of liquid nitrogen as well as Geiger counters for civil defense against the possibility of nuclear attack from the Soviet Union.

But somehow, the cows were not as fascinating to me as the mysteries of the sky. When I was around 8, my parents took me and my sister to the American Museum of Natural History in New York City, and we saw the planetarium show and the dinosaur and fish bones. My parents also read aloud to me and my sister from biographies of Darwin and Galileo. Quite an introduction to science, which looked very important and a bit dangerous!

Jumping ahead many decades, astronomers now have a coherent story to tell about the origin of today’s universe. We say there was a Big Bang 13.7 billion years ago that started everything, we have a lot of mathematics to describe how it worked, and we have elaborate computer simulations of how the primordial material would grow into the things we see today. But until recently, when the COBE satellite flew, we did not know the details of the starting point, so we did not know what computer simulations to run. The scientific impact of the COBE was to provide that starting point.

Surprise! Explosions in the Bathroom Mirror

One of the great challenges of modern science has been to work out the origins of the chemical elements. When you look in the mirror in the morning, thinking of hair and whiskers and the day ahead, you are looking at the remains of exploded stars. The Big Bang gave us only hydrogen and helium and tiny traces of lithium, and everything else has been made since then by nuclear reactions inside stars. The basic idea was explained by Fred Hoyle in 1946, and developed in great detail over the years. However, much is still unknown about this, since the details seem related to the nuclear reactions that take place during the final explosions of supernovas. Those are very difficult to calculate because the three-dimensional structure of the explosion is highly turbulent.

Looking Back in Time

Astronomers look back in time in a way that is not open to anybody else. We see things as they were when they emitted light, and that can be a long time ago if we are looking at things very far away. The speed of light, immense though it is from a human perspective, is still finite. We see the nearest star as it was 4 years ago, the center of our galaxy as it was 25,000 years ago, and if we look almost to the edge of the visible universe, we look back almost 13.7 billion years. Geologists look at old rocks, historians look at old documents, but astronomers really travel back in time with their telescopes.

Measuring Distances

Astronomers naturally need to know how far away things are, and we have two basic methods (see Figure 1.3). First, we draw triangles, just as the ancient Egyptians did. Given one side and two angles of a triangle, we can compute the other parts. The ancient Greeks, at least some of them, knew how to apply this to get the size of Earth and a rough distance to the Moon, but everything else was too far away for them to calculate. The other basic method astronomers use is the standard candle method: if two objects are known to have the same intrinsic brightness, then the fainter one is farther away, in accordance with the inverse square law. (In the expanding universe, this law has to be modified a bit.)

Measuring Velocities

Of course, we also need to know how fast things are moving. The Sun, the Moon, and the planets move

FIGURE 1.2 Looking back in time by looking at things far away.

FIGURE 1.2 Looking back in time by looking at things far away.

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