Andrew Pollachne, age 11, of Salt Lake City, Utah, for his question:
How bright is the star called Sirius?
The light rays from Sirius take more than eight and a half years to reach us. Brilliance is dimmed by distance, even that of the dazzling stars strewn across the vast oceans of space. Some are nearer to us than others, some shed far more light than others. Actually, we need two scales to estimate their brilliance. One tells how bright they appear to be in our sky, the other tells how brilliant they really are in their own celestial neighborhoods.
Sirius is the most brilliant star we ever see in the night skies above our planet. Our razzle dazzle sun, of course, appears much brighter but it shines only during the day. The brightness of a star is its magnitude and it's easy enough to see that magnitudes vary from very bright to very dim. It is possible to compare all these visible magnitudes as seen from the earth. This helps to identify them by their positions and to place them on star charts. But it gives a somewhat biased picture of their true brilliance.
What we see is the APPARENT magnitude of Sirius, how bright it appears to be from a distance of 8.7 light years. Its ABSOLUTE Magnitude deserves a capital letter because it reveals how bright it actually is in comparison with other stars. It was decided to estimate this Magnitude by eliminating the unfair competition of distance. For instance, let's pretend that all the stars are 33 light years from the earth,
At this distance, our sun would appear to be a very dim star. Sirius would be 30 times brighter than the sun, but less brilliant than many other stars. At this point it is handy to compare some of its vital statistics with our personal star. As a star, Sirius is roughly 2 1/2 times more massive than the sun, twice as wide and twice as hot. It is a first Magnitude star with a rating of 1.3. The sun is rated 4.7, which makes it a star of the fourth Magnitude.
It is surprising to pate that brighter stars have smaller numbers. What's more, one would expect a first magnitude star to be twice as bright as one of second magnitude. But the scales of Magnitude and magnitude are based on the same rather complex system of numbers. A first magnitude star appears to be about 2 1/2 times brighter than one of second magnitude and 100 times brighter than a sixth magnitude star. The same is true for Magnitudes, except that here the difference is real, not stars. They rate minus numbers. When Venus appears to shine about 30 times brighter than Sirius, it rates a magnitude of minus 4. On the same apparent magnitude scale, Sirius rates minus 1.6 and our dazzling sun rates minus 27.
On the true scale of absolute Magnitude, Sirius is a fairly brilliant star. Others appear dimmer in our skies because they are more remote. Canopus, the astronaut's star, appears to rate about minus 1. But its distance is 180 light years and its true Magnitude is minus 4.6. Many other stars that appear dimmer than Sirius are really far brighter but they are much farther away from us.