What is the magnitude scale?
The magnitude scale is a logarithmic scale in which each integral step corresponds to a change of approximately 2.5 times in brightness. Brighter objects have smaller magnitudes than dimmer ones. For example, an object with magnitude m = 1 is about 2.5 times fainter than an object with magnitude m = 0.
The magnitude scale originated with the Greeks, who designated the brightest stars in the sky as "first magnitude" and the faintest stars that the eye can see as "sixth magnitude." (The Greeks hadn't yet learned about the number "0!") Astronomers later quantified the magnitude scale, and extended it to brighter and fainter stars. With binoculars or a small telescope, you can see stars as faint as about 10th magnitude, and the Hubble Space Telescope can detect stars as faint as 30th magnitude, about 10 billion times fainter than the eye can see.
The magnitude of a star depends on two factors, the intrinsic brightness of the star and its distance from us. A star which is twice as far away as another star of the same intrinsic brightness will appear only one fourth as bright.
The novae you will discover have magnitudes ranging from roughly m=15 to m=20. They would appear somewhere between the brightest quasar and the faintest object on the scale above.
All of the stars in Andromeda are roughly at the same distance, give or take a hundred thousand light years or so (out of 2.2 million!). The brightness of the novae you discover depends their intrinsic brightness as well as the time that the observations are taken. We might catch a nova just after it explodes, when it is brightest, or days or weeks later, after it has faded.
The magnitudes of novae in Andromeda can be determined by comparing them to nearby stars of known brightness. Use the magnitude comparison images to find stars similar in brightness to the novae you discover, and estimate the magnitude of your novae.
For more information about the magnitude scale, check out these sites:
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