The Galactic Neighborhood
The Galactic Neighborhood
Goals: Explore the Milky Way's nearest neighboring galaxies.
Before starting, turn on: galaxy, local
You will be using: cb command
In our first tutorial, you will explore the Galactic neighborhood, discovering what lies outside our Galaxy and how these objects are arranged around us. Let's launch the Extragalactic Atlas and begin exploring.
Starting from Home
Upon launching the Atlas, you are just outside the Milky Way looking back on our home Galaxy. The image you see beside the “Home” label is not our Milky Way but NGC 1232, a galaxy that is believed to resemble the Milky Way.
Barely detectable from this distance is the Point of Interest marker, the multicolored Cartesian axes that mark the location of the Sun and its planets within our Galaxy. Each half-width is 1,500 light-years, about the distance to the Orion Nebula. You may have to fly in closer to see the axes clearly.
Begin by slowly orbiting this point where the Sun is located. You will notice some nearby galaxies with large labels and other, more distant galaxies that appear as green points. Focus on those near the Milky Way. At Partiview's Command Line, type
cb onto temporarily remove the more distant galaxies.
Our Nearest Neighbor
The nearest object to the Milky Way was discovered in 1994. The Sagittarius galaxy is represented by a point, but this belies the nature of this mysterious object.
You may have figured out that the object is called Sagittarius because it is seen in that constellation in the night sky. However, parts of this galaxy have been seen on both sides of the Galactic disk. So representing it with one point is really not too accurate.
The Sagittarius is a dwarf spheroidal galaxy that has been stretched and warped by our Milky Way. A dwarf spheroidal (dSph) galaxy appears as a smudge on the sky in even the largest telescopes. With a low star density, these galaxies often resemble star clusters rather than galaxies. The first dSph, Sculptor, was discovered in 1938 by Harlow Shapley.
Astronomers have come to realize that galaxies interact with one another more often than once thought. The Sagittarius dSph is in fact in the midst of an interaction. As it orbits our Milky Way, our Galaxy slowly rips the Sagittarius dSph apart, stripping away streams of stars on each pass near the Milky Way's disk. The same thing once happened with two other nearby companions, the Large and Small Magellanic Clouds.
The Magellanic Clouds
The Magellanic Clouds (labeled LMC and SMC for Large and Small), are irregular galaxies. They do not fit into the spiral or elliptical classes observed by Hubble and published in The Realm of the Nebulae in 1936. Galaxies that were outside these two common types were called irregular. These are seen in the southern sky and appear as two isolated patches of faint light, similar in appearance to the light in the band of the Milky Way as seen from Earth.
They are actually small galaxies that contain star formation, globular clusters, and the famous supernova of 1987 (in the LMC). These two galaxies are likely in the process of colliding with our Milky Way. Because our Milky Way is far more massive than either of these galaxies, they have been ripped apart by our more dominant Galaxy. Astronomers may even see a trail of gas connecting the two galaxies, but this remains uncertain.
All the galaxies in this clip box are dwarf spheroidals except the Milky Way, which is a large spiral, and the LMC and SMC, which are irregulars. What lies beyond, though?
The Local Group
Pull out farther so that you see the entire clip box, then turn the clip box off by typing
cb offat the Command Line. Turn on another box (the non-clipping variety) by pressing the Box Toggle Button. This box surrounds the galaxies of the Local Group. The Milky Way is at the center of this box, and it now encompasses another grouping of galaxies. If you inspect this group, you will find it surrounds the Andromeda Galaxy (M31).
Similar to the Milky Way, Andromeda commands attention from its smaller, nearby companions. At 2.3 million light-years, Andromeda is the largest galaxy we've seen thus far, just slightly larger than the Milky Way, and is the king of the hill among Local Group galaxies. It is the farthest object we can see with our unaided eye in a dark sky.
Like the Milky Way grouping, most of the galaxies around Andromeda are smaller dwarf spheroidals or dwarf ellipticals. One exception, M33, is the other spiral galaxy in the Local Group.
All the galaxies in this box are part of a group of galaxies that interact with one another under the group's mutual gravity. Many of the nearby galaxies are interacting with the Milky Way, but Andromeda is also on a collision course. In 3 billion to 5 billion years, the two galaxies will collide. By then, our Sun will be in its late stages, expanding its envelope as it evolves into a white dwarf star.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott