1. Good Day Sunshine

    • Artists, writers and religions have revered the Sun.

    • Without the Sun's light and heat, life would quickly vanish from our planet.

    • The Sun drives our weather, warms our globe and energizes plants by photosynthesis.

    • When compressed, dead organic material that was originally energized by the Sun has been turned into fuel for our lights, cars and trucks.

    • Radiation from the Sun includes all the colors of sunlight, which make white light when combined, as well as invisible infrared, ultraviolet, radio and X-ray radiation.

    • The Sun is the only star close enough to resolve details in its atmosphere.

    • Physical conditions capable of sustained nuclear reactions occur within the Sun and not on Earth.

    • A spectrum is a display of radiation intensity as a function of wavelength.

    • From long to short waves, the colors of sunlight are red, orange, yellow, green, blue and violet.

    • Features in the spectrum of sunlight are known as absorption lines; they are used to identify the chemical ingredients of the Sun.

    • The absorption lines seen in the Sun's light are generated as emission lines when individual elements are vaporized in the terrestrial laboratory.

    • The absorption, or emission, lines act as a bar code that identifies, or fingerprints, the element that produced them.

    • Hydrogen is the most abundant element in the Sun. Hydrogen is also the most abundant element in most other stars and interstellar space.

    • Helium, the second most abundant element in the Sun, is so rare on Earth that it was first discovered in the Sun.

    • All of the hydrogen and most of the helium in the Universe were produced in the Big Bang that sent the galaxies fleeing in an expanding Universe.

    • The Sun is a second generation star, containing elements heavier than helium that were synthesized inside former stars that seeded space with these elements during their explosive death.

    • The carbon, oxygen and iron inside your body were synthesized inside other stars before the Sun was born, but your hydrogen was produced in the Big Bang.

    • The Sun's radiation carries energy out in every direction.

    • Different types of electromagnetic radiation differ in their wavelength, although they propagate at the same speed, known as the velocity of light and usually denoted by the lower-case letter c, with a value of about 300,000 kilometers per second.

    • The light travel time is equal to the distance divided by the velocity of light. for the Sun-Earth distance, the light travel time is 8.3 minutes or 499 seconds, while it takes 4.29 years for light to travel to us from the nearest star other than the Sun.

    • The observed wavelength of radiation increases or decreases when the motion of the source is respectively away or toward the observer; this Doppler effect also applies to sound waves.

    • The product of the wavelength and the frequency of radiation is equal to the velocity of light, c. So longer wavelengths correspond to lower frequencies, and shorter wavelengths correspond to higher frequencies.

    • The ability of radiation to interact with matter is determined by the energy of its photons.

    • The photon energy of radiation increases with decreasing wavelength. So short waves have more energy.

    • The Sun emits invisible radiation at short ultraviolet and X-ray wavelengths, detected from above the Earth's atmosphere, and at long radio wavelengths observed with gigantic radio telescopes on the ground. But the total intensity of the Sun's radiation emitted at any of these wavelengths is less than that emitted in visible sunlight.

    • A hotter gas emits its most intense radiation at shorter wavelengths. Intense X-rays are emitted from a million-degree gas, visible sunlight from a 5780-kelvin gas like the visible solar disk, and radio waves from interstellar space at 100 kelvin.

    • The development of visible light, or optical, spectroscopy permitted investigation of the magnetic fields, atmospheric motions and composition of the Sun.

    • The coronagraph uses an occulting disk to block out the intense glare of the photosphere, and permit observation of the faint light of the Sun's outer atmosphere, known as the corona.

    • Radio telescopes can be used to study violent activity on the Sun, and interferometric radio telescopes can have an angular resolution that is better than that obtainable with ground-based visible light, or optical, telescopes.

    • The development of artificial satellites and other spacecraft allowed scientists to study the Sun above the Earth's obscuring atmosphere, permitting a full and continuous view of the Sun's ultraviolet and X-ray radiation, and direct sampling of energetic particles and magnetic fields flowing from the Sun.

    • Sensitive and precise instruments aboard nine solar missions, named Yohkoh, Ulysses, Wind, SOHO, ACE, TRACE, RHESSI, Hinode and STEREO, detect otherwise invisible particles, ultraviolet and X-ray radiation and magnetic fields, tracing the flow of energy and matter from down inside the Sun to the Earth and beyond.

Copyright 2010, Professor Kenneth R. Lang, Tufts University