Where exactly is the edge of the Milky Way? That question is harder to answer than one might expect. Since we’re inside of the galaxy itself, it’s obviously hard to judge the “edge” to begin with. But it gets even more complicated when defining what the edge even is — the galaxy simply gets less dense the farther away from the center it goes. A new paper by researchers originally at the University of Malta thinks they have an answer though. The “edge” can be defined as the star-forming region, and in their paper, published in Astronomy & Astrophysics, they very clearly show that “edge” to be between 11.28 and 12.15 kiloparsecs (or about 40,000 light years) from the center.
Even finding that edge was no easy task, though. The researchers had to analyze the ages of over 100,000 giant stars from the data of several different surveys, including APOGEE-DR17, LAMOST-DR3 and Gaia. In the data they found an interesting story about the evolution of the position of stars in the galaxy, and their age.
That relationship can be thought of as a U curve. In this case, the Y axis is age, and the X axis is the distance from the galaxy’s center. A picture (or graph in this case) is worth a thousand words, but in words that simply means that stars closer to the center of the galaxy are older, and get progressively younger out to a certain point, and then start getting older again. That “certain point”, according to the authors, is the end of the galaxy’s star-forming region, and hence, the “edge” of the galaxy.
