Monday, July 8, 2013

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

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!

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