1. Magnitude

2. Seeing Stars
There are about 5000 stars visible to the unaided eye on a clear moonless night.
Ancient astronomers measured certain properties.
Brightness
Color
Distance should be determined by parallax, but the stars were too far without a telescope.

3. Light Source
The energy produced by an object each second is its luminosity (L).
Watts from a light
Absolute brightness
Astronomers measure luminosity relative to the Sun.
LSun = 1 L
LSirius = 23 L
Stars have a wide range of luminosities.
Minimum L
Maximum 10,000,000 L

4. Brightness and Distance
Light travels equally in all directions from a source.
Double the sphere, four times the area
Star brightness (B) decreases as the square of the distance to the star (d).

5. Photometer Light goes out in all directions from a source.
Falls on a small area
The ratio of luminosity to area is the flux or irradiance.
Apparent brightness
Photometers measure irradiance.

6. Six Classes
In the second century BC Hipparchus grouped stars based on their brightness.
Six categories
1 for brightest, 6 for dimmest
In 1856 this system of magnitude was made mathematical by Norman Pogson.
Five points smaller = 100 times brighter
One point = 2.5 times brighter or dimmer
Logarithmic scale

7. Apparent Magnitude The apparent magnitude of stars is set to Vega.
Some bright stars m
Sun
Sirius -1.4
Alpha Centauri -0.3
Vega 0.0
Capella 0.1
Rigel 0.1
Betelgeuse 0.5
Aldebaran 0.9
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8. Distance Correction
Distance information is needed to get the luminosity.
Measure in parsecs (pc)
Alpha Centauri: 1.3 pc = 4.3 ly
The magnitude changes with distance.
Example: 2 identical stars
A is 7 pc from Earth
B is 70 pc from Earth
The apparent brightness of B is 1/100 that of A
The magnitude of B is 5 larger.

9. Absolute Magnitude
Absolute magnitude (M) measures brightness as if the stars were 10 pc away.
Distance from Earth (D)
Different from apparent magnitude (m)
Some bright stars M
Sun 4.8
Sirius 1.4
Alpha Centauri 4.1
Vega 0.6
Capella 0.4
Rigel -7.1
Betelgeuse -5.6
Aldebaran -0.3