Final answer:
The Sun has been shining for over 4.5 billion years due to nuclear fusion at its core, an energy source that was not fully understood until the twentieth century. The interplay between the Sun's increasing energy output and Earth's declining atmospheric carbon dioxide levels has helped maintain stable conditions favorable for life.
Step-by-step explanation:
The sun has been shining for more than 4.5 billion years because of the process of nuclear fusion that occurs at its core. This process converts hydrogen into helium and, in doing so, releases a prodigious amount of energy in the form of light and heat. The Sun's energy output is about 4 × 1026 watts, which is brighter than a trillion cities with a trillion 100-watt light bulbs each. Initially, nineteenth-century scientists contemplated sources such as chemical and gravitational energy, but these were insufficient to explain the Sun's longevity and energy production. Eventually, it became clear that only nuclear reactions could account for the immense and sustained energy output.
Further supporting the Sun's constant shining, the greenhouse effect played a role in maintaining Earth's temperature when the Sun emitted only about 70% of its current energy. Higher levels of carbon dioxide in the Earth's early atmosphere trapped more heat, preventing the planet from freezing. As a result, simple life forms could exist with the Sun's lower energy output. Scientists suggest that as the Sun has increased in luminosity, Earth's atmosphere has experienced a decline in carbon dioxide, stabilizing temperatures over time, making it conducive to the development of life.