Most of the stars in the Milky Way lie in approximately the same plane, meaning that our galaxy is basically a flat disc, 100,000 light years across.
Astronomers have investigated a small population of stars in the halo of the Milky Way Galaxy, finding its chemical composition to closely match that of the Galactic disk. This similarity provides compelling evidence that these stars have originated from within the disc, rather than from merged dwarf galaxies. The reason for this stellar migration is thought to be theoretically proposed oscillations of the Milky Way disc as a whole, induced by the tidal interaction of the Milky Way with a passing massive satellite galaxy.
If anyone from outer space would like to contact you via “space mail”, your cosmic address would include several more lines including “Earth”, “Solar System”, “Orion Spiral Arm” and “Milky Way Galaxy”. This position within our home galaxy gives us a front row seat to explore what is happening in such a galaxy.
However, our internal perspective presents some challenges in our quest to understand it – for example for outlining its shape and extent. And yet another problem is time: How can we interpret galactic evolution if our own life span (and that of our telescopes) is far less than the blink of the cosmic eye?
Today, we have a fairly clear picture of the broad properties of the Milky Way and how it fits among other galaxies in the Universe. Astronomers classify it as a rather average, large spiral galaxy with the majority of its stars circling its center within a disk, and a dusting of stars beyond that orbiting in the Galactic halo.