Thursday, March 19, 2009

The Polar EU Cancri, Part 2

As I described yesterday, my contribution to the cataclysmic variables conference involves my observations of a polar named EU Cancri in the open star cluster Messier 67.

So, why would my findings be considered interesting?

It's precisely because the polar may be in an open star cluster. Open clusters are groups of stars that were all born at the same time out of the same cloud of material. So, every star in the open cluster is the same age, and they all have the same mix of elements. M67 is a well-studied cluster, so we know the age (4 billion years) and we know that it has the same amount and mix of elements that the sun has (this latter bit is important, because some stars only have a thousandth the heavy elements that the sun has, some have twice or three times as many metals as the sun has, and the amount of metals affect the brightness and structure of stars.

Anyway, if the polar I looked at is in the star cluster M67, we know that the two stars had only four billion years to get into their present state. Four billion years is a long time, but, even so, this is one of only a few cataclysmic variables (and the only polar) where we know this timescale.

One important question remains. Is my polar actually in the star cluster? When we look out and see star clusters, we have to look through parts of the Milky Way galaxy, and the galaxy extends for tens of thousands of light years behind the star cluster. So, many of the stars we see in a picture of a star cluster like M67 are either in front of or behind the star cluster.

We can only estimate the distance to the polar I've looked at, we can't get a firm answer (yet). We think we know, roughly, how bright polars are. We know how bright this one appears to be. And the difference between those two gives us a distance (because the further away things are, the fainter they appear to be). From these calculations, we think that the polar is about 9000 light-years away. But Messier 67 is only about 2500 light-years away. So, it seems that the polar only appears to be in the star cluster, but in reality it is not. In this case, all of the information that the star cluster gives us is of no use.

It still remains possible that the polar is in the star cluster, if we've guessed wrong as to how bright it really is. There are other ways to tell distance, and we'll have to try some of those.

So, in some ways, it is disappointing that the polar may not be in the star cluster. There are many known polars, and most of those are much brighter (and so easier to study) than the one I looked at. If my polar is not in M67, then it probably won't get studied much anymore. But that would be a good thing: my polar is very faint and needs lots of time on big telescopes to study. This time would be worthwhile if we can use what we know about the star cluster to study the white dwarf, but this time is not worthwhile if it is just a normal polar floating alone in the galaxy.

So, I get to write a paper about our findings, and I got to learn a lot about a fun type of star. I won't become famous for my accidental observations of this star, but maybe next time.

1 comment:

  1. Anonymous8:10 PM

    what part of the galaxy do we see on clear nights?