In this article, I will discuss the subject of Stellar Magnitudes.
The magnitude system is very old, developed by the Greek astronomer, Hipparchus, (c. 190 - 12O B.C.) In these ancient times, stars were classified by how bright they appeared to human eyes, since many centuries would pass until the invention of the telescope and more sophisticated astronomical instruments.
The brightest stars were called "first magnitude," the next brightest "second magnitude", all the way to "sixth magnitude," the limit of the unaided eye.
We now call such descriptions apparent magnitudes, because they compare how bright different stars appear in our skies. Professional astronomers also employ a system of absolute magnitudes, which describe the luminosity of a star, which is the actual power output of that star. For the backyard amateur astronomer, knowing the apparent magnitude is enough for our purposes.
In our time, the magnitude system has been further refined. A difference of five magnitudes represents a difference in brightness of exactly
100, so a magnitude 1 star is 1OO times brighter than a magnitude 6 star. Notice that the system seems to work backwards; that is, the higher the number, the dimmer the star. That's just the way it was defined all those years ago, and it stuck! Modern star charts or atlases often designate magnitudes by the size of the dot or circle representing the star, with an accompanying key.
It was recognized, as time has gone by, that some objects have apparent magnitudes of zero or minus numbers, such as the brightest star in the night sky, Sirius, with an apparent magnitude of about minus 1.4. That is because they are brighter to our eyes than the originally-defined plus 1 to 6th: magnitude stars. An extreme example of minus magnitude objects is our own star, the Sun, with an apparent magnitude of minus 26.7.
Going in the opposite (or dimmer) direction, the Hubble-Space Telescope has imaged distant objects with an apparent magnitude of plus 30 !
One way of testing your ability to see stars with the unaided eye is to view the Pleiades (or Seven Sisters) open star cluster (a group of stars that were all born about the same time), which may contain 500 stars. However, depending on light pollution, the visible naked eye number may be as low as 4 or 5 stars, perhaps up to 1O or more in dark skies. Most of the time, only 6 can be reliably seen. So, why the "Seven Sisters"? An ancient myth says the seventh Sister was kidnapped by Mizar, one of the seven "brothers" of the Big Dipper asterism. The brightest star, Alcyone. in the Pleiades, has an apparent magnitude of plus 2.9 . The cluster is still visible in early March in the western sky, about 9:00 p.m., near the constellation Taurus.
May you have clear skies!!
George Drake, M.D.
ETHOS Innovation Center Volunteer,
Michiana Astronomical Society Member
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