Rubin Planetarium Video - Open Clusters

An open cluster is a group of stars that formed out of a single large molecular cloud of gas. For that reason, all of the stars in the cluster have the same age and chemical composition. Open clusters can contain more than a few thousand stars. The clusters are loosely held together by gravity, but over time the stars in a cluster will disperse.

Open clusters are particularly useful to astronomers who are studying stars. In addition to having a common age and composition, all of the stars in the cluster are essentially at the same distance from Earth. This allows astronomers to calculate the distance to the cluster, and to accurately determine properties of the cluster stars.

Since stars in a cluster are seen intermixed with other stars, it can be difficult to determine which stars belong to the cluster. One way in which cluster members are identified is by measuring their proper motions. Since they formed in the same molecular cloud, stars in a cluster will move across the sky in roughly the same speed and direction.

More than 1,100 open clusters have been discovered within the Milky Way galaxy, but there are many more to be found. LSST will be exceptionally good at finding clusters, especially in the dense stellar environment in the disk of the Milky Way. By measuring their proper motions over its ten-year survey, LSST will accurately identify cluster members. LSST will also see faint cluster stars (e.g., M stars) to distances of 30,000 light years, which is five times the distance currently possible.

Storyboard

This animation provides a visual representation of an open cluster in two different manners: first, we see the cluster as it appears at a particular moment in time. Next, we see the cluster with time sped up rapidly, over a span of millions of years, so that the stars can be seen as they move through space. The sequence was developed by qualitatively capturing the kinds of proper motion studies of open clusters that the recent GAIA dataset has produced.

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From the opening of the sequence, we experience 15 seconds of motion through a static starfield. The open cluster should be apparent towards the front of the dome because it contains an unusual abundance of stars all clumped around the same distance.

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Our motion through the 3D distribution of stars pauses momentarily to establish a halt to any geometric movement.

00:18

From this stationary position the stars begin moving, but this time following simulated proper motions over millions of years. In this time-based view, the open cluster members stand out from the ambient starfield due to their systematic motion as a group, moving le to right. This illustrates one principal way that open clusters can be identified: by observing stellar proper motions and finding groups with shared vectors.

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At this point the cluster is at the front of the dome. In addition to the overall consistent direction of movement, random components of motion cause the cluster diameter to expand over time, representing increasingly diffused associations.

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Near the end of the sequence the cluster is no longer particularly identifiable as a density of stars in a particular area, but the shared motion of cluster members still allows it to be clearly identified, even among stars at the same distance that do not share the same motion.

Additional References

https://royalsocietypublishing.org/doi/full/10.1098/rsta.2009.0257