Beyond the Milky Way
Beyond the Milky Way
Goals: Investigate what lies immediately outside the Galaxy.
Before starting, turn on: halo, galaxy
You will be using: lgrp, pul, clip and labelsize commands
Beyond our Galaxy is extragalactic space, the remainder of the observable Universe. We will cover this in the next chapter on the Extragalactic Atlas. For now, though, our last step in the Milky Way Atlas will be the Galactic neighborhood. What lies just beyond our Galaxy?
We Are a Member of the Local Group of Galaxies
Astronomers call the grouping of galaxies around the Milky Way the Local Group. Turn on the Local Group to see these galaxies. You will notice a single point for the Milky Way now. Pull away so that you see the Galaxy and its halo. Most of the nearby galaxies share their names with the constellations: Sagittarius, Ursa Major, and Draco, to name a few. Two notable exceptions are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), named for Ferdinand Magellan, the Portuguese explorer of the 16th century who first spotted these clouds while exploring the Southern Hemisphere. If you turn on the pulsars, you'll see they extend beyond the Galaxy to these two satellite galaxies.
These are called satellite galaxies because astronomers believe that they have been captured by the Milky Way and are now interacting with it. It is likely that the Milky Way collided with these small galaxies and decimated them, leaving an unstructured cloud of stars, gas, and dust.
Our nearest neighbors, the Sagittarius elliptical dwarf galaxy (SgrDEG) and the Canis Major dwarf galaxy (CMa dw), were discovered in 1994 and 2003, respectively. Both of these small galaxies are being stretched by the tidal forces of the Milky Way, ripped apart like the Magellanic Clouds as they orbit the Galaxy. Because they are both near the plane of our Galaxy, they remained elusive until astronomers began observing the galaxy with infrared telescopes.
Let's do some quick Partiview maintenance. Once again, you are pushing the limits of your graphics card and need to change your clipping planes. Type clip and Partiview should report that the near and far clipping planes are 0.1 and 1 million parsecs. We no longer need to see detail, so change the clipping planes using the command
clip 1 1e8(Again, different graphics support will react differently, and you may need to adjust these values to suit your hardware.)
Now fly farther from the Milky Way. If you have the halo on, the sphere will reduce to a small circle on your screen. Using the Slum Slider, brighten the points for the Local Group. As you orbit from this distance, you may notice two main groupings of galaxies. One group is huddled around the Milky Way, while the other is around the Andromeda Galaxy (M31). If you try increasing the labels with the Labelsize Slider, you will see that the slider is at its maximum point. Increase the label size by hand using the command
lsize *10This will increase the size of the labels by a factor of 10. The Andromeda Galaxy is the farthest object you can see with your unaided eye. At 2.36 million light-years away, if you gaze upon it in the sky tonight, you're seeing the galaxy as it looked 2.36 million years ago.
The three large galaxies in the Local Group are Andromeda, the Milky Way, and M33. All three are spiral galaxies and, Andromeda and the Milky Way in particular, have great influence on the galaxies around them. Each lies at the center of a cluster of mostly small dwarf galaxies.
There is no conclusive definition for the Local Group. You might imagine it is defined by some arbitrary distance, inside of which all galaxies are members. Or perhaps it is defined by those galaxies that share a mutual gravitation with one another. In reality, both of these criteria are used to define the size of the Local Group.
We will explore this topic more in the Extragalactic Atlas, where we discuss the local structure of the Universe and travel out to the edge of the observable Universe. Following this section are the detailed sections on each of the data groups in the Milky Way Atlas. You may choose to skip this section and continue your tutorials in the Extragalactic Atlas, but we recommend coming back to the data sections, as they contain a great deal of information that we were not able to include in the tutorials.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott