Polaris
Published on Jul 27, 2006 at 12:40 am.
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Filed under stars.
“Is that the North Star? It’s the brightest one that I see.” pointing to the southwest. “No. The North Star is that dim one right up there,” I say, pointing towards it. For some reason, the myth that the North Star is the brightest star in the sky seems to be pretty pervasive. Polaris, the North Star, isn’t really all that bright. Yeah, it is the brightest star in the constellation Ursa Minor, but Ursa Minor is not a particularly bright constellation. And, Polaris is on the list of the 100 brightest stars, but on any given night, there may be only a couple dozen stars brighter that are visible, but at magnitude 2.02 (on average) there are actually just over 50 stars brighter than Polaris. I say “on average” because Polaris is actually a variable star. I’ll say more on that later.
Polaris is actually short for Stella Polaris, which is Latin for the Pole Star. It has also sometimes been labeled as Polaris Boreallis, the Northern pole star. That is a bit superfluous, though, since there is no southern pole star. William Shakespeare and Edmund Spenser both refer to this star as the Lodestar. Some sailors gave it the name Navigatoria.
Polaris is special, because it happens that Earth’s rotational axis points very much in the direction of this star. So, as the Earth rotates, this star appears to hang in nearly one position in the sky. The alignment isn’t perfect, so Polaris actually makes tiny circles around the North Celestial Pole. But, this star did not always have this special position in the sky. Earth wobbles like a top. This motion, called precession slowly alters the point direction that Earth’s rotational axis points. Over the course of about 26,000 years, the Earth’s rotational axis traces out a cone with apex angle of 47°. Thousands of years ago, the Earth didn’t point at Polaris at all. Rather, it pointed to a spot near the star Thuban. That was the North Star then. In Biblical times, the Earth’s axis pointed to neither Thuban nor Polaris. In fact, at that time, it didn’t really point at any one star. Rather, it pointed to a point near the bowl of the Little Dipper. The two stars on the end of the bowl, called Kochab and Pherkab both orbited around the North Celestial Pole. These two stars were called the “Guardians of the Pole.†Since that time, the Earth’s rotational axis has been swinging closer and closer to Polaris. It will actually be closest, under ½ degree away, at the beginning of the 22nd Century. But, if the name Polaris comes from the position of the star, what was it called before it became the North Star? Well, like many stars, it had several names. The most common one that I have seen is Cynosura, which means the Tail of the Dog. Actually, that name was originally used interchangeably between the star and the whole constellation that we now call Ursa Minor. The name Cynosura was even used for this star as recently as Johannes Kepler’s Rudolphine Tables in the early 17th Century. I mentioned earlier that Polaris is a variable star. Variable stars have their own naming convention. It is also has a Bayer designation of Alpha Ursa Minoris and a Flamsteed number of 1 Ursa Minoris. In the Bonner Durchmusterung, it is BD+88°8. Looking at Simbad, a stellar database for professional astronomers, I can find 33 distinct names or catalog designations for Polaris. It has considerably more.
Polaris is a dying star. It has used up the hydrogen in its core, and it has ballooned in size. Polaris has nearly 27,000 times the volume of the Sun (30 times the diameter), but it has only 6 times the mass. It is rather hotter than the Sun, and it is over 2000 times as bright. That is appears so dim in our sky is a reflection of its distance, some 430 lightyears distant. Polaris pulsates slightly. The manner of its pulsation shows that it is a Cepheid variable star. Cepheid variables pulsate due to a layer, or layers, of gas in their interiors that are initially opaque and absorb light, warming and expanding, but then becoming cooler and less dense due to the expansion, which makes them transparent, and thus they contract and get more dense and opaque again. They keep doing this, and as they expand and contract, so does the exterior of the star. Like many Cepheids Polaris has displayed more than one mode of pulsation. However, over the last century, Polaris’ pulsations have drastically decreased. It may be on the verge of stopping pulsations altogether. One of its pulsation modes has, in fact, ceased. Also, over the last Century, the oscillation period has increased by about 8 seconds each year.
Eventually, Polaris will settle down, and then start to expand again. It will become unstable and lose its outer layers into space to form a cloud of gas around it, and the burnt out core, a bit over the mass of the Sun, will be left behind to form an object that we call a white dwarf. The outer layers will form a roundish cloud of gas around what is left of the star. This cloud is called a planetary nebula. A planetary nebula has nothing at all to do with planets. Rather, it gets its name from the fact that it is fuzzy like a nebula and round like a planet.
In 1780, William Herschel saw a dim companion to Polaris. Polaris is a tough double star for my students to see, because the ones that I have them look at before all have companion stars of comparable magnitude of perhaps a few times dimmer. Polaris’ companion is 1/250 as bright as Polaris itself. That makes it very dim, and so most students overlook it. If you know what you are looking for, though, it isn’t really all that hard of a double star to see. The Polaris also has an even dimmer companion so close to the primary star that you have no hope of seeing it. The Hubble Space Telescope is only able to just resolve this third star of the system.
Interestingly, it seems that in ancient times, Ptolemy categorized Polaris as 3rd magnitude, not 2nd magnitude as it is today. Polaris is variable, but it doesn’t vary by much. Ptolemy was pretty good about such things as measuring magnitudes, so it has been suggested that perhaps Polaris is actually a magnitude brighter than it was then. A difference of one magnitude is a difference of about 2.5 times in brightness. Such a change is far bigger than is normally expected of a Cepheid variable at this stage of its life. Of course, we do know that in the last century Polaris has gotten about 15% brighter, so who knows.
Anyway, such is Polaris.
-Astroprof
