How bright something appears in the sky is measured by a numerical quantity called magnitude. I use the term often, and explain it qualitatively in this post. But thought I should really quantify the concept. Magnitude is important for describing celestial objects, and a grasp of what it means will help you find your targets.
Lower magnitudes (including negative numbers) are brighter objects. You might well ask “Why not just start at zero and go up from there?” Historically, that’s not the way it worked out. If you want to understand why that happened, follow the magnitude link above. If you’d rather go straight to the equation, here it is:
|M1-M2|2.512 = the visual brightness ratio of M1/M2 [where M1 is the brighter object]
Note that the expression “|M1-M2|” designates an absolute value. So drop any negative signs before raising the difference to the 2.512 power. A star of magnitude 1 is 2.512 times as bright as a star of magnitude 2, and a star of magnitude -1 is 2.512 times as bright as a star of magnitude -2.
Magnitudes are measured on a logarithmic scale. A change of 1.0 in visual magnitude equates to a 251.2% change in brightness. Logarithmic scales are useful when you have to describe a really large range of numbers, and the range of brightness visible to the human eye is incredibly large. Scientists also use logarithmic scales to measure earthquake and sound level intensity.
The graphic above shows a comparison of the brightest celestial objects visible to astronomers. The area of the circle is proportional to the brightness, so the amount of light sent to your eye from your monitor will correspond to how bright each object appears. To include the Sun, Full Moon, and fireball in the graphic, they had to be located “out of frame” and only partially shown. Compared to other celestial objects, they’re just too bright.
I made the Iridium circle 100 pixels in diameter as a base of reference. That allowed me to simulate all the other objects of interest at a size visible on most displays. An Iridium flare is reflected sunlight from a communication satellite. If you’d like to try observing an Iridium flare, go here, enter your location, and then click on Iridium Flares. You’ll get a list of when and where to watch for them. If the terms azimuth and altitude aren’t clear to you, go here.
FYI: The Sun is sized at 1700 pixels, the Moon at 230 pixels, and the fireball at 480 pixels. But that’s an “average” fireball. Their brightness is limited only by their size. And because they’re so rare and brief, there are very few accurate brightness measurements recorded. The recent Chelyabinsk fireball hit magnitude -27.3, which means it was brighter than the Sun.
The other objects in the graphic scale down from there to the faintest star your eyes can see (labeled “star”), which is about 1 pixel on my chosen scale. Whether you can even see that “star” will depend on your monitor size and eyesight. Sirius, by comparison, is the brightest star in the sky.
The planets included are shown at their maximum brightness. You may already know that planets vary in brightness over time, depending mostly on their distance from Earth. By coincidence, Mars (which is smaller and less reflective) has about the same maximum brightness as Jupiter. This is because Mars is so much closer to Earth.
The “supernova” shown is the brightest ever recorded, and was bright enough to see during daylight. Its magnitude was estimated based on written records. The star exploded in 1006 and was at a distance of 7200 light years. Known officially as SN 1006, the expanding remnant of that explosion is still being studied by astronomers. You can see an amazing image of that supernova remnant here.
The final object chosen was the International Space Station (ISS). It also varies in brightness during any given pass depending on the angle between its solar panels (the most reflective part) and the Sun. The value used (M = -5.9) is as bright as it can get under ideal conditions. If you’ve ever seen it, you know how bright the ISS can get. If you haven’t, check out my 11 Nov 2013 Sky Lights to learn how to spot it.
Everything else you can detect with your unaided eye (stars, nebulae, other planets) is between magnitude -1 and +6, and that includes thousands of objects. But by comparing objects spanning a larger range of brightness, it’s easier to convey the meaning of “magnitude.”
Next Week in Sky Lights ⇒ Waterfall in the Sky