Color Magnitude Analysis of NGC 457
This is going to be another blog post highlighting a project done in my course Intro to Astrophysics. For this project, the goal was to capture our own data (using the college’s telescope) of a star cluster, that would allow us to plot a color-magnitude diagram, and learn more about the stars within the cluster.
This project coincided with our inquiry into stellar evolution, and discussion of the main sequence. Through our diagrams, the hope was that we would be able to identify different branches in stellar evolution, as well as give an approximate age to our cluster.
I chose the cluster NGC 457, the Owl Cluster, due to its optimal size and position within the night sky. To read more about specifically how I knew this would be a good choice, and the process that went into collecting and analyzing this data, please see the paper linked:
The commentary that I share here will briefly discuss one of the more interesting inquiries I chose to make when faced with the data, and the diagram, and needing to make sense of it.
A blue straggler and a very red giant
One task we were assigned in analyzing our data was to throw out any outlandish noise, or data points that were obviously not reasonable…
Given this prompt, two data points leapt out at me. In the top-left hand corner (approximately -0.15 B-V, 7.25 V) is a star still on the expected main sequence curve. In short, as a star cluster evolves it is expected that a star’s mass directly defines how long it takes to move off of the main sequence. As you go “down” the main sequence curve, the stars become less massive, and take longer to evolve. So why is it, then, that this star has remained here while many less massive than it have begun to work their way off of the main sequence?
Unsatisfied with the idea of writing it off as noise, I looked into other researchers’ analyses of this cluster, and found many agreeing that this star was very much real, and can be described as a “blue straggler”—a concept we had never discussed in the course. These are stars who have, for whatever reason (usually collision?) gained mass beyond what they were “born” with. This new mass, however, does not immediately set this star down the life path of a star of its new size. Instead, it will continue to evolve at the rate of stars similar to the mass it was born with. For this reason, this star is now left hanging on the main sequence beyond what would be expected of it.
The second data point of concern sat incredibly far out along the B-V axis. While this one I am much less certain of, references are made in other scientific literature to the existence of a significantly-more red giant in NGC 457.
In my paper I cite Fitzsimmons, 1993, who found this same, incredibly red supergiant in their own analysis. They classify it as a M0 Ib supergiant. Although, inconveniently, they omit it from all figures published in their article due to its inclusion requiring scaling of the rest of the relavent data that would have made it present poorly.
This is definitely a type of project that I would like to revisit later this summer with my own camera and telescope. Definitely stay tuned for that :)