THE STARS FROM EARTH
Astronomy has developed exponentially since the mid-1800s, when the universe was a fairly small and uncomplicated place, and the view of the cosmos was Sun-centric. The planets were simply little-known cousins of Earth. The stars were thought to be immutable, and galaxies did not exist.
Today, we know the planets as separate worlds, each with a unique geology and meteorology. Each star, too, displays a complex evolutionary history, and galaxies are recognized as giant stellar systems independent of our own.
In the 1860s, the development of spectroscopy—the breaking down of light into its constituent colors—started to expand astronomy from the study of the positions of celestial objects to the study of their physical properties. For the first time, astronomers could extract information from starlight. This allowed them to classify the stars and chart them from birth to death.
Giant reflecting telescopes at California's Mount Wilson and Palomar observatories were constructed in 1917 and 1947, and for decades, they were the largest optical telescopes in the world. Now, new mirror-making technologies are producing grander instruments. Each of the Keck telescopes on Mauna Kea, Hawaii, has a mirror 400 inches (10 m) in diameter.
Just as importantly, photographic cameras have now been largely replaced by electronic detectors called charge-coupled devices. CCDs capture images quickly, and are sensitive over a wide range of visible light.