed more accurate estimates of the distance from Earth to Alcor which now indicate that Mizar and Alcor could be as close as .27 light years apart at their minimum separation meaning that they are indeed close enough to orbit together and are most likely a Binary pair.
Mizar-Alcor is a classic example of there being more than meets the eye to many of the objects in the night sky. Far from just being a single star in the handle of the Big Dipper, Mizar and Alcor were important as one of the first visual Double Stars, having been known since ancient times. Mizar was important as the first Binary star observed (1650), it was the first to be photographed (1857), and it was the first to be measured with a spectroscope (1909). Even among all the attention paid to this pair over the years, we have known only since 2009 that Alcor is a Binary star and is most probably gravitationally bound to Mizar!