Re: What are the effects of the Sagittarius dwarf galaxy on the Milky Way?

Hi Sia,

The quick answer is that a galaxy the size of the Sagitarrius dwarf will have no noticeable affect upon the Milky Way galaxy. As you may know, our Galaxy is of a type called spiral galaxies. These galaxies have a few different components. The most noticeable is the disk, which has spiral arms. There's a pretty good picture of a galaxy with spectacular spiral arms at the Space Telescope site. The spiral arms contain most of the stars of the Milky Way, and all of the young stars. The disk of our Galaxy is what we see as the Milky Way in the sky at night.

Not as easily seen is the halo. It is roughly spherical in shape. It contains many fewer stars than the disk (about 10 times fewer in the case of the Milky Way). The stars are also older and fainter. Also, whereas the disk stars follow almost circular orbits around the galaxy, stars in the halo orbit at random, more like a swarm of angry bees.

Finally, there is the so-called "bulge," at the center of the Galaxy. You can begin to get a hint of the bulge in the Milky Way during the summer, if you look in the direction of constellation of Sagittarius. When you look there, you're looking towards the center of the Galaxy. The stars of the bulge are old, like those of the halo. While they don't move randomly, like halo stars, they also don't orbit in nice circles, like the disk stars.

The first place in which we'd see a change due to an encounter with another galaxy would be in the disk component of our Galaxy. This is because the disk stars orbits are so close to circular. We call such a system "dynamically cold," meaning that there isn't much random motion. Because of this, the disk of the Milky Way is quite thin, less than 3000 light years, as compared to its radius of at least 30,000 light years. An encounter with another galaxy would give the disk stars strong gravitational "kicks," which would add random motions to their orbits. The result would be that our disk would "puff up."

To seriously puff up the disk of the Milky Way, however, would require an encounter with a galaxy of about 10% of the mass of the Milky Way. The mass of the Milky Way is something like 100 billion times the mass of the Sun. The mass of the Sagittarius dwarf is much less, perhaps only one billion times the mass of the Sun. You can find these numbers in technical references. I got mine from "Galactic Astronomy," a graduate level book by Mihalas and Binney, and a technical article about Sagittarius dwarf in "The Astronomical Journal," 1997, vol. 13, page 634.

So, the Sagittarius dwarf will have no noticeable affect upon our own Galaxy. In fact, the current belief is that our Galaxy grew largely by collisions with small systems such as Sagittarius.

There are some references online to Sagittarius. There's a brief one at http:// www.seds.org/messier/more/sagdeg.html. Another is at Johns Hopkins University. You can see it captured "on film" (actually a CCD image) as an Astronomy Picture of the Day. There's a more popular-level discussion at the ABCNews web site.