An astronomer studying a galaxy finds that its spectrum shows only old, low-mass stars, and photographs of the galaxy show little or no interstellar gas or dust. what kind of galaxy is this astronomer studying?

An elliptical galaxy.

Explanation:

Elliptical galaxies are one of the 3 classes of galaxies existing throughout the universe.

Unlike spiral galaxies (such as our Milky Way), that have a distinctive spiral form, elliptic galaxies don't have any pattern or form visible from a regular telescope. To the eye, they look like a lot of small stars really close to each other, orbiting an even bigger star.

However, they are far more complex than it seems. These galaxies are almost as old as the universe itself. The time for them to form new stars have passed, due to the lack of interstellar gas or dust, which is now depleted.

Due to its age, an elliptical galaxy is home of really old stars, that have been around for billions of years, and remain as a relic of the galaxy's violent past.