Apparent Magnitude

[Magister]

What is the meaning of: "the apparent magnitude of a star is obtain with a system of 3 stars"?

Thanks

[mgb_phys]

"apparent magnitude" is how bright a star looks from earth.
It's on a rather odd log scale from 0=brightest, 6 is the about the limit with your eye, 26 is about the record for a big telescope.

Not sure about the "system of 3 stars" could mean either that you compare the unknown star with 3 known stars nearby ( is this the delta cephei experiment) .
The star Vega used to be defined as magnitude 0 but then you just use each magnitude being 2.5 times fainter than the previous - there is no standard mag=1,2,3 etc star.

[Chronos]

Sounds live an equivalent single star brightness question. The magnitude scale is logarithmic, so magnitudes do not multiply of divide linearly. For example 3 stars of apparent magnitude 3 would collectively appear about as bright as one star of apparent magnitude 1.8. The most straightforward way to obtain combined brightness of individual stars is to convert each individual magnitudes to luminosity, add them up and then convert back to magnitude.

[physicsatiztech]

"Apparent magnitude" is a kind of brightness quantity. But as you guess, brightness depends on the distance between observer and a object. Apparent magnitude is just the brightness, when you observe it from the Earth with, or from space telescope from space. But be careful, its unit is not just the same with brightness(whose is energy/time). After Hipparchus, who was the first sky watcher to determine the locations of the stars, and he also labeled the brightest star as 1 magnitude, and the faintest with 6 magnitude.