Sun

VII

History of Studying the Sun

The Sun is so important to life on Earth that humans have always paid special attention to it. The movement of the Sun across the sky helps mark time. The change throughout the year in the Sun’s daily path helps mark the seasons. Many cultures attach special significance to solar events, such as eclipses. The brightness of the Sun made studying it closely difficult for humans for many years. Looking at the Sun directly is dangerous, and even thick clouds do little to protect human eyes from the damage that direct sunlight causes. Astronomers could not make true scientific studies of the Sun until they developed techniques to observe the Sun indirectly.

The study of the Sun has both pushed and been pushed by revolutionary scientific discoveries. Early indirect observations of the Sun, using a telescope, allowed scientific study of the Sun to begin, showing that the Sun is a dynamic, changing body. The development of spectroscopy and the discovery of elementary particles and nuclear fusion allowed scientists to begin to understand the composition of the Sun and the processes that fuel it. The development of artificial satellites and other spacecraft finally allowed scientists to study the Sun from space, allowing a full view of all of the Sun’s radiation and a continuous study of the Sun.

A

Beginning of Scientific Study

Greek philosopher Aristotle was the first known person to use a device that allowed indirect observation of the Sun. Sometime between 384 bc and 322 bc Aristotle noticed that a hole in a screen would create an image of the Sun on the ground, if the screen were between the Sun and the ground. He made a simple version of a device called a camera obscura to take advantage of this effect. A camera obscura is still a popular way to observe solar eclipses.

Italian scientist Galileo observed the Sun with a telescope for the first time in 1610. Looking through a telescope directly at the Sun is even more dangerous than looking at the Sun with the naked eye, so Galileo turned the telescope into a camera obscura. He pointed it at the Sun and then set up a screen behind the eyepiece. The eyepiece projected the image of the Sun onto the screen. Galileo observed sunspots with his telescope. He saw that sunspots rotate with the Sun and change in size and shape. Galileo’s work showed that the Sun is a changing and active body.

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B

Spectroscopy

The next major breakthrough in the study of the Sun was the development of ways to study sunlight. In the mid-17th century English scientist Isaac Newton used a prism—a specially cut chunk of glass—to break sunlight down into its different colors. This range of colors is called the Sun’s spectrum, and the study of spectra is called spectroscopy. In 1802 British scientist William Wollaston found that the solar spectrum was cut by several dark gaps. By 1815 German physicist Joseph von Fraunhofer had cataloged the wavelengths of more than 300 of the gaps, called absorption lines. Fraunhofer assigned letters to the most prominent absorption lines. In the mid-19th century German scientists Gustav Kirchhoff and Robert Bunsen related the absorption lines in the Sun’s spectrum to chemical elements. In 1925 English-born American astronomer Cecilia Payne (later Cecilia Payne-Gaposchkin) compared the spectrum of the Sun to that of other stars to show that virtually all bright, middle-aged stars have the same composition.

The spectrum of the Sun’s corona was studied for the first time in the mid-19th century. During the solar eclipse of August 7, 1869, American astronomers Charles A. Young and William Harkness independently discovered that the corona’s spectrum featured an especially bright line of green light. Bright lines in a spectrum are called emission lines. They are the fingerprints of elements in the substance producing the light. The corona’s bright green emission line comes from highly ionized iron, indicating that the corona has very high temperatures.

C

Studying the Sun’s Photosphere and Sunspots

Detailed studies of the Sun’s photosphere and the sunspots began with Galileo’s telescopic camera obscura of the 17th century. The next revolution in this area occurred in the 1840s, when German scientist Heinrich Schwabe discovered that the number and positions of sunspots vary over an 11-year period. In 1859 British astronomer Richard Carrington discovered solar flares. Carrington’s discovery helped explain that geomagnetic storms (increased intensity of Earth’s magnetic field) are related to events on the Sun. In 1908 American astronomer George Ellery Hale showed that sunspots contain magnetic fields that are thousands of times stronger than Earth’s magnetic field.

D

Study of the Sun’s Energy

The Sun produces an enormous amount of energy. Scientists could not explain how something with the mass of the Sun could produce so much energy until they discovered nuclear fusion. The details of just how nuclear fusion changes hydrogen into helium nuclei were not known until discoveries in the field of elementary particles were made. Elementary particles are the tiny particles that make up all matter. The most familiar particles, the particles that make up atoms, are protons, neutrons, and electrons. Protons and neutrons are the main particles involved in nuclear fusion. Both types of particles are about the same size and mass, but protons have a positive electric charge, while neutrons are electrically neutral. New Zealand-born British physicist Ernest Rutherford discovered the proton in 1918. British physicist Sir James Chadwick discovered the neutron in 1932, and was awarded the 1935 Nobel Prize in physics for his discovery.

The first fusion reaction in a laboratory occurred in the early 1930s. In 1938 German-born American physicist Hans A. Bethe and American physicist Charles L. Critchfield demonstrated how a sequence of nuclear reactions, called the proton-proton chain, makes the Sun shine. Bethe was awarded the 1967 Nobel Prize in physics for his discoveries concerning energy production in stars.

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