Since we discussed Ursa Major last time, it makes sense that we move onto the constellation’s smaller companion, Ursa Minor. As was the case with the Greater Bear, you’re probably more familiar with the asterism within the Lesser Bear. It is aptly named the Little Dipper as its seven stars form a shape remarkably similar to the seven stars of the Big Dipper, save the former’s upturned handle. The distinction between Ursa Major and the Big Dipper, however, is rather clear. The asterism makes up the upper-left hindquarters of the larger constellation. Conversely, the Little Dipper asterism dominates the constellation of Ursa Minor. In many of the star catalogs I could find, the two are practically interchangeable. The fainter stars of the constellation at large do little to change the overall shape. For this series of posts, we will treat them interchangeably, as well.
Ursa Minor is the northernmost constellation in the northern celestial hemisphere, owed to the fact that it contains Polaris, our planet’s North Star! If you were standing at the North Pole, it would appear directly above you. Otherwise, it will appear in the northern sky. The constellation is circumpolar for much of the northern hemisphere, and it only disappears completely over the northern horizon just south of the equator (1). If you have trouble with your cardinal directions (like me), your best bet of finding Ursa Minor is to use the technique we discussed in our introduction to Ursa Major. Find the prominent Big Dipper, locate Merak and Dubhe at the end of the ladle, and follow the line they make up from the bowl to Polaris. Once you find Polaris, look for the handle of the Little Dipper, approximately 90° clockwise from the line you just followed up from the Big Dipper. From the handle, you should find the bowl rather easily, but be sure to keep your focus. The Little Dipper is far fainter than the Big Dipper, and if you aren’t looking carefully you can easily lose your place in the sky. Fear not! Just find the Big Dipper again and repeat the process, using any references you need. You’ll find Ursa Minor in no time!
Polaris is an interesting star for several reasons. Its name is derived from the Neo-Latin “stella polaris” for “the pole star” (2), but “stars” would be more accurate. Polaris is not a single star, but a system of three gravitationally bound stars known as a trinary system. Our solar system is a single star system, but multiple star systems are more common than you might think! About 50% of sun-mass stars have one or more companions, and many of the stars in the night sky are in fact multiple star systems that the human eye cannot resolve into individual components (3). The Polaris system is composed of its primary star, Polaris Aa, in a tight orbit with a fainter companion, Polaris Ab. These two stars are in turn in orbit with another faint star, Polaris B (4). Furthermore, Polaris Aa is a Cepheid variable star. Cepheids brighten and dim periodically, and by periodically I mean in very regular intervals. This process is driven by the expansion and contraction of the Cepheid’s outer layers (5). The American astronomer Henrietta Swan Leavitt (1868 - 1921) discovered that brighter Cepheid stars are associated with longer periodical intervals, so the former can be inferred if the latter is known. From brightness, the star’s distance can be determined. These stars are incredibly powerful cosmic meter-sticks, and Cepheids in galaxies beyond the Milky Way are vital for exploring the size and expansion of the universe (6). Polaris Aa is a prime candidate for studying the intrinsic physics and evolution of Cepheids. Not only is it the closest star of its type to us, about 430 lightyears from Earth, but its being a part of a multiple star system makes determining its mass all the more easy (4). Evidently, Polaris is far more than simply our stoic and unmoving North Star.
Finally, let’s touch on what a “North Star” really is as we tend to take the term for granted. Our northern axis points almost directly at Polaris, and thereby it appears near motionless in the night sky as the Earth rotates. This is most evident in long exposure images of the northern celestial hemisphere like Figure 3, where Polaris is the bright star trail closest to the center. There is, however, nothing particularly special about Polaris, besides perhaps its brightness, that makes it our North Star. It is simply a cosmic coincidence. There is no physical law that states that a planet must have North Star visible to the human eye. As a matter of fact, Polaris hasn’t even been our North Star for the entirety of recorded human history. Around 3000 B.C., that honor belonged to the dimmer star Thuban, from the Arabic “Ath-Thu’ban” for “the snake,” in the constellation Draco (7). Thuban's tenure as our pole star corresponds to the building of the Pyramids of Giza in Egypt, and some Egyptologists believe they were built using Thuban as a polar reference (8). To be clear, it was the genius and ingenuity of Egyptian astronomers that let this astro-architectural feat, not aliens. Around 1000 B.C., however, Thuban was largely retired as the pole star, but there were no other good candidates. Instead, the star Kochab, from the Arabic “Al-Kaukab” broadly for “the star,” in Ursa Minor served as a reference to the pole (7). Kochab can be found at the tip of the bowl the Little Dipper and is only slightly dimmer than Polaris, but it was never as close to the pole as Thuban was. Finally, around 500 B.C., Polaris inherited its current role as the North Star, and ever since it has moved closer and closer to the true pole. It will be closest on March 24, 2100, after which the pole will begin to move away from Polaris.
The process by which the pole moves is known as “precession,” and you're probably more familiar with it than you think. Think of a spinning top with a handle. As it continues to spin on a table, the direction of the top’s handle will enter a periodic cycle, and it does so due to the force of gravity and the frictional forces of the table acting upon the top. Similarly, Earth’s axis precesses due to the gravitational forces of the Sun and the Moon on the Earth, although the periodic cycle is far longer (9). It takes about 26,000 years to complete one precession cycle. The path the Earth’s axis follows can be seen in Figure 4, and it will continue to follow this path after the year 2100. In 2000 years, the star Errai, from the Arabic “Ar-Ra’i” for “the shepherd,” in the constellation Cepheus will serve as our pole star. In 5500 years, another star in Cepheus called Alderamin, from the Arabic “Adh-Dhira’ al-Yamin” for “the right forearm,” will succeed Errai (7). Indeed, Polaris will soon retire its title, and even its name will no longer be appropriate. There was a time, however, when Polaris was known to the Greeks as Phoenice. Perhaps it will return to this older name when the time is right. Next time, we’ll discuss the origins of Phoenice. (10)
References
Duchene, G. & Kraus, Adam. (2011). Stellar Multiplicity. Annual Review of Astronomy and Astrophysics. 51. 10.1146/annurev-astro-081710-102602.
https://hubblesite.org/contents/news-releases/2006/news-2006-02.html
http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes16.html
https://starchild.gsfc.nasa.gov/docs/StarChild/questions/cepheids.html
http://hosting.astro.cornell.edu/academics/courses/astro201/earth_precess.htm
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