| How to: Measuring Brightness ⬅︎ |
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Look up into the night sky and you’ll see a huge range of brightness. On one end is the full moon and on the other are countless stars and DSOs on the edge of visibility. To describe the brightness of an object, stargazers use a number called the apparent magnitude.
The diagram above shows that the apparent magnitude numbers that we use for stargazing range from about -13 to +14. There are some important things to know:
1) The dimmer the object, the higher the number.
2) For every five steps, there is exactly a 100x change in brightness, or about 2.5x per step.
As an example, see that Venus is close to -5 and Vega is close to 0. This means that Venus is about 100x brighter than Vega. Try to figure out how much dimmer Polaris is than Vega. The answer is below.
Notice the cutoffs for naked-eye stargazing, for 10x50 binoculars and a for a 6" telescope. The binoculars let you see about 3 magnitudes dimmer, or 16x. The telescope lets you see about 7 magnitudes dimmer, or 600x. Yes, optical aid makes a huge difference.
Because of light pollution, there is a cutoff, or limiting magnitude, of what can be seen. In a big city it may be +2.0. In a suburb it may be +4.0. At a dark site, you are only limited by what your eyes, binoculars or telescope are capable of. This is why a good location is so important.
Answer: With a 2 magnitude difference, Polaris is about 6.3x dimmer than Vega. (2.5 x 2.5). Be sure to check out the other topics below. |
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| ▶︎ Why Stargaze?
▶︎ The Night Sky
▶︎ North
▶︎ Planets
▶︎ Brightness
▶︎ Distance
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