25 Astronomy and cosmology
25.1 Standard candles
• Luminosity L is defined as the total power output of radiation emitted by a
star.
• It is measured in Watts
• The observed amount of intensity F is the observed amount of intensity, or
the radiant power transmitted normally through a surface per unit of area,
of radiation measured on defined as Earth.
• Light leaving a star can be assumed to be a uniformly spread out like a spherical
shell.
• Hence, the inverse square law of flux can therefore be calculated using
𝐿
𝐹=
4𝜋𝑑 2
Here L is the luminosity of the source (watts), d is the distance between the
star and Earth (m).
• F is measured in Wm-2
• Standard candle is defined as an astronomical object which has a known
luminosity due to a characteristic quality possessed by that class of object.
• By knowing luminosity of a source, the distance can be estimated based on how
bright it appears on Earth.
25.2 Stellar radii
• Wien’s Law states that the black body radiation curve for different
temperatures peaks at a wavelength λmax which is inversely proportional to
the temperature (T)
25.1 Standard candles
• Luminosity L is defined as the total power output of radiation emitted by a
star.
• It is measured in Watts
• The observed amount of intensity F is the observed amount of intensity, or
the radiant power transmitted normally through a surface per unit of area,
of radiation measured on defined as Earth.
• Light leaving a star can be assumed to be a uniformly spread out like a spherical
shell.
• Hence, the inverse square law of flux can therefore be calculated using
𝐿
𝐹=
4𝜋𝑑 2
Here L is the luminosity of the source (watts), d is the distance between the
star and Earth (m).
• F is measured in Wm-2
• Standard candle is defined as an astronomical object which has a known
luminosity due to a characteristic quality possessed by that class of object.
• By knowing luminosity of a source, the distance can be estimated based on how
bright it appears on Earth.
25.2 Stellar radii
• Wien’s Law states that the black body radiation curve for different
temperatures peaks at a wavelength λmax which is inversely proportional to
the temperature (T)
