The brightness of a star depends on its

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size and temperature only

size and distance from the earth

size, temperature and mass

size, temperature and distance from the earth

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This question was previously asked in

UPSC NDA-1 – 2016

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The correct option is D.

The apparent brightness of a star as observed from Earth depends on two primary factors: its intrinsic luminosity and its distance from the observer. The intrinsic luminosity of a star, which is the total energy it radiates per unit time, is determined by its size (radius) and its surface temperature. Larger and hotter stars are more luminous. Therefore, the brightness we perceive (apparent brightness) is a function of the star’s size, temperature (which together determine luminosity), and the distance the light travels to reach us. The inverse square law dictates that brightness decreases with the square of the distance.

While mass is a fundamental property of a star and influences its temperature, size, and lifespan, the most direct factors determining luminosity are size and temperature. Apparent magnitude is a measure of apparent brightness, while absolute magnitude measures intrinsic luminosity (brightness at a standard distance of 10 parsecs).

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