Object 35: Mizar and Alcor
Podcast release date: 30 November 2020
Right ascension: 13:23:55.5
Constellation: Ursa Major
Corresponding Earth location: East of Simeonof Island in Alaska
If you've done a little bit of amateur astronomy, then the name Mizar may be familiar to you. Mizar is one of the stars in the Big Dipper, which is also known as the Plough in the UK and Ireland and which also forms the bright part of the constellation Ursa Major. Mizar is also very close to the star Alcor in the sky, so I'm going to be discussing both of those stars in this podcast. If you identify the group of stars as a big ladel or spoon or "dipper", then Mizar and Alcor are the pair of bright stars that are second from the tip of the handle. If you identify the group of stars as the posterior of a bear, then Mizar and Alcor are second from the tip of the bear's tail. If you identify the group of stars as a plough, then I suppose that Mizar and Alcor are second from the tip of the handle of the plough. I don't really see a plough when I look at these stars myself, so I can't really tell you which part is supposed to be which.
The name Mizar seems to have come from medieval Arabic astronomers, although the original Arabic name was Al-Maraqq or "the loins" [1,2]. It seems like some sort of translation error resulted in the name Mizar . Rather strangely, one of the other stars in Ursa Major was also called Al-Maraqq; that other star is now called Merak [1,2]. Mizar, which has a magnitude of 2.0 [3,4], also has the designation Zeta Ursa Majoris, with Zeta (the sixth letter in the Greek alphabet) used to indicate that it is the sixth brightest star in the constellation.
Alcor is much fainter than Mizar; its magnitude is 4.0 . Since it's fainter than 24 of the other stars in the constellation, it doesn't have a Greek letter to identify it, so it has the alternate designations of g Ursa Majoris and 80 Ursa Majoris. It also had a few different Arabic names, including Suha, which means "the forgotten one", and Al-Khawwar, which means "the faint one" . That second Arabic name is most likely the origin of the modern name Alcor.
As an amateur astronomy object, Mizar and Alcor are an interesting double star that can actually be seen with the naked eye. The two stars are separated on the sky by an angular distance of slightly under 12 arcminutes, which is equivalent to slightly more than one-third the angular diameter of the Moon. Also, it's worth noting that these two stars were used at least in medieval Arabic civilizations as a type of eye test [1,6]. If people could see both stars, they were considered to have good eyesight. It's possible that other civilizations also used the stars as a form of an eye test, although I had difficulty finding reliable references for this, so I can't comment further on it.
Scientifically, Mizar and Alcor are fairly complicated. They are not just a single pair of stars.
Mizar (not Mizar and Alcor together but Mizar by itself) was identified as two separate stars by Giovanni Battista Riccioli in early telescope observations around 1650, and this is cited as the first time that a telescope was used to identify that a star was actually two separate star systems [1,7]. In 1857, Mizar was also the first pair of stars to be photographed [1,7,8]. These two star systems have been given the designations Mizar A and Mizar B (or Zeta Ursa Majoris A and Zeta Ursa Majoris B or Zeta Ursa Majoris 1 and Zeta Ursa Majoris 2). They are separated by an angular distance of about 14 arcseconds, or about 1/130 the diameter of the moon. If you want to take a look yourself, they should be visible as two separate objects in a halfway-decent amateur telescope .
In some early spectroscopic observations of Mizar A around 1890, periodic variations in the redshifting and blueshifting of the light from the star system indicated that it was actually two stars that are orbiting each other . This was the first time that spectroscopy had been used to identify that any star was actually a binary star system. The two stars have masses of about 2.5 times the mass of the Sun each, and they are also a little hotter and a little more whitish in color . They orbit each other about once every 20.5 days [11,12]. While Mizar A hasn't attracted a lot of attention from professional astronomers recently, it was featured in a science paper in 2012 that used it to test the application of new mathematical techniques for identifying and characterizing other binary star systems as well as stars with exoplanets .
Mizar B was also found to be a binary star system through observations of its spectra in the early twentieth century. Its stars have masses of 1.85 and 0.25 times the mass of the Sun, which means that one of them is a larger, more whitish version of the Sun while the other is probably a red dwarf . They orbit each other about once every 176 days [11,14]. This system has attracted the attention of astronomers in part because one of the stars in the system, which I assume is the brighter one, is chemically peculiar . In general, spectra of stars show the presence of very small amounts of various elements in their atmospheres, but chemically peculiar stars have unusual combinations of these elements. In Mizar B's case, it's atmosphere has relatively little calcium, although it does have more iron and nickel than usual . These chemically peculiar stars have a tendency to be stars with masses within a certain mass range that includes the larger star in Mizar B, and they typically appear in binary star systems . While I didn't find an explanation for what causes these stars to appear to be chemically peculiar, it seems like Mizar B will be very useful for understanding exactly what is going on.
Alcor is also a binary system, although it wasn't discovered to be a binary system until 2007 . The discovery was made in images made with the MMT telescope using an adaptive optics system that moves the secondary mirror in the telescope to correct for how light is distorted by the Earth's atmosphere. The brighter star in Alcor, which is also the dominant light source, is again a star that is hotter and more whitish than the Sun with a mass 1.85 times that of the Sun's . The fainter star is a red dwarf with a mass of about 0.3 times the mass of the Sun . Interestingly, Alcor produces a small amount of X-ray emission which seems to originate from the corona of the dwarf star .
So, to recap all of the different pairings (because, at this point, everything's probably become a little confusing), Mizar and Alcor are a pair of objects that can be seen without a telescope if you have good eyesight, Mizar is a pair of star systems called Mizar A and Mizar B, and Mizar A, Mizar B, and Alcor are all binary star systems. That's six stars in five different pairings. Apparently, if you have good eyesight when looking at anything involving Mizar or Alcor, you're supposed to have double vision.
One of the interesting questions is whether Mizar A, Mizar B, and Alcor are all located physically close to each other or whether they just happen to appear close to each other in the sky but are actually at very different distances. The geometrical technique called parallax, where the positions of nearby stars on the sky appear to shift relative to background stars, has been used by the Gaia spacecraft to measure very accurate distances to Mizar B and Alcor. The last distance measurements from the spacecraft were published just a couple of years ago, so they're fairly new. The distance from Earth to Mizar B is 80.52 light years (24.69 pc) [17,18], and the distance from Earth to Alcor is almost the same at 80.60 light years (24.71 pc) [17,18]. When the separation on the sky between these two star systems is taken into account, they appear to be 0.29 light years (0.088 pc) apart. This is close enough that Mizar B and Alcor could be at least gravitationally interacting with each other if not weakly gravitationally bound. For some reason, the GAIA spacecraft didn't measure the distance to Mizar A, but I did find a distance measurement from 1998 of 82.78 light years (25.38 pc) . If this measurement for Mizar A is trustworthy, then it would imply that Mizar A and Mizar B are separated by about 2.26 light years. Although Mizar A is definitely physically close to Mizar B and Alcor, it seems a little too far from the other stars to be in a gravitationally bound system. However, I would feel more comfortable reaching that conclusion if Mizar A's distance measurement was more up-to-date or if I had distance measurements to all three star systems from the same instrument.
Finally, it is worth mentioning that Mizar A, Mizar B, and Alcor all appear to be members of an object called the Ursa Major Moving Group [20,21]. This sound slike it should be the name of a company that helps move your furniture to a new home, but it's actually a group of stars that includes most of the other stars in the Big Dipper (or the Plough or whatever). They all appear to have similar ages of about 400 million years and all appear to be moving in the same direction, which inidcates that the stars are all physically associated with each other [20,21]. If this group was located further away, we would probably call it an open cluster. Most constellations and other collections of stars on the sky (formally called asterisms) usually are not stars that are physically associated with each other, so it is very interesting that most of the stars in the Big Dipper (or the Plough or whatever) are actually part of a larger object.
14 December 2020 Update: A week after I published this episode, the people who operate the Gaia spacecraft published new distance measurements for Mizar A, Mizar B, and Alcor , and those measurements change the distances that I calculated among these star systems. The new distances are 81.11 light years to Mizar A, 80.97 light years to Mizar B, and 80.60 light years to Alcor. When accounting for the separation between these stars on the sky, this means that Mizar A and Mizar B are separated by 0.14 light years, which means that they are probably gravitationally interacting or weakly gravitationally bound. Meanwhile, Mizar B and Alcor look like they are separated by 0.46 light years, which means that Alcor is also probably weakly gravitationally interacting with the other two star systems, but it's less likely that Alcor is actually gravitationally bound to the other stars. I kind of expected that new distance measurements would make my analysis out of date at some point. I just didn't expect that to happen a week after putting out this episode.
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