The fundamental difference between these two types relates to when and how they were formed. Younger, metal-rich stars, a.k.a. Etc. How the Milky Way … It tickles me that we know the shapes of galaxies hundreds of millions of light years distant with much more specificity and confidence than we know our own Milky Way. Older stars, a.k.a. Young, metal rich stars rotating with the disk of the Galaxy are termed Population I, while old, metal weak stars are termed Population II. The same goes for Andromeda, it has a luminosity similar to the Milky Way with even more stars. Lecture 22: The Milky Way. Population II stars, are poor in metal and are found in the halo and the central bulge. NGC891 . Milky Way as seen by the Cobe satellite in the near-infrared. Population I stars, are found in the disk. Elliptical galaxies have eight subclasses E0-E7, where eccentricity increase in the direction of E0 to E7, and E0 is roughly spherical in shape. In one calculation, the Milky Way has a mass of about 100 billion solar masses, so it is easiest to translate that to 100 billion stars. Population I The disk of the Galaxy is thought to be about 10 Gyr old. What is the difference between Spiral and Elliptical … An elliptical galaxy contains a mixture of Population I and Population II stars within the body. Detailed Shape of the Milky Way-- comparison of various views of the Milky Way with external galaxies suggests strongly that the Milky Way is a spiral galaxy . By the way, this is a very interesting report about important and innovative research. The luminosity of the Milky Way galaxy according to this is $5\times10^{36}$ Watts, but this number suggests that there are about 10 billion stars with Solar luminosities in the Milky Way, which doesn't sound right considering that the Milky Way contains 200-400 billion stars of different luminosities.. Stars observed in galaxies were originally divided into two populations by Walter Baade in the 1940s. Population II stars are older than Population I stars.