What is Luminosity?
The absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object.
Luminosity generally depends upon two factors:
- The size of the star: The larger a star is, the more energy it puts out, and the more luminous it is.
- The temperature of the star: If two stars are of the same size but have different temperatures, then the star with a higher temperature will be more luminous than the star with lower temperatures.
The Brightness of a Star
The brightness of a star which appears to our naked eyes mainly depends upon two factors:
- The luminosity of the star
- The distance of the star from the Earth
Luminosity depends on the surface area of the star. If the radius of a star is R then,
The surface area of the star = 4PR2
Two stars having the same temperature, one with radius 2R will have 4 times greater luminosity than a star with radius R.
The luminosity of a star also depends upon its temperature. When considering a star to be a completely black body, the radiation emitted per second will be according to the Stefan- Boltzmann law.
Energy emitted per second (E) = sAT4
- s= Stefan’s constant with a value of 5.7 × 10-8Wm-2K-4
- A= Surface Area of the Star
- T = absolute temperature of the star
Calculating the energy output for a star that is of the same size as the sun.
- R = 6.96×108 m
- T = 6000 K
So, surface area of sun = surface area of star,
Example of Luminosity
The Sun has a surface temperature of 6000K produces radiation with lmax = 420 nm. Find out the temperature of the Sirius if l max of Sirius is 72 nm.
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