E3 Radiation and Light in Astronomy

Topic

Electromagnetic radiation is the primary messenger of information reaching the observer from the rest of the universe. The full spectrum emitted by a body in thermal equilibrium depends solely on that body's temperature; the wavelength at which the spectrum peaks shifts toward shorter wavelengths as the emitting body gets hotterthe Suns peak emission is in visible light, whereas cooler stars peak in the infrared.

A star's total luminosity increases with the square of its radius andeven more dramaticallywith the fourth power of its temperature; consequently, even a modest rise in a star's surface temperature results in a massive increase in its total luminosity.

The magnitude scale astronomers use to measure the brightness of celestial objects is logarithmic and invertedlower magnitude corresponds to greater brightnessand is calibrated so that a difference of five magnitudes equates exactly to a factor of one hundred in the received light flux.

The distance modulus, in turn, links the magnitude as observed from Earth, the magnitude the object would have at a fixed reference distance, and the actual distance separating the object from the observer; this connection underpins the entire cosmic distance ladder: the farther away an object of known luminosity is, the fainter it appearsa relationship that scales logarithmically with the distance from the observer. Thus, from the electromagnetic radiation that opens this text to the cosmic distance ladder that concludes ittaking in the link between luminosity and temperature as well as the magnitude scaleradiation and light in astronomy allow the simple color and brightness of a point in the sky to reveal both its temperature and its actual distance from the observer.

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