(Actually, we are pretty glad about that because having many different types of stars makes the universe a much more interesting place.) But this means that if a star looks dim in the sky, we cannot tell whether it appears dim because it has a low luminosity but is relatively nearby, or because it has a high luminosity but is very far away. If it is three times farther away, it will look nine (three squared) times dimmer, and so forth.Īlas, the stars do not all have the same luminosity. If, for example, we have two stars of the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one. The energy we receive is inversely proportional to the square of the distance. To pin down this idea more precisely, recall from the Radiation and Spectra chapter that we know exactly how light fades with increasing distance. In the same way, if all the stars had the same luminosity, we could immediately infer that the brightest-appearing stars were close by and the dimmest-appearing ones were far away. In this way, you can tell which bulbs are closest to you. Those close to you appear brighter (more of their light reaches your eye), whereas those far away appear dimmer (their light has spread out more before reaching you). But from where you are standing in one corner, they do not have the same apparent brightness. Since they are all 25-watt bulbs, their luminosity (energy output) is the same. Imagine you are in a big concert hall or ballroom that is dark except for a few dozen 25-watt bulbs placed in fixtures around the walls. If all stars were the same luminosity-if they were like standard bulbs with the same light output-we could use the difference in their apparent brightnesses to tell us something we very much want to know: how far away they are. Most stars, in fact, are so dim that you need a telescope to detect them. If you look at the night sky, you see a wide range of apparent brightnesses among the stars. We call the amount of a star’s energy that reaches a given area (say, one square meter) each second here on Earth its apparent brightness. 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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