I’ve won and lost a lot of money for people in silly bar bets about Polaris. People have the common misconception that Polaris, the North Star, is the brightest star in the sky. It’s not; Sirius holds that honor. It’s not even in the top 20 brightest stars. It comes in at number 48. That’ll be five dollars, thank you.
And as my friends and family all know, if you ask me an astronomy question after a few drinks, you are not going to get a short answer. So here is the rest of the fascinating story of the North Star, Polaris.
Polaris, also named alpha Ursa Minoris, is the brightest star in the Little Dipper. It marks the end of the handle. By a twist of luck, it also happens to reside very close to the North Celestial Pole (NCP). This is the point in the sky that all the stars in the north rotate around. It’s not exactly on the NCP, in fact it’s more than a Moons width away, so it scribes out a very small circle in long exposure star trail images like this one below. To the unaided eye it appears that all the stars rotate around Polaris while it remains fixed in one spot.
This fact has been known since ancient times, and Polaris has been used for navigation for centuries. The Chinese philosopher, Confucius, remarked, “He who exercises government by means of his virtue may be compared to the north polar star, which keeps its place and all the stars turn towards it.” Not only does it tell you where north is, its angle above the horizon roughly equals your latitude on Earth.
Through binoculars Polaris looks like the diamond in a small asterism called the ‘Engagement Ring’.
Through a small telescope it is easy to see that Polaris is actually a double star, a fact discovered by William Herschel in 1780. This visual companion is known as alpha UMi B.
In 1929, another fainter and much closer companion was detected spectroscopically, but it wasn’t until 2006 that we were actually able to image this close dwarf star with the Hubble Space Telescope. This third member of the system is called alpha UMi Ab.
In spite of Shakespeare's Julius Caesar declaring, "I am as constant as the Northern Star, of whose true fixed and resting quality, there is no fellow in the firmament", not only is it not at rest in the firmament, the North Star is not constant in brightness either! Polaris is a variable star, and as it turns out, a rather interesting, unique variable star.
Polaris is a Cepheid variable. These are stars that pulsate with periods of a few days. The expansion and contraction of the outer atmosphere leads to changes in brightness. These stars are typically yellow giants or super-giants. They are huge stars, 40-180 times the radius of our Sun and much more massive. Polaris is six times as massive as our Sun and its radius is 45 times that of the Sun. Polaris is the closest of these stars, at a distance of 431 light years.
Cepheids have the unique characteristic that the period of the star, the time it takes to go from maximum light to minimum and back again, is directly proportional to the absolute magnitude (brightness) of the star. If we know the period and how bright the star ‘appears’ from earth, we can determine with a great deal of accuracy how far away the star is. In this way Cepheids have been used as benchmarks, or ‘standard candles’ to measure distances. Since these stars tend to be huge and bright, we can even see them in galaxies outside the Milky Way.
Since Cepheids are used to measure the distance to galaxies and the expansion rate of the universe, it is essential to understand their physics and evolution. Being able to image and study the exact motion of Polaris and alpha UMi Ab is a boon for astronomers who want to determine the mass of Polaris accurately. Analyzing the orbits of double stars is one of the most effective ways astronomers have for determining the mass of stars. Knowing the mass is the most important ingredient in understanding the evolution and other properties of stars.
Another interesting discovery in the last few years is that Polaris and many other Cepheids are shrouded in an envelope of gas, some 2 to 3 times the size of the stars themselves. The physical processes that have created these envelopes are still uncertain, but it is probable that these envelopes were created from matter ejected by the star itself.
As a consequence of the large amplitude oscillations of these humungous stars in a period of just a few days, material in the photospheres of these variable stars can be moving with velocities up to 100,000 km/h. It doesn't seem too unlikely that occasionally these stars might lose their gravitational grip on some of this fast moving material. Astronomers are studying the link between this pulsation, the mass loss and the formation of these envelopes.
Even stranger than all this, is the fact that Polaris has been steadily quieting down its pulsations over the last 100 years. Around 1900 the variations in brightness were about 10% of the average luminosity. During the last half of the 20th century Polaris’ variations had dropped to approximately 2%. No other Cepheid is known to have gone through this. Astronomers believed they were witnessing the evolution of the star before their very eyes, and that eventually we would see Polaris’ variations snuff out entirely.
In the course of performing this death-watch, it was discovered recently that Polaris is actually coming back to life! The amplitude of pulsations is on the rise. The evolutionary explanation of the changes in Polaris may not hold water any more, and astronomers will be scrambling to collect more data to figure out what is actually happening.
So, while she may not be the brightest star in the night sky, Polaris is one of the most intriguing.