The Milky Way does not travel through the universe alone. It is accompanied on its journey by smaller galaxies. The two largest are the Small Magellanic Cloud and the Large Magellanic Cloud, which are both visible as dusty twin smudges in the southern hemisphere.
When the Magellanic Clouds started orbiting the Milky Way billions of years ago (astronomers are not certain about the timing and the gravitational bond, it’s possible that they are still on their first approach to the Milky Way), an enormous stream of gas known as the Magellanic Stream was ripped out of them. The stream now extends across more than half the night sky. Astronomers, however, cannot explain how the stream became so massive that it contains a billion solar masses, which corresponds to one-tenth of the mass of the LMC.
Astronomers from the University of Wisconsin-Madison and their colleagues have now discovered that a halo made up of warm gas surrounding the Magellanic Clouds is probably acting something like a protective cocoon that shields the dwarf galaxies from the Milky Way’s own halo and accounts for the majority of the mass of the Magellanic Stream. When the smaller galaxies came into an area influenced by the Milky Way, parts of this halo were stretched out and scattered and formed the Magellanic Stream. The researchers published their results on September 9, 2020 in the journal, Nature.