Magnitude.

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Presentation transcript:

Magnitude

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.

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 0.0001 L Maximum 10,000,000 L

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).

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.

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

Apparent Magnitude The apparent magnitude of stars is set to Vega. Some bright stars m Sun -26.7 Sirius -1.4 Alpha Centauri -0.3 Vega 0.0 Capella 0.1 Rigel 0.1 Betelgeuse 0.5 Aldebaran 0.9 ucolick.org

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.

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