Brightness and Magnitude

Choose apparent brightness:

magnitude

m

1

magnitude

m

2

or choose specific objects to compare:

object 1

Sun

Moon

Vega

Sirius

Canopus

Alpha Centauri

Regulus

Polaris

object 2

Sun

Moon

Vega

Sirius

Canopus

Alpha Centauri

Regulus

Polaris

m

1

= -26

m

2

= 0

The first object is

 -26- 0

2.5

10

= 2.5×

10

times brighter than object 2.

The brightness ratio between two celestial objects, the flux ratio, is defined as

ϕ

1

ϕ

2

=

|

m

1

-

m

2

|

2.5

10

, where

m

1

and

m

2

are the apparent magnitudes of the objects. The magnitude scale is a much more convenient way to express brightness measurements than using actual brightness measurements that use much larger numbers. Brightness and magnitude are important introductory topics in basic astronomy.