Expert: Courtney Seligman - 2/24/2011

Question
Hello! I need some help on a question in my astronomy class, I've tried to understand this and figure it out on my own but I am now more confused than when I started and am feeling frustrated. If you could help me with the answer or guide me in how to obtain it I would GREATLY appreciate it! Thank You!
What is the intensity brightness difference between Venus at apparant magnitude of -4 and the star Regulus at an apparant magnitude of +.1?

Answer
Somewhere in the textbook discussion of magnitudes it should point out that each magnitude represents a difference of about 2.5 times in brightness, and 5 magnitudes represents a difference of 2.5 to the fifth power, or 100 times. Odds are there is even a table showing how much of a difference in brightness there is for a given difference in magnitude.

So just find the difference between Regulus (which is actually +1, not 0.1) and Venus. That is, subtract one value from the other. It doesn't make any difference which way you do it; if you subtract the larger number from the smaller one, you'll get a negative result; if you subtract the smaller one from the bigger one you'll get a positive result; but all you're concerned about is the number -- is it 2 magnitudes, 3, 5, etc? Then compare that to the table or discussion in the book, and you'll see that one of the objects is about 100 times brighter than the other. To be sure which is which, you'll need to reread the discussion of how the magnitude scale works (you don't want to say that Venus is 100 times brighter than Regulus, if it's actually the other way around).