Best picture I've seen of Comet Lovejoy

The colors you're seeing are a bit more exaggerated and aren't exactly precise. Exposure, distance and alterations of hue's can give different variations of the same image; basically, the apparent luminosity -- given the frame of reference, is relative to the equipment used and the distance at which the object is observed. Putting that aside, Comet's are often referred to as "bright," if their magnitude is larger than 12 because at that magnitude, the comet should still be visible through amateur aperture telescopes, without the need of substantial exposure times.

I'm not sure how familiar you are with the calculations involved in determining magnitudes of luminosity so i'll try my best to explain it - apparent magnitude is how "bright" something appears to us on Earth based on a scale that was created by Hipparchus, given by the formula: M1-M2 = -2.50log(B2/B1), where M1 and M2 stand for apparent magnitudes of two different "objects," and B2/B1 stands for brightness. Knowing that this is a logarithmic equation, a large range in brightness will result in a smaller range of magnitude, A.K.A larger magnitude = lower brightness. Luminosity can be calculated by knowing the relation between distance, temperature, and distance : B = L/4πd2 , where B is apparent brightness, d is distance, and L is luminosity : through basic algebra you can rewrite the expression to determine the luminosity assuming you know the distance : L = 4π/d2 B. Understanding the relationship will tell you that it's entirely possible for a small and hot star to be more luminous than a larger, cool star. That's why a feint star can appear to be brighter than an obviously more luminous star due to its distance - a good example being between the Sun and Spica. The sun has an apparent magnitude of -23.7~ and Spica has an apparent magnitude of 0.9~, although it might seem as though the sun is significantly more luminous because of its negative magnitude value, Spica is actually 12000~ times more luminous than the sun, but since it's much closer to Earth, it appears brighter.

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