What does star magnitude mean

Astronomers use a system of magnitudes to accurately measure the brightnesses of astronomical objects. An object is said to have a certain numerical magnitude. The larger the magnitude number, the fainter the object. Each object with an increased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye (no telescope) is about sixth magnitude (from dark skies) or magnitude 6.0, whereas the brightest stars are negative numbers. Sirius is the brightest appearing star in the sky and is magnitude -1.5.

Apparent magnitudes of selected objects
Sun                             -26.7
Full moon                   -12.6
Venus (maximum)     -4.7
Sirius                          -1.5
Saturn (maximum)     -0.2
Naked-eye limit         6.0
Neptune (maximum) 7.7
Quasar 3C-273         12.9
Pluto (maximum)       13.7
8-inch telescope        14.2 (limit)
Hubble Telescope     30.0 (limit)

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