![]() Consequently, only a minuscule fraction of the energy given off by a star actually reaches an observer on Earth. Stars are democratic in how they produce radiation they emit the same amount of energy in every direction in space. Apparent BrightnessĪstronomers are careful to distinguish between the luminosity of the star (the total energy output) and the amount of energy that happens to reach our eyes or a telescope on Earth. In a later chapter, we will see that if we can measure how much energy a star emits and we also know its mass, then we can calculate how long it can continue to shine before it exhausts its nuclear energy and begins to die. We use the symbol L Sun to denote the Sun’s luminosity hence, that of Sirius can be written as 25 L Sun. For example, the luminosity of Sirius is about 25 times that of the Sun. (And there are stars far more luminous than the Sun out there.) To make the comparison among stars easy, astronomers express the luminosity of other stars in terms of the Sun’s luminosity. Earlier, we saw that the Sun puts out a tremendous amount of energy every second. Perhaps the most important characteristic of a star is its luminosity-the total amount of energy at all wavelengths that it emits per second.
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