Two-Dimensional Distributions

Goals: View various data sets from Earth's perspective to see what they reveal about the Galaxy.

Before starting, turn on: stars, mwVis, gc

You will be using: oc, ob, pul, pn, h2, snr

You've seen the distribution of the globular clusters in “The Milky Way from Earth,” now let's investigate other data sets for similar trends.

Open Clusters Are Found in the Band of the Milky Way

With the globular clusters on (group gc), turn on the open clusters (oc). Open clusters are less dense, younger star clusters. In the Atlas, they are represented by green points. By turning off the stars, you have a clear view of the two types of clusters. You can make things even clearer by turning off the clusters' labels. Make oc the active data group and press the Label Toggle Button. Once you've turned off the labels for both data groups, it becomes apparent that the open clusters are concentrated in the Galactic disk, while the globulars are found mainly in one part of the sky.

This concentration of open clusters in the disk led astronomers to call them Galactic clusters. Since gaining a wider perspective on our Galaxy and the surrounding Universe, astronomers have since turned to calling them open clusters, a name that refers to the more spread-out nature of the stars within them.

Open Clusters Are Younger Than Globular Clusters

This spatial distinction between open and globular clusters reveals a story of Galactic structure and evolution. Open clusters are filled with younger stars and, therefore, are found in regions of the Galaxy where star formation occurs. Globular clusters are filled with older stars that formed very soon after the Galaxy was born.

In our Galaxy, star formation occurs in the spiral arms within the disk. The gas in the disk condenses to form stars, some of which are massive enough to explode in supernovae, thereby triggering new stars to form by compressing the surrounding gas. The older stellar population is found in the spherical component of the Galaxy, often called the halo. Most of the stars in the halo are cool, dim stars that are spherically distributed around the Galactic center.

What Do Other Data Sets Tell Us About the Galaxy?

Let's turn on another data set that shows the locations of bunches of stars called OB associations. These contain from tens to hundreds of stars and, as you can see, lie in the plane of the Galaxy. Again, these are filled with younger stars, as the associations last only a few tens of millions of years before drifting apart into individual stars.

Turn on the rest of the 3-D data groups: pulsars, planetary nebulae, HII regions, and supernova remnants. Aside from the HII regions, which signal stellar birth, the rest of these signify stellar death. HII regions, in a sense, are at the beginning of the stellar life cycle. These give birth to open clusters and OB associations, which eventually die to become planetary nebulae, pulsars, or supernova remnants. Because these are connected to the stellar life cycle, you would expect them to lie in or near the Galactic plane. For the most part, they do. You may see some pulsars or planetary nebulae up toward the Galactic poles [turn on the Galactic coordinates to see], but these are typically nearby, just above or below us in the Galactic disk.

As you scan the sky, you now see the clutter that is our Galaxy. The disk appears to be a disordered mess. This view is akin to looking at a map with too much information. One would never look at a map that contained every town and village. Cartographers must choose which towns to display and typically choose population as a filter. But they also base it on the scale of the map. In the Digital Universe, this is equivalent to changing the field of view. Center the disk of the Galaxy in the Graphics Window and use the FOV Slider to “zoom in” so that the map becomes clear. Some of the polygons may be too large, but you get the idea. By decreasing your field of view, you effectively increase your resolution, clarifying the view.

You have explored the Atlas from Earth's point of view. Now it's time to leave our home planet and fly into the solar neighborhood.

Digital Universe Atlas

Digital Universe Guide

Two-Dimensional Distributions