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The Quest to Understand the Sun’s Energy Source
For centuries, humanity has been fascinated by the question of what fuels the Sun’s seemingly endless energy. Early theories speculated wildly, considering everything from meteor impacts to chemical combustion and the Sun’s own contraction.
This image from the Solar Dynamics Observatory captures the Sun at a wavelength of 304 Ångstroms (30.4 nanometers), highlighting its upper chromosphere. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio, SDO Science Team, and the Virtual Solar Observatory.
Before the advent of Einstein’s theory of special relativity and the famous equation E = mc², what explanations were put forth regarding the Sun’s relentless energy?
William Fields
Dayton, Ohio
The inquiry into the Sun’s energy source stretches back hundreds of years. In the 19th century, some theorists proposed that a continuous bombardment of meteors could supply the necessary energy. Others dismissed various chemical processes, considering them inadequate to explain the Sun’s longevity.
During this period, William Thompson, known as Lord Kelvin, and Herman von Helmholtz independently suggested a theory centered on solar contraction, a concept now referred to as the Kelvin-Helmholtz contraction theory. They posited that the Sun was gradually contracting, which would generate heat as gravitational energy transformed into kinetic energy. Their calculations estimated the Sun’s lifespan to be between 20 million and 500 million years, a timeframe that, in many instances, proved inconsistent with geological evidence for Earth’s history.
Despite these limitations, the contraction theory represented the best understanding of solar energy until the nuclear age began to illuminate the topic. In the 1920s, Sir Arthur Stanley Eddington introduced the idea of proton-proton fusion, a process converting hydrogen into helium. Later, in 1938, Hans Bethe detailed the carbon-nitrogen-oxygen (CNO) cycle, another mechanism for fusing hydrogen into helium. Both processes result in a small fraction of mass being transformed into energy, aligning with Einstein’s revolutionary equation, E = mc², which clarified how the release of mass could generate the vast energy necessary to sustain the Sun against gravitational forces.
The proton-proton chain reaction remains the predominant method through which the Sun generates energy, fusing four hydrogen nuclei into one helium-4 atom. Meanwhile, the CNO cycle involves a series of transformations where carbon-12 converts through various isotopes, eventually returning to carbon-12 and producing helium-4 in the final step. In both processes, a minute quantity of mass is converted into energy, demonstrating the elegance of nuclear fusion. Credit: Astronomy: Roen Kelly, after Borb/Wikimedia Commons.
Sten Odenwald
Senior Outreach Coordinator, NASA HEAT Program, Kensington, Maryland
Source
www.astronomy.com