Sun

After scientists understood how the Sun produces its energy, they began developing theories to explain how the Sun’s energy travels from the core to the Sun’s atmosphere. For the first few decades after the discovery that fusion powers the Sun, scientists deduced the Sun’s structure by comparing the theoretical output of the Sun’s core to the energy actually released at the Sun’s atmosphere. In the 1960s American physicist Robert Leighton developed a camera that could record Doppler shifts in light at the Sun’s surface. A Doppler shift is a change in the wavelength of light caused by the movement of the object that is emitting the light. If the object is moving away from the observer, each wave will have to travel farther to reach the observer, making the distance between waves (the wavelength) longer. An object moving toward the observer will seem to emit light with a shorter wavelength. Leighton used this device to discover that the Sun seemed to pulsate in and out, making a complete cycle about every five minutes. See the Oscillating Sun section of this article.

Leighton’s discovery launched the field of helioseismology, or the study of the Sun’s interior by observing the vibrations of the Sun and how sound waves move through it. In the 1970s scientists demonstrated that the entire Sun is vibrating with ponderous, organized rhythms that can extend to its very core. Scientists developed models of the interior of the Sun based on vibrations at its surface.

In 1995 six observatories around the world coordinated with each other to begin observing the oscillations of the Sun as a team. This project, a collaboration of 20 nations, is called the Global Oscillation Network Group (GONG). GONG can keep constant watch on the Sun because, at any given time, daytime is being experienced by at least one of the observatories. GONG has allowed scientists to get a better idea of the interior structure of the Sun through helioseismology.

Studying the Sun from space has revolutionized solar physics. Since the first observations from space began to be made, scientists have made great advances in the study of the Sun’s surface, energy production, structure, and relationship to Earth and the solar system. Study from space began in 1957 when the Soviet satellite Sputnik 2, the second satellite to go into space, carried instruments to study the Sun. Since then, many missions have been devoted to studying the Sun. The series of missions by the Pioneer spacecraft of the United States included several experiments to study the Sun and its relationship to Earth. The Pioneer program lasted from the late 1950s to the 1970s. The U.S. Mariner 2 spacecraft, launched in 1962, used data obtained from its voyage to Venus to demonstrate that a low-speed solar wind is continuously emitted from the Sun, and also discovered high-speed streams in the Sun’s winds. In the 1960s and 1970s the U.S. Orbiting Solar Observatory (OSO) series studied the Sun over an entire cycle of solar activity. One of the satellites in the OSO series was the piloted Skylab space station, launched in 1973. Skylab astronauts used X-ray telescopes to transform our knowledge of the Sun’s corona. In the early 1980s the United States launched the Solar Maximum Mission spacecraft to study the Sun during its most active period. The joint Japanese, U.S., and British probe Yohkoh studied solar flares through the 1990s.

Two of the most productive solar spacecraft of the late 1990s and early 2000s were the Ulysses spacecraft and the Solar and Heliospheric Observatory (SOHO). Both spacecraft are joint projects of the United States and the European Space Agency (ESA). Ulysses was launched in 1990 and SOHO was launched in 1995. Ulysses’s orbit takes it over the poles of the Sun, then back to the planet Jupiter to get a gravitational boost that sends the spacecraft back to the Sun. By 2001 Ulysses had passed over the Sun’s north and south poles twice. Ulysses’s mission has contributed much knowledge about the solar wind in regions above the Sun’s poles. Findings from this mission conclusively demonstrated that the fast component of the solar wind pours out at high solar latitudes, including from polar coronal holes. The slow component of the solar wind is constrained to low latitudes near the solar equator. Ulysses also found that the Sun’s magnetic field does not warp at the poles as much as scientists expected.

SOHO is at a point in space where the Sun’s gravitational pull balances Earth’s gravitational pull, so the satellite orbits the Sun with Earth. SOHO always faces the Sun. This probe has allowed scientists to make great leaps forward in their understanding of the structure and dynamics of the solar interior, the heating mechanisms of the solar corona, and the origin and acceleration of the solar wind. SOHO has returned amazing images of the Sun, including comets hitting the Sun and features on the Sun’s surface that scientists compare to tidal waves, tornadoes, and rivers.