Open Star Clusters

Goals: Explore the 3-D distribution of the open star clusters and contrast them with the OB associations.

Before starting, turn on: stars, oc

You will be using: ob, galaxy, galGrid, galac, see command

If you're coming directly from the last tour, turn off the OB associations and turn on the open clusters. If you're starting the Milky Way Atlas fresh, turn on the 10,000-light-year grid and fly away from the Sun until you can see the entire grid.

The open clusters are represented by green points. From out here, we can see the entire data set, but let's quickly fly in toward the Sun (at the center of the grid) to familiarize ourselves with these data. As you approach the Sun, stop and orbit. The Hyades should be visible, as should a few other nearby clusters. Let's fly over to the Hyades.

Once you reach the Hyades, turn off the open cluster's polygons so that you can see the stars in the cluster. This is the best example of an open cluster in the stellar data set; it is a spherically distributed grouping of stars. Turn on the boxes for the stars group to emphasize the cluster.

Open Clusters Are Relatively Young Objects

Now pull back out and turn on the open clusters again. Once you've reached the point where you see most of the data, stop there and orbit. Again, you will notice a concentration toward the disk of the Galaxy. From our discussion in “OB Associations,” you may deduce that these are younger objects in the Galaxy. In fact, they range in age from those that are currently forming to the oldest known cluster, Berkeley 17, which is believed to be more than 12 billion years old. However, this is an anomaly among clusters, the majority of which are young.

The Youngest Clusters Lie in Spiral Arms

You might expect the younger clusters to trace out the present structure of the Galaxy and its spiral arms. Let's see if this is true. In Partiview, run the command

see young

This sets a threshold on the clusters such that all clusters more than 10 million years old will be removed from view. Now you see something that resembles arms. Turn on the OB associations to see if they correlate. Flash them on and off to see the similarities. You can see that they line up very well, confirming the placement of the local spiral arms.

You may return the full data set to view using

see all

Let's also gain a wider perspective by turning on the Milky Way image and the Galaxy coordinate grid (turn off 10kly for clarity). Viewing the Galaxy from afar, turn off the open clusters and turn on the Galactic coordinates to mark the location of the Sun (which is at the center of that 1,000-light-year sphere). Now you're ready to add other pieces to the Galaxy. Let's see what they reveal about the Milky Way.

Digital Universe Atlas

Digital Universe Guide

Open Star Clusters