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THE BRIGHTNESS OF STARS

The Sun is the nearest star to us, and its luminosity makes it special from the rest. We can see its intense brightness since it is “very close” to us, merely to 150 million kilometers away, whereas the next closest star is no less than 39,762,576,000,000 kilometers away (39 trillion kilometers). Two thousand times farther!

Most of the stars in the Universe are, in fact, smaller and brighter than our Sun. Nevertheless, it is surprising that most of the ones we can see at night are actually, larger, hotter, and brighter. It seems contradictory, but let’s imagine that we can see a light bulb or a streetlight shining from a distance. A match, however, requires to be relatively closer to see if it is lit.

Let’s review a real example. The nearest star is Proxima, located at the Constellation Centaurus. It is part of a triple-star system, where the main one is Alpha Centauri A, following by Alpha Centauri B and then Alpha Centauri C or Proxima. Of those three stars, the first two are visible to the naked eye, even though they seem to be only one because they are very close, and we need a telescope to watch them separately. Alpha Centauri A is similar to the Sun in terms of size, age, temperature, and brightness, being one of the most luminous stars in the southern hemisphere sky. Although it is slightly closer, Proxima has just under one-eighth of the Sun’s mass, and the surface temperature at almost 2800 °C (in contrast to the Sun that is over 5400 °C), which means that its brightness is less than the hundredth part of the Sun’s shine. That is why we cannot see it without help, even though it is very near. 20 • • •