Download Lesson 4. Wiens and Stefans Laws

WEIN’S LAW
T = temperature of
star in K
A black body is one
that absorbs all the
EM radiation that
falls incident on it
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
λmax T = constant
= 2.9 x 10-3 mK
Understand and explain
the characteristics of
stars
A BLACKBODY
A black body is one that absorbs all the EM radiation that
falls incident on it and reemits it.
The characteristics of the emitted light can tell us about
the temperature of that object.
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
WEIN’S LAW
λmax T = constant
= 2.9 x 10-3 mK
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
WEIN’S LAW
The peak wavelength of light will be
the colour of the star. The cooler the
star the longer the wavelength (the
redder the star)
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
STEFAN’S LAW
P =σAT4
P = power
A = surface area
T = temperature (K)
σ = stefan’s constant = 5.67 x 10-8 Wm-2K-4
Sometimes referred to as Stefan-Boltzman Law
(Stefan arrived at it experimentally, Boltzman did it
theoretically)
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
STEFAN’S LAW
Remember that the total
power radiated by the
star = luminosity
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
STEFAN’S LAW
The ratio of the power output of two stars can be used
to calculate the ratio of the diameters of the stars. (If
one of those starts has a known diameter e.g the Sun
the diameters of the other star can be determined)
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
Questions
1. The peak intensity of thermal radiation from the
Sun is at a wavelength of 500 nm, calculate the
surface temperature of the Sun.
2. A star has a power output of 6.0 x 1028 W and a
surface temperature of 3400K, calculate its radius
and the ratio to the Sun’s radius (rsun = 7 x 108 m)
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars
Questions
A red giant and a main sequence star have the
same absolute magnitude of 0. Their surface
temperatures are 3000K and 15000K
respectively. Show that the radius of the red
giant is 25 times larger than that of the main
sequence star.
Use a HR diagram to
Recall how a HR diagram
describe the life cycle of
is constructed
stars
Understand and explain
the characteristics of
stars