Saturday, June 17, 2006

Astronomy Internship - Day 8

As predicted, I spent a good deal of my time reading through the previous literature on NGC 7142. The most recent publication I was able to find was from 1991. The earliest I had printed was 1962.

So here's a brief intro to NGC 7142:

NGC 1742 is an open cluster located in Cepheus. Open clusters are assosciations of gravitationally bound stars that will have anywhere from a few thousand to tens of thousands of stars (for reference, globular clusters generally have on the order of hundreds of thousands).

In the picture above, NGC 7142 is the little knot just below and to the right of center. However, you'll probably notice the nebula just up and to the left of it. This is NGC 7129. It's a reflection nebula. This type of nebula is lit by light reflecting off clouds of gas and dust.

However, its presence in the same region of the sky gives an indication that there will be difficulties in making scientifically meaningful measurements of the cluster because it's quite likely that the cloud of dust extends in front of NGC 7142. As I discussed in one of my earlier sections, clouds such as this will dim light that passes through them. However, it does not dim all wavelengths evenly. Red light passes through more easily.

In astronomy, we frequently compare how bright a star is at two different wavelengths (I'll explain this in more detail in an upcoming post). However, since this cloud is effecting the wavelenghts differently, it becomes difficult to make a meaningful assesment of the difference.

To get any sort of meaningful science, it will be necessary to somehow take this cloud into account. The problem then becomes determining how much of an effect this cloud has (known as the reddening). Unfortunately, there's no good way to do this.

And to make matters even more difficult, its strongly believed that the thickness across the face of the cluster isn't even! This introduces relatively large uncertanties into the determinations of all properties.

The most recently determined distance to the cluster is 1900 parsecs (6200 light years or 3.7 x 1016 miles) (Crinklaw & Talbert, 1991).

Another feature many expected to see was a relatively high number of variable stars of a rare type known as "W UMa". Another open cluster, NGC 188, shows many similarities to NGC 7142 and contains many of this type. Thus, it was suspected that NGC 7142 might as well. However, of the 1093 stars observed by Crinklaw & Talbert, only one was found to be a variable. Unfortunately, not enough observations were made to obtain a detailed enough light curve to determine of what type. However, indications from the partial light curves generated suggest it is most likely an eclipsing binary. Looking at the number of images obtained that I'll be working with, it's doubtful that I'll be able to make any further determination of the type of variable.

As far as age, the best determination has been that the cluster is between 3.5 x 109 and 4.5 x 109 years old with data favouring the older estimate. This makes it one of the oldest known clusters.

Tomorrow I'll be going to the lab and trying to find a few more papers that these ones referenced (some as early as 1945). In the evening, our group is going to Mount Laguna Observatory (MLO). The observatory has a pretty live webcam:

Refreshing the page should update the image.

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