The 3-d Sky: Galaxies in VRML

We now have a database of galaxies taken from the Catalogue of Principal Galaxies, a catalog of more than 70 000 galaxies.

The data from the catalog give the galaxy's position in the sky and its velocity toward or away from the observer. This velocity is mostly due to the expansion of the universe. If this were the only cause of the expansion, the distance to a galaxy would be merely the velocity at which a galaxy moves away from us divided by a constant called the Hubble constant. Since this determines the scale of the model, a particular value is unnecessary. The Hubble constant is between 50 and 100 kilometers per second per megaparsec. This value should become better known as the Hubble Space Telescope makes more measurements.

However, things are not so simple. Galaxies have gravitational fields. If a region is denser than the surrounding area, galaxies should fall into it. Eventually, the dense region will become a cluster of galaxies. In a cluster of galaxies, the central galaxies are on orbits around the center with periods of around a billion years and galaxies farther out may be falling in. In our part of the universe, there is a region known as the Virgo cluster which contains many galaxies.

The Virgo cluster contains hundreds of galaxies within a few degrees of its center. The velocity at which these galaxies are moving away from us can be determined using the Doppler effect. On average, the galaxies are moving away at about 1100 kilometers per second. However, there is a large range of values because of the orbital motion. Some of the galaxies are even moving toward us. In our model, the distances from the Milky Way to the galaxies are based on their Doppler shift. This means that we see the Virgo cluster as a line of galaxies pointing at the Milky Way. Astronomers who analyze these models call these lines the "finger of God".

When you look at the model, our position is at the end of this "finger of God". It is indicated by a yellow sphere. Other galaxies are indicated by white spheres.

To avoid confusion, galaxies approaching the Milky Way are not plotted. There are about thirty of these known. Most of them are in the core of the Virgo cluster.

On some models, there will be other groups in view. Most of these are small groups which show less scatter in the radial velocities of their galaxies. The Milky Way is in one of these. This "Local Group" of galaxies is still contracting. The Milky Way is approaching the other large member of this group, the Andromeda Galaxy, at about 50 kilometers per second. The two galaxies will collide in about 10 billion years.

You may also notice that many of the galaxies lie in a sheet known as the Supergalactic Plane.

In the set of 19 990 galaxies in the catalog which have measured radial velocities, there are many clusters of galaxies like the Virgo cluster. The background for these pages is the Coma cluster of galaxies, which has an average radial velocity of about 7000 kilometers per second. Its mass is so large that the scatter in the radial velocities is about 2500 kilometers per second on each side of the mean.

If you've got VRML, you can fly around in one of our models. We reccomend starting with a small model and working up to bigger ones. If you don't have VRML, look at this screenshot of the Virgo cluster.

100 galaxies VRML 1
500 galaxies VRML 1
2500 galaxies VRML 1

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