1. Light of the Sun

    • The Sun is the closest star to the Earth and the Sun provides the light and heat necessary to sustain life on our planet.

    • The mean distance between the Earth and the Sun is one astronomical unit, or 1 AU for short.

    • The AU is equal to 149.6 million kilometers.

    • The time for light to travel from the Sun to the Earth at the speed of light is 499 seconds, or about 8.3 minutes.

    • The time for light to travel from the next nearest star, Proxima Centauri, to the Earth is 4.24 years.

    • Electromagnetic radiation propagates by the interplay of electric and magnetic waves.

    • Radiation has a wavelength and a frequency.

    • The product of wavelength and frequency is equal to the speed of light c = 299.8 million meters per second.

    • All radiation travels at the speed of light in empty space.

    • The radiation we see with our eyes is called visible light.

    • Invisible radiation that we do not see with our eyes includes x-rays, gamma rays, infrared rays, ultraviolet rays and radio waves.

    • Visible light and radio waves emitted by cosmic objects reach the ground and are not absorbed in the Earthís atmosphere.

    • Cosmic x-rays, gamma rays, infrared rays, and ultraviolet rays are absorbed in the Earthís atmosphere.

    • The unit of energy is the Joule, and the unit of luminosity is the Watt, or 1 Joule per second.

    • The solar constant is the amount of solar radiant energy per unit time per unit area reaching the top of our atmosphere.

    • The Sunís luminosity LĚ = 3.828 x 1026 J s-1.

    • The radius of the Sun is RĚ = 6.955 x 108 m = 695.5 million meters, or 109 times the size of the Earth.

    • The Sunís visible disk has an effective temperature of 5,780 K.

    • Any hot gas emits thermal radiation.

    • The maximum intensity of thermal radiation occurs at a wavelength that is inversely proportional to the temperature.

    • The luminosity of thermal radiation increases the fourth power of the temperature and the square of the radius; this is known as the Stefan-Boltzmann law.

    • Bigger telescopes gather more radiation, enabling the detection of fainter objects. Bigger telescopes also detect finer details with greater angular resolution.

    • Visible light telescopes collect light using a mirror or a lens. They are respectively known as reflectors and refractors.

    • An interferometer uses two connected telescopes to achieve a resolution comparable to that of a single telescope as big as the separation of the two telescopes.

Copyright 2010, Professor Kenneth R. Lang, Tufts University