Weighing Polaris
Published on Jan 8, 2008 at 10:33 pm.
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Filed under conference blogging, stars.
About a year and a half ago, I did a posting on the star Polaris. For those of you who don’t know, Polaris is the name of the North Star. You can look at my previous posting for some additional information on the star. One interesting thing about Polaris that a lot of people don’t know is that it is a variable star. It does not vary in brightness by much, but it does change some. It is a type of variable star that we call a Cepheid Variable. Cepheid variable stars pulsate, changing temperature, size, and brightness as they do so. Interestingly, though, there is a relationship between how quickly the Cepheids pulsate and how bright they are. The brighter ones take longer to pulsate. This period-luminosity relationship was discovered by Henrietta Swan Leavitt nearly a century ago. The relationship is quite good. It is so good, in fact, that if you can measure the period of a Cepheid, then you can determine its luminosity. And, if you know how bright the Cepheid really is, you can determine its distance by how bright it appears to be. This is a very powerful tool, because it can be used to determine distances to star clusters and galaxies that are too far away for their distances to be directly determined using parallax. Cepheid variables, then, can be used as measuring sticks. The actual term that astronomers use for this is “standard candle” because the actual intrinsic luminosity can be determined from a measurement of some other property observable from Earth. But, in order to be a “standard candle,” you have to be able to calibrate the period-luminosity relationship. That turns out to be tough, since most of the Cepheids are quite far away, and thus measuring the distance to them is tough, but you need to know the distance in order to calibrate the relationship.
Polaris is only about 430 lightyears away, making it one of the nearest Cepheid variables. But, for all the importance of Cepheid variables, we really don’t know nearly as much about them as we’d like to. For example, we have not really had a good measure of the mass of a Cepheid variable before. But, just how do you go about determining the mass of a star? After all, you can’t just put it on a laboratory balance and weigh it.
The only way to really “weigh” a star is to measure binary stars. When stars are in a binary pair, they move around one another under the influence of each other’s gravity. By measuring this motion, we can determine the masses of the binary stars. However, most Cepheid variables are too far away to see if they are part of binary systems, or to measure them if they are. When you look at masses of Cepheid variables, most of those masses are really best guesses. But, remember, Polaris is a Cepheid variable. And, in fairly modest amateur telescopes, it can be seen to be a binary star. The brighter star, the one that you see with the naked eye, is called Polaris A. The dimmer one is Polaris B. But, Polaris A itself, turns out to be a binary. We call these two stars Polaris Aa and Polaris Ab. Again, the brighter one is a and the dimmer one is b. The star that you see with the naked eye, the Cepheid variable, is Polaris Aa.
You can look up Polaris in various stellar lists, and sometimes a mass is listed. However, if you compare the masses in those lists, you will see a wide range of masses reported. Now, this is where I tie this posting into the AAS meeting. Earlier today, I saw a poster presented by Nancy Evans, et al, reporting on their progress in determining the mass of Polaris. Using the Hubble Space Telescope, they determined a mass for Polaris A as 5.8 solar masses. The mass of Polaris Aa appears to be about 4.5 solar masses (and Polaris Ab is, thus, 1.3 solar masses). This is a continuation of their earlier work. And, of course, there is still more work to be done. There is still a range of error in this measurement, but the more data, the more the masses can be nailed down.
-Astroprof
(image courtesy of Wikimedia)
