By Dr. Marc Rayman
Asked by our friends at the North Museum Planetarium in Lancaster, Pennsylvania.
Click play to hear me read this to you!
Of course, the star that appears the brightest to all of us on Earth is the Sun. Although it is a rather typical star, not all that different from many of the ones you see at night, we live so close to it that it outshines everything else. Even the next closest star is more than a quarter of a million times farther from Earth, so it is not surprising that the light from the Sun overwhelms that from other stars.
It might be more fun to think about how bright the stars are themselves, without regard to how they look from Earth. After all, as much as we cherish Earth, our planet is not in a special place in the universe, so it isn't really fair to rank the Sun as the brightest only because it is nearest Earth. Suppose we could put all stars at the same distance from us. Then which one would be the brightest?
Imagine you are in a boat on the ocean at night, and you see a light. How would you know how bright the light really is? Is it a weak light on a nearby boat or a brilliant one on a distant island? If you know how far it is, then you have an idea of how bright it really is. But now suppose the night is foggy. In that case, it is much harder to judge the true brightness of the light, because you don't know how much of the light is blocked by the fog.
To find out the true brightness of a star, scientists need to know how far it is. Although there are some very clever ways of gauging the distances to stars, they generally work well only for stars that are in the Sun's neighborhood of the Milky Way galaxy. The more distant stars are just so fantastically far from us, that measuring their distances accurately is too difficult. Making it still harder to know how bright a star really is, there is a kind of patchy fog between the stars - space is not truly empty. Although it is not exactly the same as the fog on Earth, gas and dust in space can dim the light of stars. Without a good way to know how much of this interstellar fog is blocking the light, there is no reliable way to discover the true brightness of a star.
Although we don't know which star truly is the brightest, we know some are remarkably bright. You can see one of them any clear night this summer. Deneb is the northeastern of the three stars that form a large and easily seen grouping called the Summer Triangle. While Deneb shines the brightest in the constellation Cygnus, 17 other stars glow brighter in our night skies. But Deneb is much farther from Earth than most of the other stars you see, and this giant is around 100,000 times brighter than the Sun. If Deneb were the same distance from Earth as Vega, another star in the Summer Triangle, not only would it outshine all the stars and planets visible at night, but it would even be bright enough to see in the daytime!
Deneb pays a precious price for shining so brilliantly. It is using up its stellar fuel at a furious rate, and it will burn itself out after a relatively brief appearance as a star of our galaxy. While the Sun (and Earth) are more than 4.5 billion years old, Deneb is a youngster at only a few million years. And it will last only a few million years more before it dies a spectacular death. In the meantime, less showy but more typical stars like the Sun will continue their more tranquil lives, ultimately shining for perhaps 1000 times as long as a powerhouse like Deneb.
You can find Deneb and other wonders of the night sky with a Star Finder. Make yours for this month here.
Remember in the winter, when you are gazing at the constellation Orion, to think about the two brightest stars there: reddish Betelgeuse and white Rigel. Like Deneb, they could outshine the Sun like a searchlight next to a match.