Our place in the universe
The renowned British astrophysicist Martin Rees looks back on 175 years with many great discoveries in astronomy and physics that changed the way we see the world. He marvels as astronomers have revealed a larger and stranger cosmos that we are far from fully understanding.
The French philosopher Auguste Comte claimed in 1835 that no one would ever know what the stars are made of: "We know how to determine their shapes, their distances, their sizes and their movements," he wrote, " whereas we shall never be able to study by any means their chemical composition or their mineralogical structure, much less the nature of organized beings that might live on their surface". Comte would be amazed at the discoveries that have been made since then. We now know that the universe is much larger and stranger than previously thought. Not only is it extending beyond the Milky Way to countless other galaxies-that alone would surprise 19th- and early 20th-century astronomers, who saw our galaxy as “the universe”-but it's also expanding faster every day. Now we can confidently trace cosmic history back 13.8 billion years to a moment just a billionth of a second after the Big Bang. Astronomers have pinned the expansion rate of our Universe, the mean density of its major constituents, and other key figures to within a percent or two. They have also devised new physical laws that govern space, general relativity and quantum mechanics, which are proving to be far stranger than the classical laws previously understood by humans. These laws, in turn, predicted cosmic oddities like black holes, neutron stars, and gravitational waves. The story of how we arrived at this knowledge is full of accidental discoveries, amazing surprises, and dogged scientists pursuing goals others thought unattainable.
Our first clue to the true nature of stars came in 1860, when Gustav Kirchhoff realized that the dark lines in the spectrum of light coming from the Sun are caused by different elements absorbing certain wavelengths. Astronomers analyzed similar features in the light of other bright stars and discovered that they were composed of the same materials found on Earth - and not some mysterious "fifth essence" as the ancients had believed.