Blogging Siriusly

The Focus of the Sky

Look up into the night sky during the Winter. What are your eyes immediately drawn to? A constellation? A star? A planet? A satellite? A passing airplane? The moon? Maybe a rock burning through in the Earth's atmosphere? For me, this is without a doubt Sirius:

This is Sirius, one of the most easily identified stars in the sky. With an apparent magnitude of -1.46 it is actually, the Sun aside, the brightest star in the sky. Again, with the Sun aside, this means that Sirius delivers the most visible light to the Earth. But what does this mean?

Well, think about it like this: The Sun's apparent magnitude is -26.74. But that's only 18 times lower than the apparent magnitude of Sirius. Obviously, the Sun appears much more than 18 times brighter than Sirius. If this weren't the case then the Earth would be frozen solid because it wouldn't get nearly enough heat from the Sun. In fact, the full moon alone is more than 30,000 times brighter than Sirius, and the moon wouldn't be able to provide enough heat to the Earth. Therefore, the magnitude doesn't really say much about the actual brightness of a star and is thus easily misinterpreted. 

Instead, we must convert magnitude into flux ratios which tells us that the Sun is almost thirteen billion times brighter to us than Sirius. It's almost impossible to conceive a thirteen billion difference. It's just crazy!

The Truth

In all reality, however, distance is key. Sirius is more than 540,000 times further away from us than the sun. Therefore, we must make adjustments to our measurements accordingly. To do so, we have to use absolute magnitude, which is the magnitude of a star at a distance of 10 parsecs. The absolute magnitude of Sirius is 1.42 and the absolute magnitude of the Sun is 4.8. That is correct: Sirius is actually brighter than the Sun. 22.9 times brighter, to be more precise. Who'd have ever guessed?