### Vega system

**(Johnson system)**
(magnitude calibration such that Vega has zero magnitude)

The **Vega system** or **Johnson system** of magnitude sets the zero
point for magnitudes, basically using the magnitude of the star
Vega as zero. This 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 have been adopted for the system
that only approximate Vega's current magnitudes.

A consequence of magnitudes being logarithmic is no magnitude
cannot represent "zero flux" since zero has no logarithm (or it
might be referred to as "minus infinity"). A brightness must be
chosen to be represented by zero magnitude, which defines the
magnitude's scale.

The *Vega system* can be thought of as part of (or the same thing as)
the UBV photometric system. The term *Vega system* might also
be used more generically as part of the description or means of
using other photometric systems, i.e., merely meaning the magnitudes
are calibrated so Vega is zero.

Other systems include the AB system, which makes no pretense
of matching any particular star.

(*measure,brightness,logarithmic,EMR,magnitude*)
**Further reading:**

http://en.wikipedia.org/wiki/Magnitude_(astronomy)

**Referenced by pages:**

AB system

Index