Polaris Part Two

How 'constant' is the northern star? Last week, I noted that the role of the northern star changes over the millennia  due to the precession of the equinoxes. But even the star we call the north star today is not so constant! Its brightness varies!

Like almost half of stars, Polaris belongs to a multiple star system. Although such systems can have more than two stars they are generally referred to as 'binary stars'.  What we see with our eyes as one star, a telescope would reveal as two (Polaris Aa and Polaris B) and a better telescope would reveal as three (Polaris Ab). There are two other stars, more widely separated from the other three, that were once thought to be part of the system, but X-Ray observations with the Chandra Space Telescope suggest these stars are older than the other three and therefore did not form with them.

Astronomers, rather than coming up with new names for each star in a binary system, keep the same name for every star in the system and use capital letters (A B C ...) to distinguish each star. A lowercase letter denotes an object which orbits or is paired  up (so orbiting a common center of mass) with only one of those objects. In this case, Polaris Aa and Polaris Ab orbit together, and Polaris B is paired with that pair of stars. The two non-related companions are still called Polaris C and D, even though they are no longer thought to be part of the system.

Images of Polaris A, Ab, and B via the Hubble Space Telescope
Credit  listed at bottom of photo original image found at hubblesite.org

Of the three stars, it is Polaris A, the star we see with our naked eyes that is the most interesting. It is a Cepheid Variable. These stars are of immense importance in modern astronomy because the amount of time it takes for one to undergo a full cycle of dimming and brightening (period) is related their luminosity. Because astronomers use luminosity as means of determining distance, this means that by observing the period of a Cepheid astronomers can determine the distance to a star, even if it is too far away to accurately determine its luminosity. In this way, we are able to determine the distance to other galaxies.
 Polaris is of particular importance because it is the closest Cepheid to Earth and because it is a binary. That Polaris is close is important because it means we can determine its distance by other methods, such as parallax, which allows us to determine it's luminosity independently of period  and fine tune the relationship between period and luminosity.  Moreover, because it is a binary, it is possible to determine the mass of the star, which helps astronomers work on the theory of why Cepheids pulse.

In later posts I'll detail more about the nature of Cepheids and describe the entire Cosmic Distance Ladder. For now, remember that Polaris, is not nearly so constant as Shakespeare had Caesar suppose!

Star of the Week: Polaris

One of the regular features I'd like to have on this blog is 'Star of the Week', in which I introduce readers to a star in the sky. It's likely that along with this, I'll talk a bit about the constellation in which the star is located,  but I'm considering having a 'constellation of the month'.

Appropriately, the first 'Star of the Week' is Polaris--the North Star.

Polaris is not an especially exceptional star. It is the brightest star in Ursa Minor (hence it has the name Alpha Ursae Minoris) but with an apparent magnitude of around 2 it is the 46th brightest of stars (in both hemispheres). Were it not for it's location, Polaris would be no more familiar than Alphard, the brightest star in the constellation Hydra, which has a similar apparent magnitude. But, what a location it has, for Polaris has a distinguished position close to celestial north pole. This means that to observers in the northern hemisphere, Polaris wobbles in place ever so slightly while all the other stars trace circles around it. It also means that at night, one can find north by finding the appropriately nicknamed North Star. Moreover, should anyone ask you to find Polaris during the day, although you cannot see it, you can confidently point north.

Stars traverse the sky at night, but not Polaris, which sits
like a king in the center of a court.
Photo : Ashley Dace under Creative Commons Attribution Share-alike

The fact that there is a star near the celestial north pole is a happy coincidence of both time and space. At the moment, Polaris happens to be located above the axis of Earth's rotation. However, the orientation of Earth's axis of rotation wobbles, a process called the precession of the equinoxes. As a result Polaris was not the North Star for the Ancient Greeks and in the coming centuries, it will be drift further and further away from north.

One can find Polaris by using a simple trick involving one of the most easily identifiable constellations, Ursa Major (also known as the Big Dipper). Using the two stars on the leading edge of the dipper section (the two stars away from the handle) follow up from the bottom star. The next bright star you find is Polaris. One can also locate the star at the edge (back end) of the handle part of the Littler Dipper (Ursa Minor) but because that constellation has many dim stars, it is difficult to find it in places with even moderate light pollution.

Besides finding north, one can also use Polaris to get a rough estimate of latitude or distance above the equator: the height of the star roughly corresponds to one's latitude. At the North Pole, it is overhead, whereas at the Equator it is skimming the horizon.

In a post later this week, I'll detail information about the nature of the star. Sneak Peak: What we call the North Star actually more than one star!