| Astrophysics (Index) | About |
The terms Vega system and Johnson system are used for a convention regarding taking the zero point of (stellar) magnitude scales such that the corresponding magnitudes of the star Vega are zero. This convention is used for a general (visible light) magnitude, bolometric magnitude, and for magnitudes associate with individual passbands of photometric systems, i.e., a star's B-magnitude would be zero if it matches the B-magnitude of Vega. Though the Vega system's principle is "Vega's magnitude is zero", in fact, absolute brightnesses are generally used that only approximate Vega's current magnitudes; though Vega was chosen as a star with stable magnitudes, all stars experience some variation and current technology senses that of Vega.
Magnitudes require some adopted zero point as a consequence of their being logarithmic: "zero flux" cannot be used since zero has no logarithm (or its logarithm might be referred to as "minus infinity"). A brightness-level must be chosen to be represented by "zero magnitude", part of the definition of the magnitude's scale.
The Vega system is associated with the UBV photometric system and is often considered part of it, or even the same thing, though the definition of passbands and the definition of magnitude zero-points are not dependent upon each other and the Vega system's magnitude scale can easily be used with passbands from other photometric systems. The current widely-used alternative to the Vega system is the AB system, which makes no pretense of pegging "zero magnitude" to any particular star. Another system, associated with Hubble Space Telescope is the STMAG system.
The term Vega system has an unrelated use: to indicate Vega's planetary system, which has been of some research interest.