Now that you understand the overall structure of the Universe, and how galaxies interact to produce dramatic structures, let's explore the structure of the Local Group. The Local Group is a sparse group of more than forty galaxies that significantly influence each other through their mutual gravitational pull, and which in turn is "falling" towards the vastly more massive Virgo cluster.

A very striking feature of the Local Group is how difficult it is to work out how many members it has, and to learn in detail about each one. Why?

We have constructed a 3D model of the Local Group using the best data available to date -- but keep in mind that the distances to many objects are very uncertain, and so the relations between objects depicted here are tentative. The plots below show the whole data set, and zoomed-in views of the vicinities local to the Milky Way and M31 (the Andromeda Galaxy):

Each point is a galaxy. Are they randomly distributed, or do you see some structure? Galaxies are color-coded as follows: spirals (blue), ellipticals (red), irregulars (green), dwarfs (cyan). All galaxies are shown as simple spheres (although the sizes are about right) except the two massive spirals in the Local Group -- The Milky Way and M 31. Why are there two distinct regions in the Local Group? Notice that, in the Milky Way region, most of the galaxies are dwarfs, but that two spirals (blue) lie close to us. These are the Large and Small Magellanic Clouds, often classified not as spirals but as irregulars. Check out these galaxies at the SEDS Local Group page.

Zooming further in on the Milky Way region eventually shows a dark point on the disk:

it's the Sun -- about 25,000 light years from the Galactic Center, and slightly out of our Galaxy's central plane. Slowly zoom back out. The galaxy on the far side of our disk is the Sagittarius dwarf. Why was that so difficult to discover and observe? Check out this galaxy at the SEDS Local Group page. 

You can hardly fail to notice a stream of gas -- represented by a scattering of blue points -- associated with the LMC and SMC (see Ex. 1.4). This is the Magellanic Stream. In fact, we have taken the liberty of putting it in a certain position -- we don't have a very good idea of where it actually is because we don't know the distance to it. We think there was a close encounter 500 million years ago that pulled gas into a bridge between the LMC and SMC. If the LMC and SMC orbit the Milky Way in a vacuum, will the gas stay in the bridge? How can you explain the trail? The stream ends far from the LMC and SMC. How might that position relate to where the close encounter occurred? Will the gas in the stream continue to move forward at the orbital speed of the LMC and SMC? If not, will the position of the stream change? 

Refer to the SEDS Local Group page for information about the different members of the Local Group. Does anything strike you about the dynamics of the group from that exploration? All these galaxies pull on each other, so you would not expect a static distribution. The Department of Physics at the University of Oregon maintains a neat Physics Applets page. Of particular interest here is the Galaxy Crash simulation. Take a few minutes an explore what might happen when galaxies collide (once the pages loads, click on the Reset button to load the data points).