An instrument used to measure the brightness and intensity of light coming from stars and other celestial objects.
From astro- (star) plus photometer (from photo- meaning light and -meter meaning measure). Developed in the 19th century to quantify what astronomers had previously only described subjectively.
Before astrophotometers, astronomers could only compare stars by eye and guess their brightness relative to each other—but these devices revealed that some of the brightest-looking stars were actually far away and intrinsically huge, while dim ones might be faint but nearby.
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