Seva Spands, age 13, of Gary, Indiana, for her question:
What is the magnitude of our sun?
You can watch a flashlight way down the road, growing bigger and brighter as it gets nearer. Something similar, on a much grander scale, happens when you survey the starry heavens. Magnitude refers to the brilliance of stars. And like beacons on the earth, the blazing stars lose their brilliance with distance.
On one scale of magnitude, our dazzling sun has a rating of minus 26.8. It is, of course, the most brilliantly luminous object ever seen in our skies and the minus sign may seem somewhat puzzling. However, when it comes to rating stellar magnitude, plus signs are dimmer than minus signs. Another ticklish problem is the fact that distance dims even the brightest star. Things up there are rarely what they seem to be. For this reason, astronomers have two scales of magnitude. Apparent magnitude rates the starry suns as they appear to human eyes. Absolute magnitude gives their actual brilliance, regardless of distance.
On both scales, the magnitudes differ by multiples of about two and a half. A star of the second magnitude is 2 1/2 times dimmer than a star of first magnitude. A first magnitude star is 100 times brighter than one of 6th magnitude. Refined instruments and ingenious calculations make it possible to rate the stellar magnitudes in whole numbers and decimals. On the scale of apparent magnitude, Aldebaran in Taurus the Bull is a first magnitude star of 1.1. Sirius the Dog Star is much brighter and rates a magnitude of minus 1.6. On the same scale, our sun rates a stupendous apparent magnitude of minus 26.8.
However, our sun is actually merely a medium star of medium brilliance. Sirius is bigger and also brighter and so is Aldebaran. But Sirius is at a distance of 8.7 light years and Aldebaran 68 light years. The fair way to estimate their true brilliance would be to consider theta from the same distance. And this is just how astronomers estimate real or absolute magnitudes. They figure how bright the stars would be if all of them were lined up at a distance of 10 parsecs, which is 32.6 light years from the earth.
Ages ago, the stars were graded in six groups of apparent magnitude and the dimmest visible stars are of 6th magnitude. Absolute magnitude was figured out with the help of precise instruments and techniques. It is based ,'on our best known unit of brilliance, the starry sun. Each degree of absolute magnitude equals the lumin¬osity of our medium bright sun. On this scale, our sun rates as a star of 5th magnitude which is quite a comedown from its minus 26.8 rating of apparent magnitude. If we could see it in the imaginary line up of stars at a distance of 10 parsecs, our sun would be just one magnitude brighter than the dimmest visible stars.
From the earth the brightest star we see is Sirus. Its apparent magnitude is minus 1.6. But on the scale of absolute magnitude it rates 1.3. As a star of first magnitude it is actually some 60 times more luminous than the 5th magnitude sun. We see Deneb as a.first magnitude star but on the scale of absolute magnitude its rating is minus 4.8. Its real magnitude is thousands of times brighter than the sun.