The brightness of a star is a basic observable quantity of an object. It is easy to observe two stars and say that star A is brighter than star B, but we need a way of quantifying this brightness so we can say that star A is x times as bright as star B. To this end the Magnitude Scale was introduced.
Common Magnitudes |
|
Bright |
|
| Sun | -26.73 |
| Full Moon | -12.60 |
| Mars | -2.90 |
| Sirius | -1.47 |
| Zero Point (Vega) | 0.00 |
| Saturn | 0.70 |
| Polaris | 2.00 |
| Naked Eye Limit | 6.00 |
| Limit of Hubble | 30.00 |
Faint |
|
History
The Greek mathematician Hipparchus is widely credited for the origin of the magnitude scale, but it was Ptolemy who popularised it and brought it to the main stream.
In his original scale, only naked eye objects were categorised (excluding the Sun), the brightest Planets were classified as magnitude 1, and the faintest objects were magnitude 6, the limit of the human eye. Each level of magnitude was considered to be twice the brightness of the previous; therefore magnitude 2 objects are twice as bright as magnitude 3 objects. This is a logarithmic magnitude scale.
With the discovery of many new objects, a modification was needed to this system in order to accurately categorise so many objects. In 1856 Norman Robert Pogson formalised the magnitude scale by defining that a first magnitude object is an object that is 100 times brighter than a sixth magnitude object, thus a first magnitude star is 2.512 times brighter than a second magnitude object.
Pogson’s scale was originally fixed by assigning Polaris a magnitude of 2. Astronomers later discovered that Polaris is slightly variable, so they first switched to Vega as the standard reference star, and later again switched to using tabulated zero points for the measured fluxes. This is the system used today.
Luminosity
Brightness is more scientifically called Luminosity, and is a measure of the amount of energy a body radiates per unit time.
Two Magnitude Scales
Going back to star A and star B, lets say that star A is magnitude 2 and star B is magnitude 3, so star A appears to be 2.512 times as luminous than star B. Here we are referring to the stars Apparent Magnitude, that is, the luminosity as seen from Earth. This is how most magnitudes are presented on TV, planetarium software and magazines.
But how do we know that Star A is actually more luminous than Star B? It is entirely possible for Star A and star B to have the same luminosity, but star B could be 2.512 times further away than star A, thus appears dimmer to us from Earth.
We now need another scale called the Absolute Magnitude, which is defined as the apparent magnitude if the object were a standard distance from the Earth. This standard distance is 10 parsecs. Absolute magnitude is often given the symbol M, while apparent magnitude is usually given the lower case m.
Our sun has an apparent magnitude of -26.73, which is obviously the brightest object visible in the sky, however the Sun would not be as bright if it was 10 parsecs away. At this distance it would only shine at absolute magnitude 4.6, so it would be quite faint in the night sky.
Sirius is the next brightest star in the sky, with an apparent magnitude of -1.47, however it only lies 2.64 parsecs away so it is relatively close. If it was a standard 10 parsecs away it would be absolute magnitude 1.4, that’s 8 times brighter that our Sun at the same distance.
In conclusion
- Apparent magnitude is a logarithmic scale of the brightness of an object as seen from Earth.
- Absolute Magnitude is the Apparent magnitude if the object was a standard distance away (10 parsecs).
- Magnitude 1 objects are 2.512 times brighter than magnitude 2 objects which are 2.512 times brighter than magnitude 3 and so on.
