How big is the Milky Way?

We thought that our Milky Way Galaxy was more than 100,000 light-years across. We were wrong.

The analysis of 100,000 larger stars of our galaxy revealed that the youngest population of stars, on average, is in the middle of the flat spiral disk. They get older towards the middle, where the central bulge is located, and older towards the outer edge.

New stars only form up to around 40,000 light-years from the center, making our Milky Way Galaxy around 80,000 light-years across

[1]

. Beyond this boundary, only old stars exist that didn’t form there but migrated over billions of years after forming closer to the center. Therefore, only very old stars exist beyond that point.

The inner flat disk is a bit older than the center of the spiral disk because it started forming stars first, when our galaxy was young.

Our Sun is also part of this huge star migration. It didn’t form 26,000 light-years from the center where it is now located. Its composition indicates it formed either in a Wolf-Rayet nebula or in nebulae enriched by winds from such stars. They are more common near the galactic bulge. Therefore, in the 4.5 billion years since our system formed, our Sun may have moved up to 10,000 light-years.

It might have moved because the central bulge of the Milky Way flattened as it became more bar-like, making our galaxy a barred spiral galaxy. Stars additionally migrate outward due to radial migration of galactic dynamics.

Alternatively, there is a theory that star formation drops at around 40,000 light-years because our galaxy is warped at that distance, probably due to interactions with the Dwarf Sagitarius Galaxy, which, on its vertical orbit, pierces the disk of our galaxy and emerges on the other side. This warp rotates roughly every 700 million years, which is almost three times as slowly as one full orbit of the Sun around the center, 230 million years.

The size of the Milky Way spiral disk doesn’t, for now, change the estimates of our galaxy’s star count, and it’s still 200 to 400 billion stars. We also still think that our galaxy is more massive than Andromeda, even though Andromeda contains 400 to 800 billion stars. This is because the Milky Way contains more dark matter.