~100 Million

Sagittarius Dwarf Galaxy Collision

Planets can have satellites and asteroids can have satellites; it turns out that even galaxies can have satellites. Our own Milky Way galaxy probably has more than 20 satellite galaxies, including the Large and Small Magellanic Clouds. These dwarf galaxy companions are all smaller than our galaxy, and take hundreds of millions to billions of years to orbit the Milky Way. And it appears that at least one of them, known as the Sagittarius Dwarf Elliptical Galaxy, has collided with the Milky Way in the past and is on a course to slam into our galaxy again in about 100 million years.

The Sagittarius Dwarf Galaxy wasn’t discovered until 1994 because much of it is hidden by the central bulge and disk of the Milky Way. It is made up of four main globular clusters and a bright arc of stars that makes a partial loop around the poles of the Milky Way. Astronomers believe that the loop outlines the path that the dwarf galaxy has taken through the plane of the Milky Way in the past, losing some mass and smearing out a bit more during each orbit. Eventually, after more passes through the plane of our galaxy, the stars in the Sagittarius Dwarf Galaxy will merge into and expand the size and mass of our already much larger Milky Way. These kinds of galactic collisions, mergers, and “cannibalistic” processes may in fact be the way that large spiral galaxies can grow so large—by consuming smaller, more ancient galaxies or galaxy clusters.

Some scientists believe that there is a connection between close galactic encounters among the Milky Way and its satellite galaxies and major extinction events or large-scale climate changes (for example, glaciations) on Earth. The idea is that these close encounters stir up distant comets and asteroids in the Oort Clouds around our Sun and nearby stars, causing more comets and asteroids to periodically hurtle inward and collide with the Earth, with potentially dramatic effects. The hypothesis is controversial, however, and difficult to test except with high-speed supercomputers. Still, the idea that not just passing stars but passing galaxies could have a profound effect on life on our planet is interesting because a detailed understanding of large-scale changes in climate and the fossil record could help reveal the past history of local galactic collisions. However, it is also humbling, because it makes us realize how lucky we are to be here at all.

2004 Hubble Space Telescope photograph of the colliding spiral galaxies NGC 2207 (top) and IC 2163 (bottom). The enormous amount of mass involved in galactic collisions generates enormous tidal forces; in this collision, the smaller galaxy is being stretched apart by the larger one.