The comet-shaped Smith Cloud contains no stars and so it's not visible in optical wavelengths. But, if it were, it would span an area of night sky as large as the constellation Orion.
When the high-velocity cloud does impact the disc of the galaxy - in about another 30 million years or so - it will be on a different (albeit neighboring) arm than the one in which our solar system resides.
The collision will likely ignite an explosion of star formation around the impact site, perhaps with enough matter and energy to create two million new solar-mass stars.
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"Our galaxy is recycling its gas through clouds, the Smith Cloud being one example, and will form stars in different places than before," said Fox. "Hubble's measurements of the Smith Cloud are helping us to visualize how active the disks of galaxies are."
What actually caused the ejection of the cloud is not yet known. One intriguing suggestion is that it's a region of dark matter that collided with the Milky Way and somehow captured a clump of gas as it went.
"There are theoretical calculations suggesting that a dark matter satellite could capture gas as is passes through the Milky Way disk and that may be the amazing circumstance we are witnessing," said research co-author and long-time Smith Cloud fan Jay Lockman, an astronomer with the National Radio Astronomy Observatory (NRAO).
The findings were published in the Jan. 1, 2016 issue of The Astrophysical Journal Letters.
Sources: NASA and NRAO