When you look at different celestial bodies from Earth (especially stars), some appear bigger and brighter than others -- and it's almost always the bigger ones that are the brightest. Eventually, we end up associating size with brightness, which is not exactly wrong, but it's missing a key aspect: distance.
A candle near your screen looks bigger, but also brighter than a candle 500 meters away. Heck, it probably looks brighter than a big lightbulb 500 meters away. When it comes to planets and stars, the distances are so colossal that they can completely distort the apparent size and brightness. So how do we measure which stars are actually brightest?
To classify who is the big winner in the brightness competition, we first need to understand how to quantify it. For thousands of years, humans have only had their eyes to work as detectors.
But now, we've got a much better understanding of the universe, and much better tools to look at things.
Table of contents
How to measure brightness?
The first systematic way to measure which objects were brighter in the sky was developed back in Ancient Greece. Their classification is called the apparent magnitude classification, and it's based on human eye sensitivity. The brightest stars in the night sky had a magnitude of 1 (m = 1), whereas the faintest were of sixth magnitude (m = 6). Funnily enough, researchers now classify stars of magnitude 0 and -1, even brighter than what the ancient Greeks knew, but that's a different story.
Centuries later, in the 19th century, a British astronomer called Norman Pogson, devised another scale -- a logarithmic scale. He initially used the Polar Star (Polaris) as a reference for his scale, but researchers then discovered that Polaris' brightness can vary, so they switched to another star: Vega. In Pogson's scale, Vega is the standard reference of zero magnitude.
A star of magnitude 1 would be approximately 2.5 brighter to the eye than a magnitude 0, and a magnitude 2 would be 2.5 times brighter than a magnitude 1, and so on. Five orders of magnitude higher means the star is about 100 times brighter. The brightest star in this system (and in any system, really) is the Sun at noon, but of course, it's more interesting to see which star is brightest aside from the Sun.
Researchers have developed several other ways to look at the brightness of stars, looking at waves from only specific wavelengths (for instance, near-infrared wavelengths), but the logarithmic approach still remains popular.
The irregulars
We've mentioned brightness as a static thing, but for many reasons, celestial bodies don’t have the same brightness forever -- and variations can be quite sudden.
First, Earth is moving around the Sun, so there are times we are further from some objects, and other times we are closer, this makes them appear different in the sky. So even from one season to the other, there can be a significant variation in apparent brightness. Then, there can be obstructions.
The Earth isn't the only thing moving in the universe -- everything is moving, and sometimes, things move in front of each other. Nearly two years ago, Jupiter and Saturn had their Great Conjunction. They came together in the sky. Planets closer to us vary in brightness as they move around the Sun. For a planet nearly behind the Sun, it gets fainter, after moving to the opposite position, we see it brighter. That is what changes Venus’ brightness over the year from nearly -5 to nearly -3. So it's important to keep in mind that apparent brightness can vary.
In fact, some stars have irregular magnitudes, and their brightness changes consistently. They are called variable stars and there are many of them.
Betelgeuse, one of the brightest stars, is a good example of this. The star is 8 million years old, just a baby compared to 4.6 billion years of our sun. However, it is much bigger than our sun, and if it would be in our solar system, it would extend to the asteroid belt. Betelgeuse is a variable star, whose brightness is known to periodically rise and fall. As Betelgeuse matures, the brightness will likely stabilize.
Other magnitudes
When it is possible to compute the object’s distance, one can find the absolute magnitude. This is not always possible, especially if the object is very distant from Earth.
A more direct feature is the AB magnitude, it requires knowledge of how much radiation is transferred from the object to the detector. Now, the detector can no longer be human eyes, but an artificial instrument capable of understanding which radiation it is receiving. The cameras are designed with filters which allow specific light to pass through.
The brightest star in the sky
So far, we've only talked about brightness and not really mentioned what the brightest star is -- as seen from Earth.
The winner is Sirius, and the star has a fitting name as Σείριος, or Seirios, means 'glowing' or 'scorching' in Greek. The star is found in the Canis Major constellation -- and it's not actually a single star, but rather a binary star system. Most of what we see is the larger star, Sirius A, a main-sequence star like the Sun, two times bigger than our star. Sirius B is a white dwarf, and it's only visible with powerful telescopes.
Even Sirius A isn't particularly large compared to some of the largest stars in the universe, but it's relatively close to the Earth. The distance from here is about 8.7 light-years, which is a lot, but it's not that much compared to the distance to other stars in our galaxy. Since distance and magnitude are related, Altair, a star similar to Sirius in the constellation of Aquila is almost two times dimmer - in terms of absolute magnitude. Altair is almost 2 times farther away than Sirius.
Other stars are very bright as well, including some of the ones we've already mentioned in this article, like Vega or Betelgeuse. Canopus is the second brightest star, the lesser known Rigil Kentaurus and Arcturus take the 3rd and 4th spots respectively. Polaris, often wrongly regarded as the brightest star, is only at 48th.
However, it's important to keep in mind that this is only from our relative perspective from the Earth. If you'd try to map the brightest stars in the universe (we're talking intrinsic brightness, regardless of observation point), you'll find stars that are over 1,000 times larger than the Sun -- and much, much brighter. For instance, a star called Stephenson 2-18 is over 2,000 times larger than the Sun, larger than the entire orbit of Saturn. In fact, we know of dozens and dozens of stars over 1,000 times larger than the Sun -- they're just far away from us.
