VII.

Toward Modern Astronomy

The telescopes used by Galileo were made with lenses that typically were only about 2.5 cm (1 in) in diameter. Over the next 400 years, developments in technology made it possible to build ever larger telescopes with greater light-gathering power to detect ever fainter objects. Mirrors replaced lenses as the main optical elements in telescopes. The largest single telescopes in the world today, the twin Keck telescopes at the Mauna Kea Observatory in Hawaii, are each 10 m (400 in) in diameter, and astronomers are developing plans to build telescopes that are 3 to 5 times larger still.

Discoveries with telescopes from the 1600s through the 1800s laid the basis for modern astronomy. Many new members of the solar system were identified, including the planet Uranus in 1781 by the British astronomer Sir William Herschel and the planet Neptune in 1846, which was discovered independently by the British astronomer John Couch Adams and the French astronomer Urbain Jean Joseph Leverrier. Using telescopes astronomers also discovered the first asteroids between the orbits of Mars and Jupiter. Newton’s colleague Edmond Halley used the new theory of gravity to calculate the orbits of comets. Based on his calculations, he noted that bright comets observed in 1531, 1607, and 1682 might well be the same comet, reaching the point in its orbit closest to the Sun every 76 years. He predicted that this comet would return in about 1758. Although Halley had died by 1758, when the comet did indeed appear as he had predicted it was given the name Halley’s Comet.

Telescopic studies of double stars, also known as binary star systems, provided evidence that gravity applies outside the solar system. The two members of a double star system follow elliptical orbits around their common center of gravity, just as the planets orbit the Sun. This proof that the law of gravity is truly universal meant that the same physical processes that we can study here on Earth can be applied to studies of distant objects, including stars.

The distances to stars were first measured in 1838. In this year, three astronomers reported distances for three different stars—61 Cygni, Alpha Centauri, and Vega. The distances were calculated from measurements of the very slight shift in position of these nearby stars relative to much more distant background stars when viewed from opposite sides of Earth’s orbit. This is the calculation that the Greeks tried to perform in order to test whether the Earth orbits the Sun. The Greeks failed because the shift in position, which is called parallax, is only about 1.5 seconds of arc for even the nearest bright star. This degree of separation is about equal to the apparent size of a quarter when viewed from a distance of 2.3 km (1.4 mi). It was much too small to be measured with the techniques available to the Greeks.

The nearest of the first three stars measured, Alpha Centauri, is at a distance of about 42 trillion km (26 trillion mi). Obviously astronomers needed a new unit to measure such large distances, and one that eventually became widely used is the light-year. One light-year is equal to the distance that light travels in one year at the speed of light, which is about 300,000 km/sec (186,000 mi/sec). So one light-year equals 9.5 trillion km (5.9 trillion mi). The distance to Alpha Centauri from Earth is about 4.4 light-years.

In the mid-1800s astronomers also obtained information about what stars are made of. They used a technique called spectroscopy. When the light from a star is spread out into its rainbow of colors and passed through an instrument known as a spectroscope, some of the colors are found to be missing. These missing colors are referred to as dark lines. Laboratory experiments showed that the pattern of dark lines can be used to identify what hot gases—hydrogen, helium, even iron—are present in the star. Each element produces its own unique pattern.

In 1864 British astronomer Sir William Huggins was the first to show that the pattern of dark lines in the spectrum of a star matched the patterns produced by elements known here on Earth. Huggins’s discovery was another important example showing that the physical processes that we study here on Earth can be used to study the whole universe. Spectroscopy also provides information about the temperatures of stars, their masses, and their motions in space.