There’s a surprising lack of young stars in the center of our galaxy.
Standard Variables
In the late 1800s Henrietta Leavitt was hired by Edward Pickering of the Harvard College Observatory. “Hired” in this case being a loose term, since Leavitt was not initially paid for her work. She was assigned the task of cataloging the brightness of variable stars from photographic plates. This is a tedious process, which is why it was done by women (known as Pickering’s Harem).
The Far Side
While we’re quite familiar with our side of the Milky Way galaxy, the far side of our galaxy is still a bit of a mystery. The reason for this is that the center of the Milky Way is filled with gas, dust and stars, so it is very difficult to see the other side of our galaxy. The central region is so cluttered with material that it sometimes referred to as the Zone of Avoidance, since we have to exclude that region from observations beyond our galaxy.
Cepheid Variables
There are stars known as Cepheid variables that change in brightness over time. The rate at which their brightness varies is proportional to their brightness, which allows us to determine the distance to galaxies millions of light years away.