Q & A...Magnitude Explained
by Jim Stanicek
Q: Just what exactly is “magnitude”, and how come extended objects such as M33 are so much dimmer than the same magnitude star!
A: Actually, when dealing with the subject of “magnitude”, one must remember that there are actually two different quantities that are referred to as “magnitude”. There is the “Absolute Magnitude” and there is the “Apparent Magnitude”. Actually, a third magnitude is sometimes considered, which is “Photographic Magnitude” the magnitude in specific wavelengths of light, such as red and blue.
Absolute Magnitude, is a measurement of how bright the object actually is. This is primarily determined by its size, and by its temperature. This figure is stated by calculating how brightly the object would shine at a fixed distance away in space, known as 10 parsecs, or 32.6 light years. The luminosity of the object is what determines its’ Absolute Magnitude.
Apparent Magnitude is how bright the object appears to us here on Earth. A certain object can have an extremely high Absolute Magnitude, but if it is far away, or shrouded in inter-steller dust, it will appear to be dim to us. Likewise, a comparatively dim star could appear to be quite bright because of its nearness to us. Keep in mind, however, that the expressions “near” and “far” are relative, because even the “nearest” star, Proxima Centuri, is over 4 light years away! That star, incidently has a very low Absolute Magnitude, so it is not a very bright star in the southern sky.
Q: But just what does this all mean? Just how is magnitude determined, and why do the brightest stars, like Sirius, actually have a NEGATIVE Magnitude assigned to them?
A: The magnitude scale is actually logarithmic in nature. According to a Wikipedia article, “It traces to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary). He classed stellar objects on how bright they appeared — the brightest were "magnitude 1", the next brightest were "magnitude 2", on down to "magnitude 6", the faintest he could see. Thus the scale is roughly 2000 years old.” Unfortunately, he ( or they ) classified several of the brightest stars as “Magnitude 0” and did not start with the very brightest star Sirius, so it had to receive a negative number, brighter than zero. The scale goes backward, so several of the brightest stars are between magnitude 0 and 1, with increasingly dimmer stars getting higher numbers, down to the dimmest stars that can be seen with the unaided eye, magnitude 8.
The invention of the telescope allowed stars down to around the 14th or 15th magnitude to be easily seen in earlier days, with darker skies. We are fortunate enough now days to see stars around the 8th to maybe 10th magnitude with most amateur scopes in today’s light-polluted skies.
Q: But you still haven’t answered, WHY do objects such as M33 ( magnitude 5.7 ) appear so dim in my telescope? I can hardly make it out, but I have no problem seeing a 6th magnitude star !
A: Ahh, now we get to the difference between seeing a point source of a certain magnitude, and seeing a diffuse object of the same magnitude.
To answer this question, perform a small experiment. Focus on a star of, let’s say, a third magnitude. It is fairly bright in your scope, isn’t it? Now, rack your eyepiece out of focus until the image is approximately the same diameter as a nebula or star cluster that you may have seen. What do you notice? The light from the point source, which was relatively bright before, has now spread out into a circle ( or maybe a donut if your telescope is a reflector ) and has become much dimmer in your vision. The star hasn’t become dimmer, but the image is, because the light from that star is spread out over a larger AREA ! The same thing happens when we look at an object such as M33. If all the light from M33 were condensed into a point, it would appear much brighter to our eye.
That is exactly the way that magnitudes were assigned to the “faint fuzzies”. The apparent diameter of the object was measured, and then a star of known magnitude was de-focused to the point where its’ light was spread out over the same area, and the magnitudes of the images were compared. The “faint fuzzy’s” magnitude would be the same as the star which produced the same intensity of light when its light was spread over the same area!
Now days, sensitive instruments are used to measure the magnitudes of stars and nebulae that are so dim that it takes DAYS of exposure time to record their images on our most sensitive instruments. But the concept remains with us to this day.