History of Astronomy

C

Hans Bethe and Solar Energy

By the mid-1900s, astronomers had finally worked out the source of the energy radiated by the Sun and stars. The Sun produces 3.86 × 10²⁶ watts of power each second, a very large number indeed. Geological evidence shows that simple forms of life have existed on Earth for nearly 4 billion years, indicating that solar energy must have been expended at about its present rate for that length of time.

In 1939 American physicist Hans Bethe advanced the theory that solar energy is produced by the fusion of four hydrogen atoms to form helium. In that process, some mass is converted to energy according to the famous equation E = mc² formulated by Einstein. In this equation, E stands for energy, m for mass, and c for the speed of light. Since the speed of light is a very large number, very little mass is required to keep the Sun shining for billions of years. Building on the work of Bethe, the American astronomer William Fowler, along with British astronomers Sir Fred Hoyle and Geoffrey and Margaret Burbidge, showed in 1957 that the heavy chemical elements, such as carbon, nitrogen, and oxygen, are made in stars as a result of nuclear fusion processes (see Nucleosynthesis). Astronomers thus discovered that all the heavy elements in the universe originated in stars.

Understanding nuclear fusion within stars also enabled astronomers to obtain a better grasp of a star’s evolution. Knowing the mass of a star, astronomers could calculate its stellar lifetime. The Indian American astrophysicist Subrahmanyan Chandrasekhar calculated the amount of mass, known as the Chandrasekhar limit, that would determine a star’s fate. Stars with masses less than 1.4 times the mass of the Sun when fusion ended could complete their evolution as white dwarf stars. More massive stars would implode and end their lives as either neutron stars or black holes. Rapidly spinning neutron stars were later detected by British radio astronomers Jocelyn Bell, who was then a graduate student, and her adviser, Antony Hewish.

IX

The Golden Age of Astronomy

The second half of the 20th century was truly a golden age for astronomy. Rapid advances in technology made it possible to build very large optical telescopes on the ground. By the early 21st century astronomers were using telescopes with mirrors larger than 8 m (300 in) in diameter. Because it is much cheaper to build telescopes on the ground than in space, large ground-based telescopes with their ability to gather large amounts of light (think of a telescope as a bucket for collecting light; the bigger the bucket, the more light collected) are particularly valuable for studying the faintest objects. The most distant objects tend to be very faint, but they are very important for understanding the evolution of the universe. Since light takes a long time to reach us, the universe gives us a kind of time machine so that we can see what it was like when it was much younger than it is now. For the most distant objects observed so far, it took nearly 13 billion years for their light to reach Earth, so we are seeing them as they existed 13 billion years ago.

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Radio astronomy is also best done from the ground. All forms of electromagnetic radiation with wavelengths longer than infrared wavelengths are called radio waves. Radio waves are not sound waves like the ones you hear when you listen to your MP3 player. In fact, we cannot detect them with our senses but must use electronic equipment. In a radio telescope, radio waves are reflected by a metallic surface and brought to a focus. They are then sent to an electronic receiver, where they can be recorded and analyzed. Radio astronomy is especially useful for studying spectral lines produced by cold gas atoms and molecules and also for studying high-energy particles moving rapidly in strong magnetic fields.

A

Radio Astronomy and the Big Bang

Radio astronomy proved to be instrumental in verifying the big bang theory of the origin of the universe. In the 1940s the Russian American theoretical physicist George Gamow proposed that the universe originated in a hot, dense state from which it exploded, setting off the observed expansion of the universe. British astronomer Fred Hoyle dismissed the theory derisively as a “big bang” in contrast to his own theory of a steady-state universe, which assumed that the universe was eternal and unchanging with time. Two of Gamow’s students—Ralph Alpher and Robert Herman—predicted that a relic of this explosive event would take the form of radiation emanating at a uniform temperature from all directions in the sky. In 1965, using a radio telescope, American astrophysicists Arno Penzias and Robert Wilson detected and identified this cosmic background radiation, providing the first observational evidence for the big bang theory.

B

New Windows on the Universe and New Mysteries

The ability to launch spacecraft opened up new windows on the universe. Astronomical objects not only give off radio waves and light of the kind that our eyes are sensitive to. They also emit other forms of energy—electromagnetic radiation—ranging from high-energy gamma rays and X rays, to infrared or heat radiation. Much of this electromagnetic radiation is absorbed by Earth’s atmosphere and does not reach the ground. However, technology again came to the rescue by making it possible to launch telescopes above Earth’s atmosphere to observe these different types of electromagnetic radiation.

During the last quarter of the 20th century, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) launched many spacecraft designed to exploit the advantages of being outside Earth’s atmosphere. Particularly powerful were three great observatories: the Chandra X-ray Observatory, the Spitzer Space Telescope, and the Hubble Space Telescope (HST). Turbulence in the Earth’s atmosphere blurs astronomical images. Because the Hubble Space Telescope is unaffected by this blurring, it can take superbly sharp images and has given astronomers both scientifically important and stunningly beautiful images of planets, star clusters, and galaxies.

The pace of discovery enabled by these new facilities, both in space and on the ground, has been truly remarkable. Astronomers not only know that the expansion of the universe began about 13.7 billion years ago, but they have also learned that the expansion is not occurring at a steady pace but is accelerating (increasing its speed) as the universe ages. Some form of energy is powering this acceleration. Since no physical theory predicted the existence of this form of energy, scientists call it dark energy. There is also dark matter in the universe—dark in the sense that it gives off no electromagnetic radiation but does exert a gravitational force. One of the challenges for astronomers in the 21st century will be to try to determine the properties of both dark matter and dark energy.

Astronomers know that stars are found in giant systems called galaxies, which are held together by gravity. Stars in each galaxy orbit around the center of the galaxy, obeying Newton’s law of gravity. The Milky Way is the galaxy that contains our own sun and solar system. Our sun is, however, only one rather ordinary star among the 100 billion or so stars that make up the Milky Way. And our galaxy is only one of billions of galaxies in the universe.

Astronomers have also verified that black holes exist in large numbers. Predicted by Einstein’s theory of general relativity, a black hole is a region in space where matter is very highly concentrated and the force of gravity is so great that nothing—neither matter nor light—that ventures too close can escape from its gravitational pull. The existence of black holes can be detected by measuring the motions of objects orbiting nearby but just out of reach. Black holes are commonly found at the centers of galaxies and provide the explanation for another curious class of objects discovered in the 1960s—the quasars. Quasars are at the distances of galaxies and produce more energy than typical galaxies in a volume of space no bigger than our own solar system. Astronomers have shown that the engine that powers the quasar is a black hole surrounded by swirling gas heated to a very high temperature as it spirals toward the black hole. Eventually this gas will be swallowed up by the black hole and disappear from view.

We know that the first stars began to form about 13.4 billion years ago and that star formation continues to the present day. Stars form from dense clouds of dust and gas. A region of slightly higher density within a large cloud can begin to attract dust and gas from nearby and eventually collapse to form a star. The nearest stellar nursery is in the direction of the constellation Orion, where there are hundreds of stars (so faint they can be seen only with a telescope) that are no more than a few hundred thousand years old.

Closer to home, NASA has now sent spacecraft to orbit or fly by all of the major planets. The dwarf planet known as Pluto, which was formerly classified as a planet, has not yet been visited by a spacecraft. Pluto was discovered by American astronomer Clyde Tombaugh in 1930. A spacecraft launched in 2006 is expected to rendezvous with Pluto in 2015.

Perhaps most exciting of all, we have discovered that our own solar system is not the only one. Astronomers have found more than 300 planets orbiting other stars. About 10 percent of the nearby stars with compositions like that of our own Sun have at least one planet. The first techniques used to detect such planets meant that massive planets the size of Jupiter or larger were much easier to find than relatively low-mass rocky planets more like Earth in size. Space telescopes such as COROT and Kepler have been specially designed to look for such low-mass planets. Over the next few decades, we should have new technologies that will allow us to learn whether planets suitable for life are common or rare. See also Extrasolar Planets; Planet.

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