In busy cities, you may only be able to see the brightest stars against light pollution, even on a clear night.
On the other hand, in a really dark location, you may see thousands of stars glittering above you. But even then, some stars will be noticeably brighter than others, and the most luminous ones will be the same select few that city-dwellers see.
These brightest stars have been a familiar sight to humans since ancient times, and we’ll look at a selection of them here.
There are several reasons why some stars look brighter than others. Perhaps the most obvious is that they’re all located at different distances from us. It’s no surprise the nearer stars tend to look brighter than ones that are further away. In fact, our closest star — by a long way — is the sun, and when it’s visible in the sky it shines so brightly that we can’t see the other stars at all.
A second reason for differences in brightness is that some stars are larger than others. Our own sun is in the prime of its life — called the “main sequence” of stellar evolution — but stars that are further advanced in their life cycles can grow to giant proportions. Not surprisingly, a giant star will be much brighter than a main sequence star seen at the same distance.
The brightness of stars is usually expressed in terms of a “magnitude” value, and — confusingly — this number actually gets bigger for fainter stars. The system originated in the ancient world when people called the very brightest stars “first magnitude”, the next brightest “second magnitude” and so on. By the 19th century astronomers needed a more precise scale, so they defined magnitude 2 as 2.5 times fainter than magnitude 1, magnitude 3 as 2.5 times fainter than that, and so on. This allowed the system to be extended to the fractional and negative numbers that we see today.
Classification of stars
There’s one other key factor that makes some stars brighter than others, and that’s their temperature. Hot stars tend to produce more light than cooler ones. We can get a rough idea of a star’s temperature based on its color. Blue stars are the hottest, followed by white, yellow and orange, and then finally red stars are the coolest.
To put this sequence on a more scientific footing, astronomers refer to the “spectral type” of a star, designated by a letter followed by a digit between 0 and 9. The letters, from the blue end to the red end, run O, B, A, F, G,…