| | Open star clusters are groups of recently formed stars. They are sometimes referred to as "Galactic" clusters because star formation occurs in the disk of our Galaxy. They are called "open" because the stars are free to leave the cluster - not like the way that the Earth is bound to the Sun. Our Sun was formed in an open cluster long ago and its brothers and sisters have all moved away. Open star clusters are typically found hundreds of light years from Earth.
Open clusters appear as an unusually dense patch of stars. They typically contain on the order of hundreds of stars. Since the group of stars is young the brightest ones will most likely be the hot, blue early type stars (remember your HR Diagram lab?!). In some cases, as in the Pleiades, you can see the stars illuminating the gas and dust from which they formed.
Guiding Questions:
- What range of colors can you see in the members of the cluster?
- Approximately how many stars are in your field of view?
- Is the density of stars uniform throughout the cluster?
- What is the range of brightness in the stellar population?
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