Measuring the brightness of celestial objects

A photometer is an instrument that measures and quantifies the brightness of a celestial body (a galaxy, star, comet or planet, for example). The brightness depends on the luminosity of the object and on the distance that separates the object from the photometer, as well as the amount of matter between the two that can obscure the object. A very luminous star could appear dim if it is far enough away or is partly obscured by gaseous clouds and interstellar dust.

A photometer has several applications in astronomy. In the case of a star, for example, a photometer can be used to determine its temperature, its distance and even its age. Photometers can also be used to determine if planets are orbiting about certain stars.

Let’s take a look at the case of temperature. A hot star will emit a lot of light in the blue portion of the spectrum (relatively energetic wavelengths), whereas a cold star will mainly emit red light (less energetic wavelengths). By using a photometer to measure the amount of light emitted at blue and red wavelengths (that is, the “colour” of the star), temperature can be estimated.

The brightness of a celestial object is a measure of its luminous flux: the amount of incoming light. The luminous flux of a star is dependent on a combination of temperature, the size of its surface, and its distance from Earth. If we know its temperature (by its colour) and its distance (by other methods), we can calculate its surface area and thus its size.

Astronomers can use photometers to precisely measure the amount of light emitted at specific wavelengths that correspond to the emission lines produced by heated chemical elements. This information is used to establish the chemical composition of a star.

Many different methods have been developed for estimating the brightness of celestial bodies. All the initial attempts, however, had a major drawback: the estimate had to be made visually and could vary greatly from one observer to another.
ASTROLab of Mont-Mégantic National Park

© 2006 An original idea and a realization of the ASTROLab of Mont-Mégantic National Park

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