In 1609 Galileo began using a new kind of instrument that magnified distant
objects: a telescope. When he trained it on the heavens, he saw countless stars
and other faint objects never before seen. Suddenly, the Universe was no longer
limited to what the naked eye could see. As telescopes improved, astronomers
continued to push back the boundaries of the known Universe, peering ever deeper
into the surrounding sea of stars known as the Milky Way.
By the late 1800s, photography was changing the way astronomers studied the
Universe. The telescope had become a high-powered camera that recorded images
of objects on photographic plates. These images were more accurate than hand
drawings and revealed objects and details far too faint for the eye to detect.
Astronomers would soon discover that the Milky Way was only one of countless
galaxies, each one a vast swirl of stars.
While astronomers were starting to use photography to capture light from celestial
objects, they were also learning how to analyze light itself. They found that
an object's spectrum, the rainbow of colors that forms when light passes through
a prism, could tell them what the object was made of and how it moved. By the
late 1920s the use of spectroscopy, creating and studying spectra, produced
an amazing discovery: the Universe of galaxies was expanding.