Sometimes it’s amazing how much settled science, even something as longstanding as the size of our galaxy, can get called into question: The Milky Way May be 50 Percent Bigger Than Thought : Discovery News.
A ring-like filament of stars wrapping around the Milky Way may actually belong to the galaxy itself, rippling above and below the relatively flat galactic plane. If so, that would expand the size of the known galaxy by 50 percent and raise intriguing questions about what caused the waves of stars.
…Evidence that the so-called Monoceros Ring, located more than 65,000 light-years from the center of the galaxy, actually is part of the Milky Way surprised Newberg, who was on a team that discovered the ring in 2002.
I have to admit that I hadn’t really heard of the Monoceros Ring, or if I had, it hadn’t stuck in my consciousness. Reading about it now, it appears to be a controversial ring of stars either surrounding our galaxy, or on the outer edges of it. The controversy involves whether the stars were gravitationally torn from another galaxy that passed by, or are stars from our galaxy separated from us in a wave like configuration that may have been caused by the gravity of another dwarf galaxy passing through.
The debate about whether or not it is part of the Milky Way, in some ways, reminds me of the debate about whether or not Pluto is a planet. It should be remembered that our galaxy is as big as it is because, over the history of the universe, it has merged with a lots of other galaxies. Galaxies aren’t static things, but are constantly evolving over billions of years.
When I was a kid, everyone thought that the space between galaxies was basically an empty void, but we now know that it’s thinly filled with stars and gases. In addition, there are lots of dwarf galaxies in orbit of ours, which it seems we’re more likely to call star clusters as they get closer to the main galaxy’s halo. The edge of the galaxy is much hazier than it used to be, so exactly where we draw the line between the inside or outside of it seems a much more arbitrary matter than it used to be.